Noop round trip through elf_update() causes segfaults

["Daniel Xu" <dxu@dxuuu.xyz>]

Hi,

I was working on code that adds an ELF section containing custom
metadata to ELF binaries when I started getting odd segfaults
in the added-to binary.

I've managed to create a minimal reproducer with a couple interesting
discoveries. The reproducer is available here:

        https://github.com/danobi/elf-segfault

Basically it does a noop round trip between elf_begin() and elf_update().
But the resulting binary, when run, outputs:

        $ ./testprog_copy
        fish: Job 1, './testprog_copy' terminated by signal SIGSEGV (Address boundary error)

Furthermore, I built and ran tests/addsections.c [0] against my testbinary
and I still get:

        $ ./testprog_copy_elfutils
        fish: Job 1, './testprog_copy_elfutils' terminated by signal SIGSEGV (Address boundary error)
 
I've also tried linking against upstream libelf built from source
with the same results.

This leads me to believe I'm doing something very wrong or
I'm hitting a bug.

If it's helps, I'm using elfutils on archlinux with the following
package information:

        $ pacman -Qi libelf
        Name            : libelf
        Version         : 0.190-1
        Description     : Handle ELF object files and DWARF debugging information (libraries)
        Architecture    : x86_64
        URL             : https://sourceware.org/elfutils/
        [...]

[0]: https://sourceware.org/git/?p=elfutils.git;a=blob;f=tests/addsections.c;hb=HEAD

Thanks,
Daniel