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Date: Fri, 5 Mar 2021 14:53:41 -0800
From: Michael Forney <mforney@...rney.org>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: ld-musl-* and empty .eh_frame
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?
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
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.
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