[PATCH] Fix section type of .eh_frame on Linux x86_64

[H.J. Lu]

On Fri, Mar 13, 2020 at 5:28 AM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> On 11/03/2020 18:27, Fangrui Song via Libc-alpha wrote:
> > Clang since https://reviews.llvm.org/D73999 will error for the wrong
> > sh_type.
>
> It will make eh_frame section of sigaction object to have the
> SHT_X86_64_UNWIND, but it seems that 'ld.bfd' at least ignore and
> set the resulting eh_frame sh_type for libc.so to SHT_PROGBITS
> anyway. The 'as' also still generate SHT_PROGBITS for code that
> requires a eh_frame (C++ with exception handling that emits a
> gcc_except_table section, for instance).

SHT_X86_64_UNWIND was added so that linker can group all
SHT_X86_64_UNWIND sections together without checking
.eh_frame section name.   Other than that, linker treats .eh_frame like
other SHT_PROGBITS sections.   Since .eh_frame is also listed as a
special section, similar to .got and .plt, SHT_X86_64_UNWIND isn't
really required.

> Setting the eh_frame in assembly routines is not a common practice,
> the only other code that I could find that actually does it is
> Linux.  For i686 vDSO is also uses 'progbits':
>
> arch/x86/entry/vdso/vdso32/sigreturn.S
>
>  36         .section .eh_frame,"a",@progbits
>  37 .LSTARTFRAMEDLSI1:
>
> The change should be ok, but I would like to understand better why
> exactly SHT_X86_64_UNWIND should be used for eh_frame, why binutils
> does not seems to use it to eh_frame, and why clang is now not
> accepting the eh_frame with SHT_PROGBITS type.
>

Linker should check .eh_frame section name, and optionally check
SHT_X86_64_UNWIND.   Because of this, i386 psABI only specifies
special .eh_frame section without SHT_386_UNWIND.

So I think there is no need to change glibc and lld should be changed.

-- 
H.J.