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Date: Fri, 5 Mar 2021 17:30:37 -0800
From: Michael Forney <mforney@...rney.org>
To: musl@...ts.openwall.com
Cc: Rich Felker <dalias@...c.org>
Subject: Re: ld-musl-* and empty .eh_frame
On 2021-03-05, Fangrui Song <i@...kray.me> wrote:
> Can you clarify how GNU ld creates an empty .eh_frame?
Sure, see the shell snippet in my original post.
> 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
There is no PT_GNU_EH_FRAME in the executables. Based on Rich's
explanation earlier, my understanding is that the issue is that GNU ld
creates a empty PT_LOAD segment for the empty .eh_frame section.
I attached the output of readelf -l in case that is helpful.
> 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 empty PT_LOAD segment is the issue here, not PT_GNU_EH_FRAME.
> 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 you saying that GNU ld might create PT_LOAD segments with zero
size, and unaligned p_vaddr? If that's the case, I think Rich might
have a valid concern here.
View attachment "t-readelf.txt" of type "text/plain" (1891 bytes)
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