extract source code from ELF with debug symbols

Question

I'm trying to extract the original source code from an ELF with debug symbols.

$ sed -n "14,16p" main.c
    for (int p=start;p<end;p++)
        if (isPrime(p))
            printf("%d\n",p);

I want the "closest", preceeding source line from a given address:

$ gcc -g -O0 main.c -o main
$ objdump -D -S main > main.asm
$ sed -n "451,457p" main.asm
        if (isPrime(p))
 6ba:   8b 45 f4                mov    -0xc(%rbp),%eax
 6bd:   89 c7                   mov    %eax,%edi
 6bf:   e8 86 ff ff ff          callq  64a <isPrime>
 6c4:   85 c0                   test   %eax,%eax
 6c6:   74 16                   je     6de <main+0x49>
            printf("%d\n",p);

So given the 0x6bf call instruction, I would like to extract if (isPrime(p)) .

This seems possible since objdump does it (right?)

Answer 1

It turns out that it can be quite easily done with pyelftools :

import elftools
from elftools.elf.elffile import ELFFile as ELF

def addr_2_line(ELFname: str, addr: int) -> int:
    with open(ELFname, "rb") as fl:
        elf = ELF(fl)
        dwarf_info = elf.get_dwarf_info()
        for cu in dwarf_info.iter_CUs():
            line = 1
            for entry in dwarf_info.line_program_for_CU(cu).get_entries():
                if entry.state:
                    if addr > entry.state.address:
                        line = entry.state.line
                    else:
                        return line
address = addr_2_line("main", 0x6bf)
print(f"src code[ 0x6bf ] = {address}")

When I run it it indeed gives the desired line:

src code[ 0x6bf ] = 15

It is probably worth checking if no adjustments are needed when there are more than just one compilation unit ( cu )

Answer 2

I'm trying to extract the original source code from an ELF with debug symbols.

That's impossible: the ELF with debug symbols contains no original source code.

What you appear to be after is source code location , ie file and line. Armed with file name, line number, and the original source, you can trivially print that line.

To recover file/line info, you could use addr2line -e main 0x6bf .