Re: Empty section flags

[Fangrui Song <i@maskray.me>]

On 2020-04-04, H.J. Lu wrote:
>On Tue, Mar 3, 2020 at 1:20 PM Fangrui Song <i@maskray.me> wrote:
>>
>> On Sun, Feb 9, 2020 at 9:21 PM Alan Modra <amodra@gmail.com> wrote:
>> >
>> > On Thu, Feb 06, 2020 at 05:25:33PM +0000, bd1976 llvm wrote:
>> > > Hi Alan, thanks for the input here. I wonder if it wouldn't be more
>> > > consistent to error in all cases - even in the case of different group
>> > > signatures. The only exception would need to be for the special section
>> > > names (.text, .debug_str, etc...) that the assembler has special knowledge
>> > > of (as you explained).
>> >
>> > Yes, let's see how that goes.
>> > https://sourceware.org/ml/binutils/2020-02/msg00129.html
>> >
>> > > I wonder why creating multiple sections with the
>> > > same name for section directives with different group signatures was
>> > > implemented - why not just require the use of a distinct section name for
>> > > these?
>> >
>> > I think plain ".text" for a group's text section is fine.  Distict
>> > names would just be yet another thing to track for a group.
>> >
>> > > Or, now that GNU has the ",unique,N" assembly extension (
>> > > https://sourceware.org/ml/binutils/2020-02/msg00028.html) that could be
>> > > used if the section name is fixed - it would then be explicit in the source
>> > > code that another section with the same name will be created.
>> >
>> > Perhaps, but we aren't designing a new toolchain.  Backwards
>> > compatibility can't be discarded without compelling reasons.
>> >
>> > --
>> > Alan Modra
>> > Australia Development Lab, IBM
>>
>> For empty flags, should there be an error as well?
>>
>>   .section .foo,"ax",@progbits; .byte 1
>>   .section .foo,"",@progbits; .byte 2  # no diagnostic
>>   .section .foo,"a",@progbits; .byte 3  # Error: changed section
>> attributes for .foo
>>
>> Context: https://github.com/ClangBuiltLinux/linux/issues/913
>>
>> I lean toward an error for consistency, and I will try making the LLVM
>> MC side rule stick.
>
>[hjl@gnu-cfl-2 tmp]$ cat x.s
>.section .foo,"",@progbits; .byte 2
>[hjl@gnu-cfl-2 tmp]$ gcc -c x.s
>[hjl@gnu-cfl-2 tmp]$ readelf -SW x.o | grep foo
>  [ 4] .foo              PROGBITS        0000000000000000 000040
>000001 00      0   0  1
>[hjl@gnu-cfl-2 tmp]$
>
>Unless it is disallowed by gABI/psABI, assembler should allow it.
>Sometimes, I found a need to create odd object files, like zero-sized
>relocation section,  for linker test.  Assembler should have more
>flexibilities within gABI/psABI.
>
>-- 
>H.J.

Declaring a section with empty flags is fine.
My question is about re-declaring with empty flags when the first declaration has other flags:

.section .foo,"ax",@progbits; .byte 1
.section .foo,"",@progbits; .byte 2   # no diagnostic
.section .foo,"a",@progbits; .byte 3  # Error: changed section

This is about the follow-up of
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=33176d912add7680277ad5e18af0e6303d9a7af8