Hi Mark, On Sat, Dec 30, 2023, at 11:41 AM, Mark Wielaard wrote: > Hi Daniel, > > On Wed, Dec 27, 2023 at 08:40:09PM -0600, Daniel Xu wrote: >> 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. > > You aren't doing something very wrong, but libelf does something you > aren't expecting. When you are calling elf_update () it will rearrange > the elf sections making sure there are no unnecessary gaps between the > sections in the file, that alignment is correct, etc. > > libelf only cares about the section headers. It doesn't know/care > about the program headers. The program headers describe how the > segments have to be loaded at runtime. Since some data has moved > around the program data isn't loaded correctly anymore which causes > the crash. > > To prevent libelf from doing this, and take responsibility of how the > sections are layed out yourself you have to call: > > elf_flagelf (elf, ELF_C_SET, ELF_F_LAYOUT); > > Before calling elf_update. Note that in that case you are responsible > for setting/updating the sh_offset fields of the Shdrs yourself. > > See for example the elfutils src/elfcompress.c program to see what it > does in case the Elf file has program headers. > > Hope that helps,
Thanks for taking a look! I did not know about this behavior - this was indeed helpful. Daniel
