Libelf creates corrupted output file, even when changing nothing

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I'm trying to use libelf to edit some things in ELF binaries, but so far I'm unable to even write the binary out without corrupting it. This sample:

#include <libelf.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <assert.h>
#include <gelf.h>

int main() {
    elf_version(EV_CURRENT);
    int fd = open("elftest", O_RDWR, 0);
    assert(fd >= 0);
    Elf *elf = elf_begin(fd, ELF_C_RDWR, NULL);
    assert(elf != NULL);
    assert(elf_kind(elf) == ELF_K_ELF);
    assert(elf_update(elf, ELF_C_WRITE) != -1);
    elf_end(elf);
    close(fd);
}

which should just read in elftest and write it back out unchanged instead converts a working C hello world into a program that segfaults immediately (according to gdb, even before main is called). The first discrepancy I noticed with readelf -h was that it moved the start of section headers back somewhat, and also reports that

readelf: Warning: the .dynamic section is not contained within the dynamic segment

What is causing libelf to alter the executable even when nothing is actually changed?

elf c
1 Answers

which should just read in elftest and write it back out unchanged instead converts a working C hello world into a program that segfaults immediately

This sure looks like a bug in libelf.

Adding this line immediately after elf_open() fixes your program:

    Elf *elf = elf_begin(fd, ELF_C_RDWR, NULL);
    elf_flagelf(elf, ELF_C_SET, ELF_F_LAYOUT);  // add this

but I don't think it should be necessary.

P.S. I know this is just an example, but putting functional parts of your program inside assert() is a really bad idea. Doing this:

  assert(elf_update(elf, ELF_C_WRITE) != -1);

will make you sorry sooner or later.

Employed_Russian_506170
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