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Date: Fri, 5 Mar 2021 17:39:02 -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, 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.
p_memsz==0 PT_LOAD is invalid. ld.bfd -z separate-code should not create
such PT_LOAD. But this is more of implementation convention (many libc
implementation error on this case or mmap will fail), not something
regulated by the specification.
(In LLD, we drop such PT_LOAD:
https://github.com/llvm/llvm-project/blob/main/lld/ELF/Writer.cpp#L163)
The empty .eh_frame is suspicious, though. There may be two problems:
1. Why do you have an empty .eh_frame in an object file
2. Why does ld.bfd create empty .eh_frame in that case (I have tried
simple examples like `.section .eh_frame,"a"` and I cannot reproduce
the empty output .eh_frame)
If you don't share the other files, it is difficult to locate the
problem. 2. deserves a http://sourceware.org/ bug.
>
>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
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