From: Michael Forney <mforney@mforney.org>
To: musl@lists.openwall.com
Cc: Rich Felker <dalias@libc.org>
Subject: Re: [musl] ld-musl-* and empty .eh_frame
Date: Fri, 5 Mar 2021 17:30:37 -0800 [thread overview]
Message-ID: <CAGw6cBsHaJez_rmGY8Gvkou4gR6bY1p7=ykKeyZkZ5UFaWuPog@mail.gmail.com> (raw)
In-Reply-To: <20210306011405.g5t2pncdcbh4ebij@gmail.com>
[-- Attachment #1: Type: text/plain, Size: 1514 bytes --]
On 2021-03-05, Fangrui Song <i@maskray.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.
[-- Attachment #2: t-readelf.txt --]
[-- Type: text/plain, Size: 1891 bytes --]
Elf file type is EXEC (Executable file)
Entry point 0x401020
There are 9 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x00000000000001f8 0x00000000000001f8 R 0x8
INTERP 0x0000000000000238 0x0000000000400238 0x0000000000400238
0x0000000000000019 0x0000000000000019 R 0x1
[Requesting program interpreter: /lib/ld-musl-x86_64.so.1]
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x0000000000000348 0x0000000000000348 R 0x1000
LOAD 0x0000000000001000 0x0000000000401000 0x0000000000401000
0x0000000000000060 0x0000000000000060 R E 0x1000
LOAD 0x0000000000002000 0x0000000000402000 0x0000000000402000
0x0000000000000000 0x0000000000000000 R 0x1000
LOAD 0x0000000000002e90 0x0000000000402e90 0x0000000000402e90
0x0000000000000190 0x0000000000000190 RW 0x1000
DYNAMIC 0x0000000000002e90 0x0000000000402e90 0x0000000000402e90
0x0000000000000160 0x0000000000000160 RW 0x8
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RWE 0x10
GNU_RELRO 0x0000000000002e90 0x0000000000402e90 0x0000000000402e90
0x0000000000000170 0x0000000000000170 R 0x1
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .gnu.hash .dynsym .dynstr .rela.dyn .rela.plt
03 .plt .text
04 .eh_frame
05 .dynamic .got .got.plt
06 .dynamic
07
08 .dynamic .got
next prev parent reply other threads:[~2021-03-06 1:30 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-05 3:18 Michael Forney
2021-03-05 15:07 ` Rich Felker
2021-03-05 16:12 ` Rich Felker
2021-03-05 22:53 ` Michael Forney
2021-03-06 1:14 ` Fangrui Song
2021-03-06 1:28 ` Rich Felker
2021-03-06 1:30 ` Michael Forney [this message]
2021-03-06 1:39 ` Fangrui Song
2021-03-06 2:04 ` Michael Forney
2021-03-06 2:09 ` Fangrui Song
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