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.section directives with the same name but different fields

Alan Modra amodra@gmail.com
Thu Feb 6 14:09:00 GMT 2020 
On Thu, Feb 06, 2020 at 01:19:14AM -0800, Fangrui Song wrote:
> 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)?

No.

-- 
Alan Modra
Australia Development Lab, IBM
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