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

[H.J. Lu]

On Fri, Mar 13, 2020 at 7:04 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 13/03/2020 10:12, H.J. Lu wrote:
> > 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.
>
> Thanks for the explanation.
>
> >
> >> 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.
> >
>
> I was confused why binutils was not throwing a similar issue.  So
> the idea is SHT_X86_64_UNWIND is optional on eh_frame definitions,
> not mandatory, and being optional I agree that compiler/linker
> should not emit an error in such case.

It is unfortunate that x86-64 psABI specifies both SHT_X86_64_UNWIND
and .eh_frame.  Only one is needed, not both.

-- 
H.J.