Re: Empty section flags

["H.J. Lu" <hjl.tools@gmail.com>]

On Sat, Apr 4, 2020 at 9:38 AM Fangrui Song <i@maskray.me> wrote:
>
> 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

[hjl@gnu-cfl-2 tmp]$ cat y.s
.section .foo,"ax",@progbits
.byte 1
.section .foo
.byte 2
.section .foo,"",@progbits
.byte 3
.section .foo,"ax"
.byte 3
[hjl@gnu-cfl-2 tmp]$ gcc -c y.s
[hjl@gnu-cfl-2 tmp]$ readelf -SW y.o | grep foo
  [ 4] .foo              PROGBITS        0000000000000000 000040
000004 00  AX  0   0  1
[hjl@gnu-cfl-2 tmp]$

If section flags or type is unspecified/empty, GNU assembler uses the
previous one if it exists.  I think this behavior is quite reasonable.

-- 
H.J.