Re: [musl] ld-musl-* and empty .eh_frame

[Rich Felker <dalias@libc.org>]

On Fri, Mar 05, 2021 at 05:14:05PM -0800, Fangrui Song wrote:
> On 2021-03-05, Michael Forney wrote:
> >On 2021-03-05, Rich Felker <dalias@libc.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 topic in question is PT_LOAD.

> >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.

Are these together valid? In that case, p_memsz==0 but p_vaddr %
p_align != 0 would result in mapping an unused page, no? Or is this
somehow a special case where the mapping size is not supposed to be
expanded out to page-aligned?

Rich