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

[Fangrui Song <i@maskray.me>]

On 2021-03-05, Michael Forney wrote:
>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.

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