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

[Fangrui Song]

On 2020-03-13, H.J. Lu wrote:
>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.
>> >

clang integrated assembler is more rigid.
https://github.com/llvm/llvm-project/blob/master/llvm/lib/MC/MCObjectFileInfo.cpp#L488
.eh_frame is a pre-allocated recognized section.
"Redeclaring" (via .section directive) it with different sh_flags or sh_type will error.

>> 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.

I know little about GNU as internals
(https://sourceware.org/legacy-ml/binutils/2020-02/msg00129.html),
but it seems .cfi* generated .eh_frame contents don't conflict with
.section declared .eh_frame

Though, .section .eh_frame,"a",@unwind; .section .eh_frame,"a",@progbits
will error as well.

https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d2b2c203e1ff2daf8cfeab0906084f9389e66246 (2004) added SHT_X86_64_UNWIND to binutils.
There was no further information why it was added, though.