GDB scripting and Indirect functions

// in `samples/puts.c`

#include <stdio.h>

int main() {
    puts("Hello from C");
    return 0;
}

$ cd samples/
$ gcc puts.c -o puts
$ ../target/debug/elk ./puts
Loading "/home/amos/ftl/elk/samples/puts"
Loading "/usr/lib/libc-2.32.so"
Fatal error: Could not read symbols from ELF object: Parsing error: String("Unknown SymType 10 (0xa)"):
input: 1a 00 10 00 a0 bf 0b 00 00 00 00 00 c1 00 00 00 00 00 00 00

// in `samples/nolibc.c`

int main() {
    return 0;
}

$ # in samples/
$ gcc -nostartfiles -nodefaultlibs nolibc.c -o nolibc
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000001000

$ ./nolibc
[1]    9994 segmentation fault (core dumped)  ./nolibc

// in `samples/nolibc.c`

int _start() {
    return 0;
}

$ gcc -nostartfiles -nodefaultlibs nolibc.c -o nolibc
$

$ ./nolibc
[1]    10208 segmentation fault (core dumped)  ./nolibc

// in `samples/nolibc.c`

#include <stdlib.h>

void _start() {
    exit(42);
}

$ gcc -nostartfiles -nodefaultlibs nolibc.c -o nolibc
/usr/bin/ld: /tmp/ccKtXh1Q.o: in function `_start':
nolibc.c:(.text+0xa): undefined reference to `exit'
collect2: error: ld returned 1 exit status

$ nm -D /usr/lib/libc-2.32.so | grep "T exit"
000000000003e730 T exit

// in `samples/nolibc.c`

void _start() {
    // note: GCC inline assembly syntax is kinda funky, just go with it
    __asm__ ("movq $42,%rdi\n\t"
             "mov $60,%rax\n\t"
             "syscall");
}

$ gcc -nostartfiles -nodefaultlibs nolibc.c -o nolibc
$ ./nolibc
$

$ ./nolibc
$ echo $?
42

$ ../target/debug/elk ./nolibc
Loading "/home/amos/ftl/elk/samples/nolibc"
$ echo $?
42

// in `samples/hello-nolibc.c`

// This mimics libc's `exit` function, but has a different name,
// because *even* with -nostartfiles and -nodefaultlibs, and no #include
// directives, GCC will complain that `exit`, which is declared *somewhere*
// as "noreturn", does in fact return.
void ftl_exit(int code) {
    // note: in AT&T syntax, it's "mov src, dst"
    // the exact opposite of what we're used to in this series!
    // also, the "%" in register names like "%edi" needs to be
    // escaped, that's why it's doubled as "%%"
    __asm__ (
            " \
            mov     %[code], %%edi \n\t\
            mov     $60, %%rax \n\t\
            syscall"
            : // no outputs
            : [code] "r" (code)
            );
}

void ftl_print(char *msg) {
    // this is a little ad-hoc "strlen"
    int len = 0;
    while (msg[len]) {
        len++;
    }

    __asm__ (
            " \
            mov      $1, %%rdi \n\t\
            mov      %[msg], %%rsi \n\t\
            mov      %[len], %%edx \n\t\
            mov      $1, %%rax \n\t\
            syscall"
            // outputs
            :
            // inputs
            : [msg] "r" (msg), [len] "r" (len)
            );
}

// here's our fake `main()` function
int main() {
    ftl_print("Hello from C!\n");
    return 0;
}

// and here's the *actual* entry point
void _start() {
    ftl_exit(main());
}

$ gcc -nostartfiles -nodefaultlibs hello-nolibc.c -o hello-nolibc
$ ./hello-nolibc
Hello from C!

$ ../target/debug/elk ./hello-nolibc
Loading "/home/amos/ftl/elk/samples/hello-nolibc"
Hello from C!

$ ../target/debug/elk ./puts
Loading "/home/amos/ftl/elk/samples/puts"
Loading "/usr/lib/libc-2.32.so"
Fatal error: Could not read symbols from ELF object: Parsing error: String("Unknown SymType 10 (0xa)"):
input: 1a 00 10 00 a0 bf 0b 00 00 00 00 00 c1 00 00 00 00 00 00 00

// in `samples/ifunc-nolibc.c`

// this used to be part of `ftl_print`.
int ftl_strlen(char *s) {
    int len = 0;
    while (s[len]) {
        len++;
    }
    return len;
}

void ftl_print(char *msg) {
    int len = ftl_strlen(msg);

    // I just wanted to show off that, in the input/output mappings,
    // you can use any old C expression - here, a call to `ftl_strlen`
    __asm__ (
            " \
            mov      $1, %%rdi \n\t\
            mov      %[msg], %%rsi \n\t\
            mov      %[len], %%edx \n\t\
            mov      $1, %%rax \n\t\
            syscall"
            // outputs
            :
            // inputs
            : [msg] "r" (msg), [len] "r" (ftl_strlen(msg))
            );
}

// same as before
void ftl_exit(int code) {
    __asm__ (
            " \
            mov     %[code], %%edi \n\t\
            mov     $60, %%rax \n\t\
            syscall"
            : // no outputs
            : [code] "r" (code)
            );
}

// Here's the implementation of `get_msg` for the root user
char *get_msg_root() {
    return "Hello, root!\n";
}

// Here's the implementation of `get_msg` for a regular user
char *get_msg_user() {
    return "Hello, regular user!\n";
}

// C function pointer syntax is.. well, "funky" doesn't even
// start to cover it, so let's make a typedef with the type
// of `get_msg`:
typedef char *(*get_msg_t)();

// Here's our selector for `get_msg` - it'll return the
// right implementation based on the current "uid" (user ID).
static get_msg_t resolve_get_msg() {
    int uid;

    // make a `getuid` syscall. It has no parameters,
    // and returns in the `%rax` register.
    __asm__ (
            " \
            mov     $102, %%rax \n\t\
            syscall \n\t\
            mov     %%eax, %[uid]"
            : [uid] "=r" (uid)
            : // no inputs
            );

    if (uid == 0) {
        // UID 0 is root
        return get_msg_root;
    } else {
        // otherwise, it's a regular user
        return get_msg_user;
    }
}

// And here's our `get_msg` declaration, finally!
// Using the GCC-specific `ifunc` attribute.
char *get_msg() __attribute__ ((ifunc ("resolve_get_msg")));

int main() {
    // print whatever `get_msg` returns
    ftl_print(get_msg());
    return 0;
}

void _start() {
    ftl_exit(main());
}

$ gcc -nostartfiles -nodefaultlibs ifunc-nolibc.c -o ifunc-nolibc
$ ./ifunc-nolibc
Hello, regular user!

$ gcc -nostartfiles -nodefaultlibs ifunc-nolibc.c -o ifunc-nolibc
$ sudo ./ifunc-nolibc
[sudo] password for coolbear
Hello, root!

$ ../target/debug/elk ./ifunc-nolibc
Loading "/home/amos/ftl/elk/samples/ifunc-nolibc"
[1]    21183 segmentation fault (core dumped)  ../target/debug/elk ./ifunc-nolibc

$ gdb --args ../target/debug/elk ./ifunc-nolibc
(gdb) break jmp
Breakpoint 1 at 0x25539: file src/main.rs, line 49.
(gdb) r
Starting program: /home/amos/ftl/elk/target/debug/elk ./ifunc-nolibc
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
Loading "/home/amos/ftl/elk/samples/ifunc-nolibc"

Breakpoint 1, elk::jmp (addr=0x7ffff7fc810a "UH\211\345\270\000") at src/main.rs:49
49          let fn_ptr: fn() = std::mem::transmute(addr);

void _start() {
    ftl_exit(main());
}

    ; function prologue
    push rbp
    mov rbp, rsp

    ; not sure why this is necessary
    mov eax, 0x0
    ; call `ftl_main`
    call 0x7ffff7fc40ed

    ; mov return value of `main` to first argument of `ftl_exit`
    mov edi, eax
    ; call `ftl_exit`
    call 0x7ffff7fc4091

(gdb) info sym 0x7ffff7fc40ed
No symbol matches 0x7ffff7fc40ed.
(gdb) info sym 0x7ffff7fc40ed
No symbol matches 0x7ffff7fc40ed.

(gdb) info proc
process 26835
cmdline = '/home/amos/ftl/elk/target/debug/elk ./ifunc-nolibc'
cwd = '/home/amos/ftl/elk/samples'
exe = '/home/amos/ftl/elk/target/debug/elk'

$ cat /proc/26835/maps | grep nolibc
7ffff7fc3000-7ffff7fc4000 r--p 00000000 08:30 564454                     /home/amos/ftl/elf-series/samples/ifunc-nolibc
7ffff7fc4000-7ffff7fc5000 r-xp 00001000 08:30 564454                     /home/amos/ftl/elf-series/samples/ifunc-nolibc
7ffff7fc5000-7ffff7fc6000 r--p 00002000 08:30 564454                     /home/amos/ftl/elf-series/samples/ifunc-nolibc
7ffff7fc6000-7ffff7fc8000 rw-p 00002000 08:30 564454                     /home/amos/ftl/elf-series/samples/ifunc-nolibc

$ readelf -a ./ifunc-nolibc
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                 0x0000000000000268 0x0000000000000268  R      0x8
  INTERP         0x00000000000002a8 0x00000000000002a8 0x00000000000002a8
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000340 0x0000000000000340  R      0x1000
  LOAD           0x0000000000001000 0x0000000000001000 0x0000000000001000
                 0x0000000000000122 0x0000000000000122  R E    0x1000
  LOAD           0x0000000000002000 0x0000000000002000 0x0000000000002000
                 0x00000000000001b4 0x00000000000001b4  R      0x1000
  LOAD           0x0000000000002ef0 0x0000000000003ef0 0x0000000000003ef0
                 0x0000000000000130 0x0000000000000130  RW     0x1000

(gdb) p/x 0x7ffff7fc40ed - 0x7ffff7fc4000 + 0x1000
$1 = 0x10ed
(gdb) p/x 0x7ffff7fc4091 - 0x7ffff7fc4000 + 0x1000
$2 = 0x1091

$ nm samples/ifunc-nolibc | grep -E '(10ed|1091)'
0000000000001091 T ftl_exit
00000000000010ed T main

$ readelf --sections ifunc-nolibc | head -10
There are 18 section headers, starting at offset 0x3480:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .interp           PROGBITS         00000000000002a8  000002a8
       000000000000001c  0000000000000000   A       0     0     1
(cut)
  [ 8] .text             PROGBITS         0000000000001020  00001020
       0000000000000102  0000000000000000  AX       0     0     1

# in samples/autosym.py

pid = gdb.selected_inferior().pid
print("the inferior's PID is %d" % pid)

(gdb) source autosym.py
the inferior's PID is 21562

maps = open("/proc/%d/maps" % pid).read()
print(maps)

$ cd elk/
$ cargo add argh@0.1
cargo add argh
      Adding argh v0.1.4 to dependencies

// in `elk/src/main.rs`

use argh::FromArgs;

#[derive(FromArgs, PartialEq, Debug)]
/// Top-level command
struct Args {
    #[argh(subcommand)]
    nested: SubCommand,
}

#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand)]
enum SubCommand {
    Autosym(AutosymArgs),
    Run(RunArgs),
}

#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand, name = "autosym")]
/// Given a PID, spit out GDB commands to load all .so files
/// mapped in memory.
struct AutosymArgs {
    #[argh(positional)]
    /// the PID of the process to examine
    pid: u32,
}

#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand, name = "run")]
/// Load and run an ELF executable
struct RunArgs {
    #[argh(positional)]
    /// the absolute path of an executable file to load and run
    exec_path: String,
}

// in `src/elk/main.rs`

// this is unchanged - it's just our way of using the `Display` trait rather
// than `Debug` when an `Error` bubbles all the way up. Since we use the
// `thiserror` crate, this gives us nicer output.
fn main() {
    if let Err(e) = do_main() {
        eprintln!("Fatal error: {}", e);
    }
}

fn do_main() -> Result<(), Box<dyn Error>> {
    // we now have several subcommands to run so, after `argh`
    // parsed them, we need to run the right one.
    let args: Args = argh::from_env();
    match args.nested {
        SubCommand::Run(args) => cmd_run(args),
        SubCommand::Autosym(args) => cmd_autosym(args),
    }
    // Note that everything is strongly typed: we couldn't call
    // `cmd_run` with `AutosymArgs`, or the other way around.
    //
    // Note also that rustc does an exhaustiveness check here -
    // if `SubCommand` had another variant, it would tell us
    // (at compile time) to handle it, or add a catch-all arm
    // to our `match`.
}

fn cmd_autosym(args: AutosymArgs) -> Result<(), Box<dyn Error>> {
    todo!()
}

// this function is largely what `do_main` used to be...
fn cmd_run(args: RunArgs) -> Result<(), Box<dyn Error>> {
    let mut proc = process::Process::new();
    // ...except we now take `exec_path` from the `args` argument instead of
    // calling `std::env::args()` ourselves!
    let exec_index = proc.load_object_and_dependencies(args.exec_path)?;
    proc.apply_relocations()?;
    proc.adjust_protections()?;

    let exec_obj = &proc.objects[exec_index];
    let entry_point = exec_obj.file.entry_point + exec_obj.base;
    unsafe { jmp(entry_point.as_ptr()) };

    Ok(())
}

$ cargo build -q
$ ../target/debug/elk
One of the following subcommands must be present:
    help
    autosym
    run

$ ../target/debug/elk help
../target/debug/elk help
Usage: ../target/debug/elk <command> [<args>]

Top-level command

Options:
  --help            display usage information

Commands:
  autosym           Given a PID, spit out GDB commands to load all .so files
                    mapped in memory.
  run               Load and run an ELF executable

$ ../target/debug/elk help run
Usage: ../target/debug/elk run <exec_path>

Load and run an ELF executable

Options:
  --help            display usage information

$ ../target/debug/elk run ./hello-dl
Loading "/home/amos/ftl/elk/samples/hello-dl"
hi there

// in `elk/src/main.rs`

fn cmd_autosym(args: AutosymArgs) -> Result<(), Box<dyn Error>> {
    let maps = std::fs::read_to_string(format!("/proc/{}/maps", args.pid))?;
    println!("maps = \n{}", maps);

    todo!()
}

$ cd elk/
$ cargo add nom@5
      Adding nom v5.1.2 to dependencies

// in `elk/src/main.rs`

mod procfs;

// in `elk/src/procfs.rs`

use nom::{
    branch::alt,
    bytes::complete::{tag, take_while, take_while1},
    combinator::{all_consuming, map, opt, value},
    error::ParseError,
    multi::many0,
    sequence::{delimited, preceded, separated_pair, terminated, tuple},
    IResult, InputTakeAtPosition,
};
use std::fmt;

/// returns true if a character is a (lower-case) hexadecimal digit
fn is_hex_digit(c: char) -> bool {
    "0123456789abcdef".contains(c)
}

/// parses 0 or more spaces and tabs
fn whitespace<I, E>(i: I) -> IResult<I, I, E>
where
    I: InputTakeAtPosition<Item = char>,
    E: ParseError<I>,
{
    take_while(|c| " \t".contains(c))(i)
}

/// execute and return the child parser's result, ignoring leading and trailing
/// spaces and tabs
fn spaced<I, O, E>(f: impl Fn(I) -> IResult<I, O, E>) -> impl Fn(I) -> IResult<I, O, E>
where
    I: InputTakeAtPosition<Item = char> + Clone + PartialEq,
    E: ParseError<I>,
{
    preceded(whitespace, terminated(f, whitespace))
}

/// parses a lower-case hexadecimal number as a delf::Addr
fn hex_addr(i: &str) -> IResult<&str, delf::Addr> {
    // `take_while1` requires at least one character
    let (i, num) = take_while1(is_hex_digit)(i)?;
    // FIXME: reckless use of expect
    let u = u64::from_str_radix(num, 16).expect("our hex parser is wrong");
    Ok((i, u.into()))
}

/// parses a delf::Addr range in the form 0000-ffff
fn hex_addr_range(i: &str) -> IResult<&str, std::ops::Range<delf::Addr>> {
    let (i, (start, end)) = separated_pair(hex_addr, tag("-"), hex_addr)(i)?;
    Ok((i, start..end))
}

/// memory mapping permission bits
pub struct Perms {
    /// readable
    pub r: bool,
    /// writable
    pub w: bool,
    /// executable
    pub x: bool,
    /// not sure, tbh
    pub p: bool,
}

impl fmt::Debug for Perms {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let bit = |val, display| {
            if val {
                display
            } else {
                "-"
            }
        };
        write!(
            f,
            "{}{}{}{}",
            bit(self.r, "r"),
            bit(self.w, "w"),
            bit(self.x, "x"),
            bit(self.p, "p"),
        )
    }
}

/// parses mapping permissions as seen in `/proc/:pid/maps`
fn perms(i: &str) -> IResult<&str, Perms> {
    /// parses a single permission bit. for example, the readable
    /// bit can be either "r" or "-".
    fn bit(c: &'static str) -> impl Fn(&str) -> IResult<&str, bool> {
        move |i: &str| -> IResult<&str, bool> {
            alt((value(false, tag("-")), value(true, tag(c))))(i)
        }
    }
    let (i, (r, w, x, p)) = tuple((bit("r"), bit("w"), bit("x"), bit("p")))(i)?;
    Ok((i, Perms { r, w, x, p }))
}

/// parses a decimal number as an u64
fn dec_number(i: &str) -> IResult<&str, u64> {
    let (i, s) = take_while1(|c| "0123456789".contains(c))(i)?;
    // FIXME: reckless use of expect
    let num: u64 = s.parse().expect("our decimal parser is wrong");
    Ok((i, num))
}

/// parses a hexadecimal number as an u64
fn hex_number(i: &str) -> IResult<&str, u64> {
    let (i, s) = take_while1(|c| "0123456789abcdefABCDEF".contains(c))(i)?;
    // FIXME: reckless use of expect
    let num = u64::from_str_radix(s, 16).expect("our hexadecimal parser is wrong");
    Ok((i, num))
}

/// a Linux device number
pub struct Dev {
    pub major: u64,
    pub minor: u64,
}

impl fmt::Debug for Dev {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}:{}", self.major, self.minor)
    }
}

/// parses a Linux device number in form major:minor, where
/// major and minor are hexadecimal numbers
fn dev(i: &str) -> IResult<&str, Dev> {
    let (i, (major, minor)) = separated_pair(hex_number, tag(":"), hex_number)(i)?;
    Ok((i, Dev { major, minor }))
}

/// Source for a mapping: could be special (stack, vdso, etc.),
/// a file, or an anonymous mapping
#[derive(Debug)]
pub enum Source<'a> {
    /// not backed by a file
    Anonymous,
    /// not backed by a file either, *and* special-purpose
    Special(&'a str),
    /// backed by a file
    File(&'a str),
}

impl<'a> Source<'_> {
    pub fn is_file(&self) -> bool {
        matches!(self, Self::File(_))
    }
}

fn source(i: &str) -> IResult<&str, Source<'_>> {
    fn is_path_character(c: char) -> bool {
        // kinda jank, and won't support files with actual spaces in them,
        // but largely good enough for our use case.
        c != ']' && !c.is_whitespace()
    }

    fn path(i: &str) -> IResult<&str, &str> {
        take_while(is_path_character)(i)
    }

    alt((
        map(delimited(tag("["), path, tag("]")), Source::Special),
        map(path, |s| {
            if s.is_empty() {
                Source::Anonymous
            } else {
                Source::File(s)
            }
        }),
    ))(i)
}

#[derive(Debug)]
pub struct Mapping<'a> {
    pub addr_range: std::ops::Range<delf::Addr>,
    pub perms: Perms,
    pub offset: delf::Addr,
    pub dev: Dev,
    pub len: u64,
    pub source: Source<'a>,
    pub deleted: bool,
}

fn mapping(i: &str) -> IResult<&str, Mapping> {
    let (i, (addr_range, perms, offset, dev, len, source, deleted)) = tuple((
        spaced(hex_addr_range),
        spaced(perms),
        spaced(hex_addr),
        spaced(dev),
        spaced(dec_number),
        spaced(source),
        spaced(map(opt(tag("(deleted)")), |o| o.is_some())),
    ))(i)?;
    let res = Mapping {
        addr_range,
        perms,
        offset,
        dev,
        len,
        source,
        deleted,
    };
    Ok((i, res))
}

pub fn mappings(i: &str) -> IResult<&str, Vec<Mapping>> {
    all_consuming(many0(terminated(spaced(mapping), tag("\n"))))(i)
}

// in `elk/src/main.rs`

fn cmd_autosym(args: AutosymArgs) -> Result<(), Box<dyn Error>> {
    let maps = std::fs::read_to_string(format!("/proc/{}/maps", args.pid))?;

    match procfs::mappings(&maps) {
        Ok((_, mappings)) => {
            println!("Found {} mappings", mappings.len());
            for mapping in &mappings {
                println!("{:?}", mapping);
            }
        }
        Err(e) => panic!("parsing failed: {:?}", e),
    };

    Ok(())
}

$ cargo build
$ ../target/debug/elk autosym 27113
Found 45 mappings
Mapping { addr_range: 0000555555554000..0000555555562000, perms: r--p, offset: 0000000000000000, dev: 8:1, len: 3336791, source: File("/home/amos/ftl/elk/target/debug/elk"), deleted: true }
Mapping { addr_range: 0000555555562000..0000555555630000, perms: r-xp, offset: 000000000000e000, dev: 8:1, len: 3336791, source: File("/home/amos/ftl/elk/target/debug/elk"), deleted: true }
Mapping { addr_range: 0000555555630000..0000555555665000, perms: r--p, offset: 00000000000dc000, dev: 8:1, len: 3336791, source: File("/home/amos/ftl/elk/target/debug/elk"), deleted: true }
Mapping { addr_range: 0000555555666000..000055555566c000, perms: r--p, offset: 0000000000111000, dev: 8:1, len: 3336791, source: File("/home/amos/ftl/elk/target/debug/elk"), deleted: true }
Mapping { addr_range: 000055555566c000..000055555566d000, perms: rw-p, offset: 0000000000117000, dev: 8:1, len: 3336791, source: File("/home/amos/ftl/elk/target/debug/elk"), deleted: true }
Mapping { addr_range: 000055555566d000..000055555568e000, perms: rw-p, offset: 0000000000000000, dev: 0:0, len: 0, source: Special("heap"), deleted: false }
Mapping { addr_range: 00007ffff7d85000..00007ffff7d87000, perms: rw-p, offset: 0000000000000000, dev: 0:0, len: 0, source: Anonymous, deleted: false }
Mapping { addr_range: 00007ffff7d87000..00007ffff7d8a000, perms: r--p, offset: 0000000000000000, dev: 8:1, len: 790795, source: File("/usr/lib/libgcc_s.so.1"), deleted: false }
Mapping { addr_range: 00007ffff7d8a000..00007ffff7d9b000, perms: r-xp, offset: 0000000000003000, dev: 8:1, len: 790795, source: File("/usr/lib/libgcc_s.so.1"), deleted: false }
Mapping { addr_range: 00007ffff7d9b000..00007ffff7d9f000, perms: r--p, offset: 0000000000014000, dev: 8:1, len: 790795, source: File("/usr/lib/libgcc_s.so.1"), deleted: false }
Mapping { addr_range: 00007ffff7d9f000..00007ffff7da0000, perms: r--p, offset: 0000000000017000, dev: 8:1, len: 790795, source: File("/usr/lib/libgcc_s.so.1"), deleted: false }
Mapping { addr_range: 00007ffff7da0000..00007ffff7da1000, perms: rw-p, offset: 0000000000018000, dev: 8:1, len: 790795, source: File("/usr/lib/libgcc_s.so.1"), deleted: false }
Mapping { addr_range: 00007ffff7da1000..00007ffff7da8000, perms: r--p, offset: 0000000000000000, dev: 8:1, len: 801521, source: File("/usr/lib/libpthread-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7da8000..00007ffff7db8000, perms: r-xp, offset: 0000000000007000, dev: 8:1, len: 801521, source: File("/usr/lib/libpthread-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7db8000..00007ffff7dbd000, perms: r--p, offset: 0000000000017000, dev: 8:1, len: 801521, source: File("/usr/lib/libpthread-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dbd000..00007ffff7dbe000, perms: r--p, offset: 000000000001b000, dev: 8:1, len: 801521, source: File("/usr/lib/libpthread-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dbe000..00007ffff7dbf000, perms: rw-p, offset: 000000000001c000, dev: 8:1, len: 801521, source: File("/usr/lib/libpthread-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dbf000..00007ffff7dc3000, perms: rw-p, offset: 0000000000000000, dev: 0:0, len: 0, source: Anonymous, deleted: false }
Mapping { addr_range: 00007ffff7dc3000..00007ffff7dc4000, perms: r--p, offset: 0000000000000000, dev: 8:1, len: 800745, source: File("/usr/lib/libdl-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dc4000..00007ffff7dc5000, perms: r-xp, offset: 0000000000001000, dev: 8:1, len: 800745, source: File("/usr/lib/libdl-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dc5000..00007ffff7dc6000, perms: r--p, offset: 0000000000002000, dev: 8:1, len: 800745, source: File("/usr/lib/libdl-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dc6000..00007ffff7dc7000, perms: r--p, offset: 0000000000002000, dev: 8:1, len: 800745, source: File("/usr/lib/libdl-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dc7000..00007ffff7dc8000, perms: rw-p, offset: 0000000000003000, dev: 8:1, len: 800745, source: File("/usr/lib/libdl-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dc8000..00007ffff7ded000, perms: r--p, offset: 0000000000000000, dev: 8:1, len: 800644, source: File("/usr/lib/libc-2.32.so"), deleted: false }
Mapping { addr_range: 00007ffff7ded000..00007ffff7f3a000, perms: r-xp, offset: 0000000000025000, dev: 8:1, len: 800644, source: File("/usr/lib/libc-2.32.so"), deleted: false }
Mapping { addr_range: 00007ffff7f3a000..00007ffff7f84000, perms: r--p, offset: 0000000000172000, dev: 8:1, len: 800644, source: File("/usr/lib/libc-2.32.so"), deleted: false }
Mapping { addr_range: 00007ffff7f84000..00007ffff7f85000, perms: ---p, offset: 00000000001bc000, dev: 8:1, len: 800644, source: File("/usr/lib/libc-2.32.so"), deleted: false }
Mapping { addr_range: 00007ffff7f85000..00007ffff7f88000, perms: r--p, offset: 00000000001bc000, dev: 8:1, len: 800644, source: File("/usr/lib/libc-2.32.so"), deleted: false }
Mapping { addr_range: 00007ffff7f88000..00007ffff7f8b000, perms: rw-p, offset: 00000000001bf000, dev: 8:1, len: 800644, source: File("/usr/lib/libc-2.32.so"), deleted: false }
Mapping { addr_range: 00007ffff7f8b000..00007ffff7f91000, perms: rw-p, offset: 0000000000000000, dev: 0:0, len: 0, source: Anonymous, deleted: false }
Mapping { addr_range: 00007ffff7fc7000..00007ffff7fc8000, perms: r--p, offset: 0000000000000000, dev: 8:1, len: 3304895, source: File("/home/amos/ftl/elk/samples/ifunc-nolibc"), deleted: false }
Mapping { addr_range: 00007ffff7fc8000..00007ffff7fc9000, perms: r-xp, offset: 0000000000001000, dev: 8:1, len: 3304895, source: File("/home/amos/ftl/elk/samples/ifunc-nolibc"), deleted: false }
Mapping { addr_range: 00007ffff7fc9000..00007ffff7fca000, perms: r--p, offset: 0000000000002000, dev: 8:1, len: 3304895, source: File("/home/amos/ftl/elk/samples/ifunc-nolibc"), deleted: false }
Mapping { addr_range: 00007ffff7fca000..00007ffff7fcc000, perms: rw-p, offset: 0000000000002000, dev: 8:1, len: 3304895, source: File("/home/amos/ftl/elk/samples/ifunc-nolibc"), deleted: false }
Mapping { addr_range: 00007ffff7fcc000..00007ffff7fce000, perms: rw-p, offset: 0000000000000000, dev: 0:0, len: 0, source: Anonymous, deleted: false }
Mapping { addr_range: 00007ffff7fce000..00007ffff7fd1000, perms: r--p, offset: 0000000000000000, dev: 0:0, len: 0, source: Special("vvar"), deleted: false }
Mapping { addr_range: 00007ffff7fd1000..00007ffff7fd2000, perms: r-xp, offset: 0000000000000000, dev: 0:0, len: 0, source: Special("vdso"), deleted: false }
Mapping { addr_range: 00007ffff7fd2000..00007ffff7fd4000, perms: r--p, offset: 0000000000000000, dev: 8:1, len: 795324, source: File("/usr/lib/ld-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7fd4000..00007ffff7ff3000, perms: r-xp, offset: 0000000000002000, dev: 8:1, len: 795324, source: File("/usr/lib/ld-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7ff3000..00007ffff7ffb000, perms: r--p, offset: 0000000000021000, dev: 8:1, len: 795324, source: File("/usr/lib/ld-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7ffc000..00007ffff7ffd000, perms: r--p, offset: 0000000000029000, dev: 8:1, len: 795324, source: File("/usr/lib/ld-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7ffd000..00007ffff7ffe000, perms: rw-p, offset: 000000000002a000, dev: 8:1, len: 795324, source: File("/usr/lib/ld-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7ffe000..00007ffff7fff000, perms: rw-p, offset: 0000000000000000, dev: 0:0, len: 0, source: Anonymous, deleted: false }
Mapping { addr_range: 00007ffffffde000..00007ffffffff000, perms: rw-p, offset: 0000000000000000, dev: 0:0, len: 0, source: Special("stack"), deleted: false }
Mapping { addr_range: ffffffffff600000..ffffffffff601000, perms: --xp, offset: 0000000000000000, dev: 0:0, len: 0, source: Special("vsyscall"), deleted: false }

// in `src/elk/main.rs`

fn cmd_autosym(args: AutosymArgs) -> Result<(), Box<dyn Error>> {
    let maps = std::fs::read_to_string(format!("/proc/{}/maps", args.pid))?;

    match procfs::mappings(&maps) {
        Ok((_, mappings)) => {
            println!("Found {} mappings for PID {}", mappings.len(), args.pid);

            println!("Executable file mappings:");
            let xmappings = mappings
                .iter()
                .filter(|m| m.perms.x && m.source.is_file())
                .collect::<Vec<_>>();
            for mapping in &xmappings {
                println!("{:?}", mapping);
            }
        }
        Err(e) => panic!("parsing failed: {:?}", e),
    };

    Ok(())
}

$ cargo build
$ ../target/debug/elk autosym 27113
Found 45 mappings for PID 27113
Executable file mappings:
Mapping { addr_range: 0000555555562000..0000555555630000, perms: r-xp, offset: 000000000000e000, dev: 8:1, len: 3336791, source: File("/home/amos/ftl/elk/target/debug/elk"), deleted: true }
Mapping { addr_range: 00007ffff7d8a000..00007ffff7d9b000, perms: r-xp, offset: 0000000000003000, dev: 8:1, len: 790795, source: File("/usr/lib/libgcc_s.so.1"), deleted: false }
Mapping { addr_range: 00007ffff7da8000..00007ffff7db8000, perms: r-xp, offset: 0000000000007000, dev: 8:1, len: 801521, source: File("/usr/lib/libpthread-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7dc4000..00007ffff7dc5000, perms: r-xp, offset: 0000000000001000, dev: 8:1, len: 800745, source: File("/usr/lib/libdl-2.30.so"), deleted: false }
Mapping { addr_range: 00007ffff7ded000..00007ffff7f3a000, perms: r-xp, offset: 0000000000025000, dev: 8:1, len: 800644, source: File("/usr/lib/libc-2.32.so"), deleted: false }
Mapping { addr_range: 00007ffff7fc8000..00007ffff7fc9000, perms: r-xp, offset: 0000000000001000, dev: 8:1, len: 3304895, source: File("/home/amos/ftl/elk/samples/ifunc-nolibc"), deleted: false }
Mapping { addr_range: 00007ffff7fd4000..00007ffff7ff3000, perms: r-xp, offset: 0000000000002000, dev: 8:1, len: 795324, source: File("/usr/lib/ld-2.30.so"), deleted: false }

// in `delf/src/lib.rs`

#[derive(Debug)]
pub struct File {
    // (cut: existing fields)
    // new:
    pub shstrndx: usize,
}

impl File {
    const MAGIC: &'static [u8] = &[0x7f, 0x45, 0x4c, 0x46];

    #[allow(unused_variables)]
    pub fn parse(i: parse::Input) -> parse::Result<Self> {
        // cut: parser

        let res = Self {
            // cut: existing fields
            shstrndx: sh_nidx as _,
        };
        Ok((i, res))
    }
}

// in `delf/src/lib.rs`

impl File {
    pub fn get_section_name<'a>(
        &self,
        file_contents: &'a [u8],
        offset: Addr,
    ) -> Result<&'a [u8], GetStringError> {
        use GetStringError as E;

        let tab_start = self.section_headers[self.shstrndx].off + offset;
        let tab_slice = &file_contents[tab_start.into()..];
        let string_slice = tab_slice
            .split(|&c| c == 0)
            .next()
            .ok_or(E::StringNotFound)?;
        Ok(string_slice)
    }
}

// in `elk/src/main.rs`

fn cmd_autosym(args: AutosymArgs) -> Result<(), Box<dyn Error>> {
    let maps = std::fs::read_to_string(format!("/proc/{}/maps", args.pid))?;

    match procfs::mappings(&maps) {
        Ok((_, mappings)) => {
            let xmappings = mappings
                .iter()
                .filter(|m| m.perms.x && m.source.is_file())
                .collect::<Vec<_>>();

            fn analyze(mapping: &procfs::Mapping) -> Result<(), Box<dyn Error>> {
                if mapping.deleted {
                    // skip deleted mappings
                    return Ok(());
                }

                let path = match mapping.source {
                    procfs::Source::File(path) => path,
                    _ => return Ok(()),
                };

                let contents = std::fs::read(path)?;
                let file = match delf::File::parse_or_print_error(&contents) {
                    Some(x) => x,
                    _ => return Ok(()),
                };

                let section = match file
                    .section_headers
                    .iter()
                    .find(|sh| file.get_section_name(&contents, sh.name).unwrap() == b".text")
                {
                    Some(section) => section,
                    _ => return Ok(()),
                };

                let textaddress = mapping.addr_range.start - mapping.offset + section.off;
                println!("add-symbol-file {:?} 0x{:?}", path, textaddress);

                Ok(())
            }

            for mapping in &xmappings {
                analyze(mapping)?;
            }
        }
        Err(e) => panic!("parsing failed: {:?}", e),
    };

    Ok(())
}

# in samples/autosym.py

import subprocess

pid = gdb.selected_inferior().pid

cmd = ["elk", "autosym", "%d" % pid]
lines = subprocess.check_output(cmd).decode("utf-8").split("\n")

for line in lines:
    gdb.execute(line)

$ cd elk/
$ cargo install --path .

$ cd elk/samples/
$ gdb --args ../target/debug/elk run ./ifunc-nolibc
(gdb) break jmp
(gdb) r
(gdb) source autosym.py
add symbol table from file "/home/amos/ftl/elk/target/debug/elk" at
        .text_addr = 0x555555562150
add symbol table from file "/usr/lib/libgcc_s.so.1" at
        .text_addr = 0x7ffff7d8a020
add symbol table from file "/usr/lib/libpthread-2.30.so" at
        .text_addr = 0x7ffff7da8b30
add symbol table from file "/usr/lib/libdl-2.30.so" at
        .text_addr = 0x7ffff7dc4210
add symbol table from file "/usr/lib/libc-2.32.so" at
        .text_addr = 0x7ffff7ded670
add symbol table from file "/home/amos/ftl/elk/samples/ifunc-nolibc" at
        .text_addr = 0x7ffff7fc8020
add symbol table from file "/usr/lib/ld-2.30.so" at
        .text_addr = 0x7ffff7fd4100
(gdb) stepi
(gdb) stepi
(gdb) stepi

(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000000000001016 in ?? ()
Cannot access memory at address 0x1016
(gdb) bt
#0  0x0000000000001016 in ?? ()
#1  0x00007ffff7fc80fb in main ()
#2  0x00007ffff7fc8118 in _start ()
#3  0x0000555555587857 in elk::jmp (addr=0x7ffff7fc810a <_start> "UH\211\345\270\000") at src/main.rs:144
#4  0x0000555555587713 in elk::cmd_run (args=...) at src/main.rs:125
#5  0x0000555555586240 in elk::do_main () at src/main.rs:60
#6  0x000055555558602c in elk::main () at src/main.rs:52

(gdb) frame 1

# in `elk/gdb-elk.py`

import subprocess


class AutoSym(gdb.Command):
    """Load symbols for all executable files mapped in memory, through elk"""

    def __init__(self):
        super(AutoSym, self).__init__("autosym", gdb.COMMAND_USER)

    def invoke(self, arg, from_tty):
        pid = gdb.selected_inferior().pid
        if pid == 0:
            print("No inferior.")
            return

        cmd = ["elk", "autosym", str(pid)]
        lines = subprocess.check_output(cmd).decode("utf-8").split("\n")

        for line in lines:
            gdb.execute(line)


AutoSym()


class Dig(gdb.Command):
    """Display all the information ELK can find about a memory address for the current inferior"""

    def __init__(self):
        super(Dig, self).__init__(
            "dig", gdb.COMMAND_USER, gdb.COMPLETE_EXPRESSION)

    def invoke(self, arg, from_tty):
        if arg == "":
            print("Usage: dig ADDR")
            return

        addr = int(arg, 0)

        pid = gdb.selected_inferior().pid
        if pid == 0:
            print("No inferior.")
            return

        cmd = ["elk", "dig", "--pid", str(pid), "--addr", str(addr)]

        # note: `check_call` would print stdout directly, but this somehow
        # breaks GDB TUI, so we print every line ourselves
        lines = subprocess.check_output(cmd).decode("utf-8").split("\n")

        for line in lines:
            print(line)

Dig()

# in `~/.gdbinit`

# those are from earlier - if you didn't have these,
# now's your chance
set history save on
set disassembly-flavor intel

# this is the important bit:
source ~/ftl/elk/gdb-elk.py

$ gdb --args ../target/debug/elk run ./ifunc-nolibc
(gdb) break jmp
(gdb) r
(gdb) autosym
add symbol table from file "/home/amos/ftl/elk/target/debug/elk" at
        .text_addr = 0x555555562150
add symbol table from file "/usr/lib/libgcc_s.so.1" at
        .text_addr = 0x7ffff7d8a020
add symbol table from file "/usr/lib/libpthread-2.30.so" at
        .text_addr = 0x7ffff7da8b30
add symbol table from file "/usr/lib/libdl-2.30.so" at
        .text_addr = 0x7ffff7dc4210
add symbol table from file "/usr/lib/libc-2.32.so" at
        .text_addr = 0x7ffff7ded670
add symbol table from file "/home/amos/ftl/elk/samples/ifunc-nolibc" at
        .text_addr = 0x7ffff7fc8020
add symbol table from file "/usr/lib/ld-2.30.so" at
        .text_addr = 0x7ffff7fd4100
(gdb) stepi
(gdb) stepi
(gdb) stepi
(gdb) bt
#0  0x00007ffff7fc810a in _start ()
#1  0x0000555555587857 in elk::jmp (addr=0x7ffff7fc810a <_start> "UH\211\345\270\000") at src/main.rs:144
#2  0x0000555555587713 in elk::cmd_run (args=...) at src/main.rs:125
#3  0x0000555555586240 in elk::do_main () at src/main.rs:60
#4  0x000055555558602c in elk::main () at src/main.rs:52
(gdb) dig
Usage: dig ADDR
(gdb) dig 0x00007ffff7fc810a
Unrecognized argument: dig

Python Exception <class 'subprocess.CalledProcessError'> Command '['elk', 'dig', '--pid', '19658', '--addr', '140737353908490']' returned non-zero exit status 1.:
Error occurred in Python: Command '['elk', 'dig', '--pid', '19658', '--addr', '140737353908490']' returned non-zero exit status 1.

// in `elk/src/main.rs`

#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand)]
enum SubCommand {
    Autosym(AutosymArgs),
    Run(RunArgs),
    // new:
    Dig(DigArgs),
}

#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand, name = "dig")]
/// Shows information about an address in a memory's address space
struct DigArgs {
    #[argh(option)]
    /// the PID of the process whose memory space to examine
    pid: u32,
    #[argh(option)]
    /// the address to look for
    addr: u64,
}

type AnyError = Box<dyn Error>;

fn do_main() -> Result<(), AnyError> {
    let args: Args = argh::from_env();
    match args.nested {
        SubCommand::Run(args) => cmd_run(args),
        SubCommand::Autosym(args) => cmd_autosym(args),
        // new:
        SubCommand::Dig(args) => cmd_dig(args),
    }
}

use thiserror::*;

#[derive(Error, Debug)]
enum WithMappingsError {
    #[error("parsing failed: {0}")]
    Parse(String),
}

// We're doing this in both `autosym` and `dig`, so it makes
// sense to have a helper for it.
// "Mapping<'a>" is annoying to manipulate, so we can't really
// return it, but we *can* take a closure that operates on it!
fn with_mappings<F, T>(pid: u32, f: F) -> Result<T, AnyError>
where
    F: Fn(&Vec<procfs::Mapping<'_>>) -> Result<T, Box<dyn Error>>,
{
    let maps = std::fs::read_to_string(format!("/proc/{}/maps", pid))?;
    match procfs::mappings(&maps) {
        Ok((_, maps)) => f(&maps),
        Err(e) => {
            // parsing errors borrow the input, so we wouldn't be able
            // to return it. to prevent that, format it early.
            Err(Box::new(WithMappingsError::Parse(format!("{:?}", e))))
        }
    }
}

// Does the same as previously, just refactored to use `with_mappings`:
fn cmd_autosym(args: AutosymArgs) -> Result<(), Box<dyn Error>> {
    fn analyze(mapping: &procfs::Mapping) -> Result<(), AnyError> {
        if mapping.deleted {
            // skip deleted mappings
            return Ok(());
        }

        let path = match mapping.source {
            procfs::Source::File(path) => path,
            _ => return Ok(()),
        };

        let contents = std::fs::read(path)?;
        let file = match delf::File::parse_or_print_error(&contents) {
            Some(x) => x,
            _ => return Ok(()),
        };

        let section = match file
            .section_headers
            .iter()
            .find(|sh| file.get_section_name(&contents, sh.name).unwrap() == b".text")
        {
            Some(section) => section,
            _ => return Ok(()),
        };

        let textaddress = mapping.addr_range.start - mapping.offset + section.off;
        println!("add-symbol-file {:?} 0x{:?}", path, textaddress);

        Ok(())
    }

    with_mappings(args.pid, |mappings| {
        for mapping in mappings.iter().filter(|m| m.perms.x && m.source.is_file()) {
            analyze(mapping)?;
        }
        Ok(())
    })
}

// Here's our new command:
fn cmd_dig(args: DigArgs) -> Result<(), Box<dyn Error>> {
    let addr = delf::Addr(args.addr);

    with_mappings(args.pid, |mappings| {
        for mapping in mappings {
            if mapping.addr_range.contains(&addr) {
                println!("Found mapping: {:#?}", mapping);
                return Ok(());
            }
        }
        Ok(())
    })
}

$ cd elk/
$ cargo install --force --path .
  Installing elk v0.1.0 (/home/amos/ftl/elk)
    Updating crates.io index
    Finished release [optimized] target(s) in 0.30s
   Replacing /home/amos/.cargo/bin/elk
    Replaced package `elk v0.1.0 (/home/amos/ftl/elk/)` with `elk v0.1.0 (/home/amos/ftl/elk)` (executable `elk`)

$ gdb --args ../target/debug/elk run ./ifunc-nolibc
(gdb) break jmp
(gdb) r
(gdb) autosym
(gdb) bt
#0  elk::jmp (addr=0x7ffff7fc810a <_start> "UH\211\345\270\000") at src/main.rs:143
#1  0x0000555555587713 in elk::cmd_run (args=...) at src/main.rs:125
#2  0x0000555555586240 in elk::do_main () at src/main.rs:60
#3  0x000055555558602c in elk::main () at src/main.rs:52
(gdb) dig 0x0000555555587713
Found mapping: Mapping {
    addr_range: 0000555555562000..000055555564b000,
    perms: r-xp,
    offset: 000000000000e000,
    dev: 8:1,
    len: 3337208,
    source: File(
        "/home/amos/ftl/elk/target/debug/elk",
    ),
    deleted: false,
}
(gdb) dig 0x7ffff7fc810a
Found mapping: Mapping {
    addr_range: 00007ffff7fc8000..00007ffff7fc9000,
    perms: r-xp,
    offset: 0000000000001000,
    dev: 8:1,
    len: 3304895,
    source: File(
        "/home/amos/ftl/elk/samples/ifunc-nolibc",
    ),
    deleted: false,
}

// in `delf/src/lib.rs`

pub struct ProgramHeader {
    pub r#type: SegmentType,
    pub flags: BitFlags<SegmentFlag>,
    pub offset: Addr,
    pub vaddr: Addr,
    pub paddr: Addr,
    pub filesz: Addr,
    pub memsz: Addr,
    pub align: Addr,
    pub contents: SegmentContents,

    // note: the `data` field is gone
}

// in `delf/src/lib.rs`

#[derive(Debug, Clone, Copy, PartialEq, Eq, TryFromPrimitive)]
#[repr(u32)]
pub enum SectionType {
    Null = 0,
    ProgBits = 1,
    SymTab = 2,
    StrTab = 3,
    Rela = 4,
    Hash = 5,
    Dynamic = 6,
    Note = 7,
    NoBits = 8,
    Rel = 9,
    ShLib = 10,
    DynSym = 11,
    InitArray = 14,
    FiniArray = 15,
    PreinitArray = 16,
    Group = 17,
    SymTabShndx = 18,
    Num = 19,
    GnuAttributes = 0x6ffffff5,
    GnuHash = 0x6ffffff6,
    GnuLiblist = 0x6ffffff7,
    Checksum = 0x6ffffff8,
    GnuVerdef = 0x6ffffffd,
    GnuVerneed = 0x6ffffffe,
    GnuVersym = 0x6fffffff,
}

impl_parse_for_enum!(SectionType, le_u32);

#[derive(Debug)]
pub struct SectionHeader {
    pub name: Addr,
    // was: u32
    pub r#type: SectionType,
    // flags are still untyped, we don't really care about
    // them for now
    pub flags: u64,
    pub addr: Addr,
    // was: `off`
    pub offset: Addr,
    pub size: Addr,
    pub link: u32,
    pub info: u32,
    pub addralign: Addr,
    pub entsize: Addr,
}

// in `delf/src/lib.rs`

impl SectionHeader {
    /**
     * File range where the section is stored
     */
    pub fn file_range(&self) -> Range<Addr> {
        self.offset..self.offset + self.size
    }

    /**
     * Memory range where the section is mapped
     */
    pub fn mem_range(&self) -> Range<Addr> {
        self.addr..self.addr + self.size
    }
}

#[derive(Debug, TryFromPrimitive, Clone, Copy)]
#[repr(u8)]
pub enum SymType {
    None = 0,
    Object = 1,
    Func = 2,
    Section = 3,
    // new:
    File = 4,
    IFunc = 10,
}

pub struct File<'a> {
    // omitted: other fields
    contents: &[u8],
}

pub struct File {
    // omitted: other fields
    contents: Vec<u8>,
}

// in `delf/src/lib.rs`

#[derive(Debug)]
pub struct File<I>
where
    I: AsRef<[u8]>,
{
    pub input: I,
    pub contents: FileContents,
}

impl<I> std::ops::Deref for File<I>
where
    I: AsRef<[u8]>,
{
    type Target = FileContents;
    fn deref(&self) -> &Self::Target {
        &self.contents
    }
}

#[derive(Debug)]
pub struct FileContents {
    pub r#type: Type,
    pub machine: Machine,
    pub entry_point: Addr,
    pub program_headers: Vec<ProgramHeader>,
    pub section_headers: Vec<SectionHeader>,
    pub shstrndx: usize,
}

impl FileContents {
    const MAGIC: &'static [u8] = &[0x7f, 0x45, 0x4c, 0x46];

    #[allow(unused_variables)]
    pub fn parse(i: parse::Input) -> parse::Result<Self> {
        let full_input = i;

        use nom::{
            bytes::complete::{tag, take},
            error::context,
            sequence::tuple,
        };
        let (i, _) = tuple((
            // -------
            context("Magic", tag(Self::MAGIC)),
            context("Class", tag(&[0x2])),
            context("Endianness", tag(&[0x1])),
            context("Version", tag(&[0x1])),
            context("OS ABI", nom::branch::alt((tag(&[0x0]), tag(&[0x3])))),
            // -------
            context("Padding", take(8_usize)),
        ))(i)?;

        use nom::{
            combinator::verify,
            number::complete::{le_u16, le_u32},
        };

        let (i, (r#type, machine)) = tuple((Type::parse, Machine::parse))(i)?;
        let (i, _) = context("Version (bis)", verify(le_u32, |&x| x == 1))(i)?;
        let (i, entry_point) = Addr::parse(i)?;

        use nom::combinator::map;
        let u16_usize = map(le_u16, |x| x as usize);

        let (i, (ph_offset, sh_offset)) = tuple((Addr::parse, Addr::parse))(i)?;
        let (i, (flags, hdr_size)) = tuple((le_u32, le_u16))(i)?;
        let (i, (ph_entsize, ph_count)) = tuple((&u16_usize, &u16_usize))(i)?;
        let (i, (sh_entsize, sh_count, sh_nidx)) = tuple((&u16_usize, &u16_usize, &u16_usize))(i)?;

        let ph_slices = (&full_input[ph_offset.into()..]).chunks(ph_entsize);
        let mut program_headers = Vec::new();
        for ph_slice in ph_slices.take(ph_count) {
            let (_, ph) = ProgramHeader::parse(full_input, ph_slice)?;
            program_headers.push(ph);
        }

        let sh_slices = (&full_input[sh_offset.into()..]).chunks(sh_entsize);
        let mut section_headers = Vec::new();
        for sh_slice in sh_slices.take(sh_count) {
            let (_, sh) = SectionHeader::parse(sh_slice)?;
            section_headers.push(sh);
        }

        let res = Self {
            machine,
            r#type,
            entry_point,
            program_headers,
            section_headers,
            shstrndx: sh_nidx as usize,
        };
        Ok((i, res))
    }
}

impl FileContents {
    /// Returns the first segment of a given type
    pub fn segment_of_type(&self, r#type: SegmentType) -> Option<&ProgramHeader> {
        self.program_headers.iter().find(|ph| ph.r#type == r#type)
    }

    /// Returns the first section of a given type
    pub fn section_of_type(&self, r#type: SectionType) -> Option<&SectionHeader> {
        self.section_headers.iter().find(|sh| sh.r#type == r#type)
    }

    /// Attempts to find a Load segment whose memory range contains the given virtual address
    pub fn segment_containing(&self, addr: Addr) -> Option<&ProgramHeader> {
        self.program_headers
            .iter()
            .find(|ph| ph.r#type == SegmentType::Load && ph.mem_range().contains(&addr))
    }

    /// Attempts to find the Dynamic segment and return its entries as a slice
    pub fn dynamic_table(&self) -> Option<&[DynamicEntry]> {
        match self.segment_of_type(SegmentType::Dynamic) {
            Some(ProgramHeader {
                contents: SegmentContents::Dynamic(entries),
                ..
            }) => Some(entries),
            _ => None,
        }
    }

    /// Returns an iterator of all dynamic entries with the given tag.
    /// Especially useful with DynamicTag::Needed
    pub fn dynamic_entries(&self, tag: DynamicTag) -> impl Iterator<Item = Addr> + '_ {
        self.dynamic_table()
            .unwrap_or_default()
            .iter()
            .filter(move |e| e.tag == tag)
            .map(|e| e.addr)
    }

    /// Returns the value of the first dynamic entry with the given tag, or None
    pub fn dynamic_entry(&self, tag: DynamicTag) -> Option<Addr> {
        self.dynamic_entries(tag).next()
    }

    /// Returns the value of the first dynamic entry with the given tag, or an error
    pub fn get_dynamic_entry(&self, tag: DynamicTag) -> Result<Addr, GetDynamicEntryError> {
        self.dynamic_entry(tag)
            .ok_or(GetDynamicEntryError::NotFound(tag))
    }
}

// in `delf/src/lib.rs`

impl<I> File<I>
where
    I: AsRef<[u8]>,
{
    pub fn parse_or_print_error(input: I) -> Option<Self> {
        match FileContents::parse(input.as_ref()) {
            Ok((_, contents)) => Some(File { input, contents }),
            Err(nom::Err::Failure(err)) | Err(nom::Err::Error(err)) => {
                use nom::Offset;

                eprintln!("Parsing failed:");
                for (input, err) in err.errors {
                    let offset = input.as_ref().offset(input);
                    eprintln!("{:?} at position {}:", err, offset);
                    eprintln!("{:>08x}: {:?}", offset, HexDump(input));
                }
                None
            }
            Err(_) => panic!("unexpected nom error"),
        }
    }
}

// in `delf/src/lib.rs`

impl<I> File<I>
where
    I: AsRef<[u8]>,
{
    /// Returns a slice of the input, indexed by file offsets
    pub fn file_slice(&self, addr: Addr, len: usize) -> &[u8] {
        &self.input.as_ref()[addr.into()..len]
    }

    /// Returns a slice of the input corresponding to the given section
    pub fn section_slice(&self, section: &SectionHeader) -> &[u8] {
        self.file_slice(section.file_range().start, section.file_range().end.into())
    }

    /// Returns a slice of the input corresponding to the given segment
    pub fn segment_slice(&self, segment: &ProgramHeader) -> &[u8] {
        self.file_slice(segment.file_range().start, segment.file_range().end.into())
    }

    /// Returns a slice of the input, indexed by virtual addresses
    pub fn mem_slice(&self, addr: Addr, len: usize) -> Option<&[u8]> {
        self.segment_containing(addr).map(|segment| {
            let start: usize = (addr - segment.mem_range().start).into();
            &self.segment_slice(segment)[start..start + len]
        })
    }

    /// Returns an iterator of string values (or rather, u8 slices) of
    /// dynamic entries for the given tag.
    pub fn dynamic_entry_strings(&self, tag: DynamicTag) -> impl Iterator<Item = &[u8]> + '_ {
        self.dynamic_entries(tag)
            .map(move |addr| self.dynstr_entry(addr))
    }

    /// Read relocation entries from the table pointed to by `DynamicTag::Rela`
    pub fn read_rela_entries(&self) -> Result<Vec<Rela>, ReadRelaError> {
        use DynamicTag as DT;
        use ReadRelaError as E;

        let addr = match self.dynamic_entry(DT::Rela) {
            Some(addr) => addr,
            None => return Ok(vec![]),
        };

        let len = self.get_dynamic_entry(DT::RelaSz)?;
        let i = self
            .mem_slice(addr, len.into())
            .ok_or(E::RelaSegmentNotFound)?;

        let n: usize = len.0 as usize / Rela::SIZE;
        match nom::multi::many_m_n(n, n, Rela::parse)(i) {
            Ok((_, rela_entries)) => Ok(rela_entries),
            Err(nom::Err::Failure(err)) | Err(nom::Err::Error(err)) => {
                Err(E::ParsingError(format!("{:?}", err)))
            }
            _ => unreachable!(),
        }
    }

    /// Read symbols from the given section (internal)
    fn read_symbol_table(&self, section_type: SectionType) -> Result<Vec<Sym>, ReadSymsError> {
        let section = match self.section_of_type(section_type) {
            Some(section) => section,
            None => return Ok(vec![]),
        };

        let i = self.section_slice(section);
        let n = i.len() / section.entsize.0 as usize;
        use nom::multi::many_m_n;

        match many_m_n(n, n, Sym::parse)(i) {
            Ok((_, syms)) => Ok(syms),
            Err(nom::Err::Failure(err)) | Err(nom::Err::Error(err)) => {
                Err(ReadSymsError::ParsingError(format!("{:?}", err)))
            }
            _ => unreachable!(),
        }
    }

    /// Read symbols from the ".dynsym" section (loader view)
    pub fn read_dynsym_entries(&self) -> Result<Vec<Sym>, ReadSymsError> {
        self.read_symbol_table(SectionType::DynSym)
    }

    /// Read symbols from the ".symtab" section (linker view)
    pub fn read_symtab_entries(&self) -> Result<Vec<Sym>, ReadSymsError> {
        self.read_symbol_table(SectionType::SymTab)
    }

    /// Returns a null-terminated "string" from the ".shstrtab" section as an u8 slice
    pub fn shstrtab_entry(&self, offset: Addr) -> &[u8] {
        let section = &self.contents.section_headers[self.contents.shstrndx];
        let slice = &self.section_slice(section)[offset.into()..];
        slice.split(|&c| c == 0).next().unwrap_or_default()
    }

    /// Get a section by name
    pub fn section_by_name(&self, name: &[u8]) -> Option<&SectionHeader> {
        self.section_headers
            .iter()
            .find(|sh| self.shstrtab_entry(sh.name) == name)
    }

    /// Returns an entry from a string table contained in the section with a given name
    fn string_table_entry(&self, name: &[u8], offset: Addr) -> &[u8] {
        self.section_by_name(name)
            .map(|section| {
                let slice = &self.section_slice(section)[offset.into()..];
                slice.split(|&c| c == 0).next().unwrap_or_default()
            })
            .unwrap_or_default()
    }

    /// Returns a null-terminated "string" from the ".strtab" section as an u8 slice
    pub fn strtab_entry(&self, offset: Addr) -> &[u8] {
        self.string_table_entry(b".strtab", offset)
    }

    /// Returns a null-terminated "string" from the ".dynstr" section as an u8 slice
    pub fn dynstr_entry(&self, offset: Addr) -> &[u8] {
        self.string_table_entry(b".dynstr", offset)
    }
}

#[derive(CustomDebug)]
pub struct Object {
    // cut: other fields

    #[debug(skip)]
    pub file: delf::File<Vec<u8>>, // was: just delf::File
}

// in `elk/src/process.rs`
// in `impl Object`
// in `fn load_object_and_dependencies`

        while !a.is_empty() {
            use delf::DynamicTag::Needed;
            a = a
                .into_iter()
                .map(|index| &self.objects[index].file)
                // this now returns &[u8] with the same lifetime as &self
                .flat_map(|file| file.dynamic_entry_strings(Needed))
                .collect::<Vec<_>>()
                .into_iter()
                // this needs exclusive access to self (because we need &mut self)
                // so it doesn't work.
                .map(|dep| self.get_object(&dep))
                .collect::<Result<Vec<_>, _>>()?
                .into_iter()
                .filter_map(GetResult::fresh)
                .collect();
        }

                // same as before
                .flat_map(|file| file.dynamic_entry_strings(Needed))
                // new:
                .map(|s| String::from_utf8_lossy(s).to_string())
                // same as before
                .collect::<Vec<_>>()

// in `elk/src/process.rs`
// in `impl Process`
// in `fn load_object`

        println!("Loading {:?}", &path);

        // was: &input[..]
        let file = delf::File::parse_or_print_error(input)
            .ok_or_else(|| LoadError::ParseError(path.clone()))?;

// in `elk/src/process.rs`
// in `impl Process`
// in `fn load_object`

        self.search_path.extend(
            file.dynamic_entry_strings(delf::DynamicTag::RPath)
                .map(|path| String::from_utf8_lossy(path)) // new
                .map(|path| path.replace("$ORIGIN", &origin))
                .inspect(|path| println!("Found RPATH entry {:?}", path))
                .map(PathBuf::from),
        );

// previously in `elk/src/process.rs`

        let syms = file.read_syms()?;

        let strtab = file
            .get_dynamic_entry(delf::DynamicTag::StrTab)
            .unwrap_or_else(|_| panic!("String table not found in {:?}", path));
        let syms: Vec<_> = syms
            .into_iter()
            .map(|sym| unsafe {
                let name = Name::from_addr(base + strtab + sym.name);
                NamedSym { sym, name }
            })
            .collect();

// now in `elk/src/process.rs`:

        let syms = file.read_dynsym_entries()?;
        let syms: Vec<_> = if syms.is_empty() {
            vec![]
        } else {
            let strtab = file
                .get_dynamic_entry(delf::DynamicTag::StrTab)
                .unwrap_or_else(|_| panic!("String table not found in {:?}", path));
            syms.into_iter()
                .map(|sym| unsafe {
                    let name = Name::from_addr(base + strtab + sym.name);
                    NamedSym { sym, name }
                })
                .collect()
        };

fn cmd_autosym(args: AutosymArgs) -> Result<(), Box<dyn Error>> {
    fn analyze(mapping: &procfs::Mapping) -> Result<(), AnyError> {
        if mapping.deleted {
            // skip deleted mappings
            return Ok(());
        }

        let path = match mapping.source {
            procfs::Source::File(path) => path,
            _ => return Ok(()),
        };

        let contents = std::fs::read(path)?;
        let file = match delf::File::parse_or_print_error(&contents) {
            Some(x) => x,
            _ => return Ok(()),
        };

        let section = match file
            .section_headers
            .iter()
            // 👇 this was "get_section_name", and it used to be fallible
            .find(|sh| file.shstrtab_entry(sh.name) == b".text")
        {
            Some(section) => section,
            _ => return Ok(()),
        };

        // 👇 the `SectionHeader` field used to be called `off`, not `offset`
        let textaddress = mapping.addr_range.start - mapping.offset + section.offset;
        println!("add-symbol-file {:?} 0x{:?}", path, textaddress);

        Ok(())
    }

    with_mappings(args.pid, |mappings| {
        for mapping in mappings.iter().filter(|m| m.perms.x && m.source.is_file()) {
            analyze(mapping)?;
        }
        Ok(())
    })
}

fn cmd_dig(args: DigArgs) -> Result<(), Box<dyn Error>> {
    let addr = delf::Addr(args.addr);

    with_mappings(args.pid, |mappings| {
        if let Some(mapping) = mappings.iter().find(|m| m.addr_range.contains(&addr)) {
            println!("Mapped {:?} from {:?}", mapping.perms, mapping.source);
            println!(
                "(Map range: {:?}, {:?} total)",
                mapping.addr_range,
                Size(mapping.addr_range.end - mapping.addr_range.start)
            );

            // we've used this pattern a bunch of times already, but in case you don't
            // see the point: this avoids deep indentation. If we didn't use that, we'd
            // soon find ourselves several levels deep into `if let` statements, whereas
            // really, if we get a `None` or an `Err`, we just want to bail out early
            // and gracefully.
            let path = match mapping.source {
                procfs::Source::File(path) => path,
                // if it's not a file mapping, bail out
                _ => return Ok(()),
            };

            let contents = std::fs::read(path)?;
            let file = match delf::File::parse_or_print_error(&contents) {
                Some(x) => x,
                // if we couldn't parse the file, a message was printed,
                // and we can bail out
                _ => return Ok(()),
            };

            let offset = addr + mapping.offset - mapping.addr_range.start;

            // Segments (loader view, `delf::ProgramHeader` type) determine what parts
            // of the ELF file get mapped where, so we try to determine which
            // segment this mapping corresponds to.
            let segment = match file
                .program_headers
                .iter()
                .find(|ph| ph.file_range().contains(&offset))
            {
                Some(s) => s,
                None => return Ok(()),
            };

            // This is the main thing I wanted `elk dig` to do - display
            // the virtual address *for this ELF object*, so that it matches
            // up with the output from `objdump` and `readelf`
            let vaddr = offset + segment.vaddr - segment.offset;
            println!("Object virtual address: {:?}", vaddr);

            // But we can go a bit further: we can find to which section
            // this corresponds, and show *where* in this section the
            // dug address was.
            let section = match file
                .section_headers
                .iter()
                .find(|sh| sh.mem_range().contains(&vaddr))
            {
                Some(s) => s,
                None => return Ok(()),
            };

            let name = file.shstrtab_entry(section.name);
            let sect_offset = vaddr - section.addr;
            println!(
                "At section {:?} + {} (0x{:x})",
                String::from_utf8_lossy(name),
                sect_offset.0,
                sect_offset.0
            );

            // And, even further, we can try to map it to a symbol. This is all
            // stuff GDB does in its `info addr 0xABCD` command, but isn't it
            // satisfying to re-implement it ourselves?
            match file.read_symtab_entries() {
                Ok(syms) => {
                    for sym in &syms {
                        let sym_range = sym.value..(sym.value + delf::Addr(sym.size));
                        // the first check is for zero-sized symbols, since `sym_range`
                        // ends up being a 0-sized range.
                        if sym.value == vaddr || sym_range.contains(&vaddr) {
                            let sym_offset = vaddr - sym.value;
                            let sym_name = String::from_utf8_lossy(file.strtab_entry(sym.name));
                            println!(
                                "At symbol {:?} + {} (0x{:x})",
                                sym_name, sym_offset.0, sym_offset.0
                            );
                        }
                    }
                }
                Err(e) => println!("Could not read syms: {:?}", e),
            }
        }
        Ok(())
    })
}

struct Size(pub delf::Addr);

use std::fmt;
impl fmt::Debug for Size {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        const KIB: u64 = 1024;
        const MIB: u64 = 1024 * KIB;

        let x = (self.0).0;
        #[allow(overlapping_patterns)]
        #[allow(clippy::clippy::match_overlapping_arm)]
        match x {
            0..=KIB => write!(f, "{} B", x),
            KIB..=MIB => write!(f, "{} KiB", x / KIB),
            _ => write!(f, "{} MiB", x / MIB),
        }
    }
}

$ cd elk/
$ cargo install --force --path .

$ cd elk/samples
$ gdb --args ../target/debug/elk run ifunc-nolibc
(gdb) break jmp
(gdb) r
(gdb) autosym

(gdb) dig 0x7ffff7fc4010
Mapped r-xp from File("/home/amos/ftl/elf-series/samples/ifunc-nolibc")
(Map range: 00007ffff7fc4000..00007ffff7fc5000, 4 KiB total)
Object virtual address: 0000000000001010
At section ".plt" + 16 (0x10)

$ objdump -dR --section=.plt ifunc-nolibc

ifunc-nolibc:     file format elf64-x86-64


Disassembly of section .plt:

0000000000001000 <.plt>:
    1000:       ff 35 02 30 00 00       push   QWORD PTR [rip+0x3002]        # 4008 <_GLOBAL_OFFSET_TABLE_+0x8>
    1006:       ff 25 04 30 00 00       jmp    QWORD PTR [rip+0x3004]        # 4010 <_GLOBAL_OFFSET_TABLE_+0x10>
    100c:       0f 1f 40 00             nop    DWORD PTR [rax+0x0]
    1010:       ff 25 02 30 00 00       jmp    QWORD PTR [rip+0x3002]        # 4018 <_GLOBAL_OFFSET_TABLE_+0x18>
    1016:       68 00 00 00 00          push   0x0
    101b:       e9 e0 ff ff ff          jmp    1000 <.plt>

(gdb) x/1xg 0x7ffff7fc4010 - 0x1010 + 0x4018
0x7ffff7fc7018: 0x0000000000001016

0000000000001000 <.plt>:
    1000:       ff 35 02 30 00 00       push   QWORD PTR [rip+0x3002]        # 4008 <_GLOBAL_OFFSET_TABLE_+0x8>
    1006:       ff 25 04 30 00 00       jmp    QWORD PTR [rip+0x3004]        # 4010 <_GLOBAL_OFFSET_TABLE_+0x10>
    100c:       0f 1f 40 00             nop    DWORD PTR [rax+0x0]
    1010:       ff 25 02 30 00 00       jmp    QWORD PTR [rip+0x3002]        # 4018 <_GLOBAL_OFFSET_TABLE_+0x18>
    ; 👇
    1016:       68 00 00 00 00          push   0x0
    101b:       e9 e0 ff ff ff          jmp    1000 <.plt>

$ readelf -a ifunc-nolibc

(cut)

  [ 6] .rela.plt         RELA             0000000000000328  00000328
       0000000000000018  0000000000000018  AI       4    13     8

(cut)

Dynamic section at offset 0x2ef0 contains 12 entries:
  Tag        Type                         Name/Value
 0x000000006ffffef5 (GNU_HASH)           0x2e8
 0x0000000000000005 (STRTAB)             0x320
 0x0000000000000006 (SYMTAB)             0x308
 0x000000000000000a (STRSZ)              1 (bytes)
 0x000000000000000b (SYMENT)             24 (bytes)
 0x0000000000000015 (DEBUG)              0x0
 0x0000000000000003 (PLTGOT)             0x4000
 0x0000000000000002 (PLTRELSZ)           24 (bytes)
 0x0000000000000014 (PLTREL)             RELA
 0x0000000000000017 (JMPREL)             0x328        <----------
 0x000000006ffffffb (FLAGS_1)            Flags: PIE
 0x0000000000000000 (NULL)               0x0

// in `delf/src/lib.rs`
// in `impl File`

    // we're going to be reading relocations twice, and I don't want any
    // code duplication, so here's a re-usable internal helper:
    fn read_relocations(
        &self,
        addr_tag: DynamicTag,
        size_tag: DynamicTag,
    ) -> Result<Vec<Rela>, ReadRelaError> {
        use ReadRelaError as E;

        let addr = match self.dynamic_entry(addr_tag) {
            Some(addr) => addr,
            None => return Ok(vec![]),
        };

        let len = self.get_dynamic_entry(size_tag)?;
        let i = self
            .mem_slice(addr, len.into())
            .ok_or(E::RelaSegmentNotFound)?;

        let n: usize = len.0 as usize / Rela::SIZE;
        match nom::multi::many_m_n(n, n, Rela::parse)(i) {
            Ok((_, rela_entries)) => Ok(rela_entries),
            Err(nom::Err::Failure(err)) | Err(nom::Err::Error(err)) => {
                Err(E::ParsingError(format!("{:?}", err)))
            }
            _ => unreachable!(),
        }
    }

    /// Read relocation entries from the table pointed to by `DynamicTag::Rela`
    pub fn read_rela_entries(&self) -> Result<Vec<Rela>, ReadRelaError> {
        // those are the ones we already knew about:
        self.read_relocations(DynamicTag::Rela, DynamicTag::RelaSz)
    }

    /// Read relocation entries from the table pointed to by `DynamicTag::JmpRel`
    pub fn read_jmp_rel_entries(&self) -> Result<Vec<Rela>, ReadRelaError> {
        // those we *just* learned about:
        self.read_relocations(DynamicTag::JmpRel, DynamicTag::PltRelSz)
    }

// in `elk/src/process.rs`
// in `impl Process`
// in `fn load_object`

        // previously:
        let rels = file.read_rela_entries()?;

        // now:
        let mut rels = Vec::new();
        rels.extend(file.read_rela_entries()?);
        rels.extend(file.read_jmp_rel_entries()?);

$ cd elk/samples/
$ cargo b
$ ../target/debug/elk run ifunc-nolibc
Loading "/home/amos/ftl/elk/samples/ifunc-nolibc"
Fatal error: Could not read relocations from ELF object: Parsing error: String("Unknown RelType 37 (0x25)"):
input: 25 00 00 00 00 00 00 00 c3 10 00 00 00 00 00 00

// in `delf/src/lib.rs`

#[derive(Debug, TryFromPrimitive, Clone, Copy, PartialEq, Eq)]
#[repr(u32)]
pub enum RelType {
    _64 = 1,
    Copy = 5,
    GlobDat = 6,
    JumpSlot = 7,
    Relative = 8,
    // new:
    IRelative = 37,
}

// in `elk/src/process.rs`
// in `impl Process`
// in `fn apply_relocation`

        match reltype {
            RT::_64 => unsafe {
                objrel.addr().set(found.value() + addend);
            },
            RT::Relative => unsafe {
                objrel.addr().set(obj.base + addend);
            },
            // new!
            RT::IRelative => unsafe {
                objrel.addr().set(obj.base + addend);
            },
            RT::Copy => unsafe {
                objrel.addr().write(found.value().as_slice(found.size()));
            },
            _ => return Err(RelocationError::UnimplementedRelocation(reltype)),
        }

$ cargo b
$ ../target/debug/elk run ifunc-nolibc
Loading "/home/amos/ftl/elk/samples/ifunc-nolibc"
$

$ ../target/debug/elk run ifunc-nolibc | xxd
00000000: 4c6f 6164 696e 6720 222f 686f 6d65 2f61  Loading "/home/a
00000010: 6d6f 732f 6674 6c2f 656c 6b2f 7361 6d70  mos/ftl/elk/samp
00000020: 6c65 732f 6966 756e 632d 6e6f 6c69 6263  les/ifunc-nolibc
00000030: 220a 5548 89e5 488d 054d 0f              ".UH..H..M.

$ ../target/debug/elk run ifunc-nolibc | dd bs=1 skip=$((0x32)) | ndisasm -b 64 -
9+0 records in
9+0 records out
9 bytes copied, 0,000529187 s, 17,0 kB/s
00000000  55                push rbp
00000001  4889E5            mov rbp,rsp
00000004  48                rex.w
00000005  8D                db 0x8d
00000006  05                db 0x05
00000007  4D                rex.wrb
00000008  0F                db 0x0f

$ gdb --args ../target/debug/elk run ifunc-nolibc
(gdb) break jmp
(gdb) r
(gdb) autosym

(gdb) x/xg 0x7ffff7fc7018
0x7ffff7fc7018: 0x00007ffff7fc40c3

(gdb) info addr 0x00007ffff7fc40c3
No symbol "0x00007ffff7fc40c3" in current context.

(gdb) dig 0x00007ffff7fc40c3
Mapped r-xp from File("/home/amos/ftl/elf-series/samples/ifunc-nolibc")
(Map range: 00007ffff7fc4000..00007ffff7fc5000, 4 KiB total)
Object virtual address: 00000000000010c3
At section ".text" + 163 (0xa3)
At symbol "resolve_get_msg" + 0 (0x0)
At symbol "get_msg" + 0 (0x0)

static get_msg_t resolve_get_msg() {
    int uid;

    // make a `getuid` syscall. It has no parameters,
    // and returns in the `%rax` register.
    __asm__ (
            " \
            mov     $102, %%rax \n\t\
            syscall \n\t\
            mov     %%eax, %[uid]"
            : [uid] "=r" (uid)
            : // no inputs
            );

    if (uid == 0) {
        // UID 0 is root
        return get_msg_root;
    } else {
        // otherwise, it's a regular user
        return get_msg_user;
    }
}

(gdb) bt
#0  0x00007ffff7fc40e4 in resolve_get_msg ()
#1  0x00007ffff7fc40fb in main ()
#2  0x00007ffff7fc4118 in _start ()
#3  0x00005555555c6db4 in elk::jmp (addr=0x7ffff7fc410a <_start>) at elk/src/main.rs:295
#4  0x00005555555c6422 in elk::cmd_run (args=...) at elk/src/main.rs:80
#5  0x00005555555c5f78 in elk::do_main () at elk/src/main.rs:66
#6  0x00005555555c5d6c in elk::main () at elk/src/main.rs:58

(gdb) x/10xb $rax
0x7ffff7fc40b6 <get_msg_user>:  0x55    0x48    0x89    0xe5    0x48    0x8d    0x05    0x4d
0x7ffff7fc40be <get_msg_user+8>:        0x0f    0x00

$ ../target/debug/elk run ifunc-nolibc | xxd
(cut)
00000030: 220a 5548 89e5 488d 054d 0f              ".UH..H..M.
               ^^^^ ^^^^ ^^^^ ^^^^ ^^

// in `elk/src/process.rs`
// in `impl Process`
// in `fn apply_relocation`

        match reltype {
            RT::_64 => unsafe {
                objrel.addr().set(found.value() + addend);
            },
            RT::Relative => unsafe {
                objrel.addr().set(obj.base + addend);
            },
            RT::IRelative => unsafe {
                // new! and *very* unsafe!
                let selector: extern "C" fn() -> delf::Addr =
                    std::mem::transmute(obj.base + addend);
                objrel.addr().set(selector());
            },
            RT::Copy => unsafe {
                objrel.addr().write(found.value().as_slice(found.size()));
            },
            _ => return Err(RelocationError::UnimplementedRelocation(reltype)),
        }

$ ../target/debug/elk run ifunc-nolibc
Loading "/home/amos/ftl/elk/samples/ifunc-nolibc"
[1]    324 segmentation fault (core dumped)  ../target/debug/elk run ifunc-nolibc

$ gdb --args ../target/debug/elk run ifunc-nolibc
(gdb) r
Starting program: /home/amos/ftl/elk/target/debug/elk run ifunc-nolibc
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
Loading "/home/amos/ftl/elk/samples/ifunc-nolibc"

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7fc80c3 in ?? ()

(gdb) bt
#0  0x00007ffff7fc40c3 in ?? ()
#1  0x000055555559a75f in elk::process::Process::apply_relocation (self=0x7fffffffd600, objrel=...) at elk/src/process.rs:365
#2  0x000055555559a19e in elk::process::Process::apply_relocations (self=0x7fffffffd600) at elk/src/process.rs:313
#3  0x00005555555c6148 in elk::cmd_run (args=...) at elk/src/main.rs:75
#4  0x00005555555c5fb8 in elk::do_main () at elk/src/main.rs:66
#5  0x00005555555c5dac in elk::main () at elk/src/main.rs:58

(gdb) autosym
add symbol table from file "/home/amos/ftl/elf-series/target/debug/elk" at
        .text_addr = 0x555555564080
add symbol table from file "/usr/lib/libpthread-2.32.so" at
        .text_addr = 0x7ffff7db1a70
add symbol table from file "/usr/lib/libgcc_s.so.1" at
        .text_addr = 0x7ffff7dcf020
add symbol table from file "/usr/lib/libc-2.32.so" at
        .text_addr = 0x7ffff7e0c650
add symbol table from file "/usr/lib/libdl-2.32.so" at
        .text_addr = 0x7ffff7fb0210
add symbol table from file "/usr/lib/ld-2.32.so" at
        .text_addr = 0x7ffff7fd2090

(gdb) dig 0x00007ffff7fc40c3
Mapped rw-p from File("/home/amos/ftl/elf-series/samples/ifunc-nolibc")
(Map range: 00007ffff7fc3000..00007ffff7fc6000, 12 KiB total)
Object virtual address: 00000000000010c3
At section ".text" + 163 (0xa3)
At symbol "resolve_get_msg" + 0 (0x0)
At symbol "get_msg" + 0 (0x0)

At symbol "resolve_get_msg" + 0 (0x0)