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.section directives with the same name but different fields

Fangrui Song i@maskray.me
Thu Feb 6 09:19:00 GMT 2020 
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)?
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