Re: .section directives with the same name but different fields

[Fangrui Song <i at maskray dot me>]

On 2020-02-06, Alan Modra wrote:
> On Wed, Feb 05, 2020@11:38:37PM -0800, Fangrui Song wrote:
> > ## Different sh_entsize
> >
> > .section .foo,"aM",@progbits,4
> > .section .foo,"aM",@progbits,8
> >
> > GNU as emits a warning `Warning: ignoring changed section entity size for .foo`
>
> I think this one probably should be an error rather than a warning.

An error is fine, but it can bring up some implementation difficulties.
If an implementation does a one-pass scan over global variables to
emit .section directives and variable labels, it may not know sh_entsize
when it is about the emit the first .section directive.

> > The output sh_entsize is 4. If the second .section defines an object, the object may get corrupted after merging
> > (https://bugs.llvm.org/show_bug.cgi?id=43457 )
> > For this case, we have several choices:
> >
> > 1. (Status quo) Emit one section. Set sh_entsize to 4 and emit a warning.
> > 2. Emit two sections, i.e. sh_entsize is a differentiator.
>
> If you do, the linker won't do merging of values for those sections.
>
> > 3. Emit one section. Set sh_entsize to 0. Should the assembler emit a warning?
>
> And remove SHF_MERGE too I guess.  That's an option but I think it's
> better just to error.
>
> > ## Different sh_flags
> >
> > .section .foo,"aw"
> > .section .foo,"a" # Warning: ignoring changed section attributes for .foo
> >
> > Shall we emit two sections?
>
> I don't think so.  User assembly often gets section attributes wrong
> or leaves them off entirely for special sections known to the
> assembler.  ie. the first .section .foo above is a built-in rather
> than user input.
>
> > ## Different sh_type
> >
> > .section .foo,"a",@progbits
> > .section .foo,"a",@nobits   # Warning: ignoring changed section type for .foo
> >
> > Shall we emit two sections?
>
> Again we should continue to handle the case where .foo is a special
> section of known type.  So I think a warning rather than creating two
> sections is appropriate.

Do you think the warnings should be upgraded to errors (for sh_flags and
sh_type)?