Date: Fri, 5 Mar 2021 17:14:05 -0800 From: Fangrui Song <i@...kray.me> To: musl@...ts.openwall.com Cc: Rich Felker <dalias@...c.org> Subject: Re: ld-musl-* and empty .eh_frame On 2021-03-05, Michael Forney wrote: >On 2021-03-05, Rich Felker <dalias@...c.org> wrote: >>> The section itself isn't the problem; rather the linker making a >>> dedicated PROT_READ segment with no non-zero-length sections in it is. >>> It really should have collapsed that out. (Also it would not happen >>> without the separate-text option, which mcm disables because it makes >>> lots of problems.) > >Ah, that makes more sense. It explains why my attempt to strip >.eh_frame from the executable did not have an effect; only stripping >it from the objects before linking fixed the issue. > >>> With that said, there's no good reason we should error out on this; >>> it's syntactically and semantically valid just pointless for the >>> linker to emit. I think adding if (!n) return p; at the top of >>> mmap_fixed in dynlink.c fixes it. >> >> In practice this probably does, but there's also something of a >> question what to do if the zero-size segment is not page aligned. This >> is not actually a mmap error since it will be automatically expanded >> out to page boundaries in both directions, but if ld is capable of >> emitting such segments they may be semantically wrong (mapping over >> top of something else they're not intended to). Can you confirm that >> ld isn't doing anything awful here? Can you clarify how GNU ld creates an empty .eh_frame? The program header PT_GNU_EH_FRAME is created from .eh_frame_hdr, which is created by ld --eh-frame-hdr. If .eh_frame is empty, from my observation GNU ld does not create .eh_frame_hdr https://maskray.me/blog/2020-11-08-stack-unwinding#eh_frame_hdr-and-pt_eh_frame >I'm not too familiar with the binutils codebase, but I can try. As far >as I can tell, the alignment is set to at least the maximum page size: >https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf.c;h=84a5d942817a9a54b1170fbbb594787c5839aa54;hb=f35674005e609660f5f45005a9e095541ca4c5fe#l5601 max-page-size is for layouting PT_LOAD. It is unrelated to PT_GNU_EH_FRAME. >The offset to make p_vaddr page-aligned is computed here: >https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf.c;h=84a5d942817a9a54b1170fbbb594787c5839aa54;hb=f35674005e609660f5f45005a9e095541ca4c5fe#l5622 > >and p_vaddr is adjusted here: >https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf.c;h=84a5d942817a9a54b1170fbbb594787c5839aa54;hb=f35674005e609660f5f45005a9e095541ca4c5fe#l5688 >So it seems to me that these segments will be page-aligned, but it >could very well be the case that I am not following the code correctly >(or I am just looking at the wrong part of this 13k line file). >Someone more familiar with binutils should probably confirm. The requirement is http://www.sco.com/developers/gabi/latest/ch5.pheader.html "p_vaddr should equal p_offset, modulo p_align." p_vaddr % p_align != 0 is valid. p_memsz can be zero.
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