[PATCH] add support for -Wmismatched-dealloc

[Florian Weimer]

* Martin Sebor via Libc-alpha:

>> realpath only returns a pointer to the heap if RESOLVED is null, so
>> the annotation is wrong here.

> This is intentional.  When realpath() returns the last argument
> (when it's nonnull) passing the returned pointer to free will not
> be diagnosed but passing it to some other deallocator not associated
> with the function will be.  That means for example that passing
> a pointer allocated by C++ operator new() to realpath() and then
> deleting the pointer returned from the function as opposed to
> the argument will trigger a false positive.  I decided this was
> an okay trade-off because unless the function allocates memory
> I expect the returned pointer to be ignored (similarly to how
> the pointer returned from memcpy is ignored).  If you don't like
> the odds I can remove the attribute from the function until we
> have one that captures this conditional return value (I'd like
> to add one in GCC 12).

Maybe David can comment on how this interacts with his static analyzer
work.  In all other cases, the attribute means that the pointer needs to
be freed to avoid a resource leak.  If we suddenly apply it pointers
which can only conditionally be freed, that reduces the value of those
annotations, I think.

Thanks,
Florian
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