Re: clang-22 -Walloc-size in mm/kfence/kfence_test.c in 6.6 and 6.1

[Matthew Wilcox <willy@infradead.org>]

On Tue, Sep 02, 2025 at 05:07:52PM -0700, Nathan Chancellor wrote:
> Hi kfence folks,
> 
> After [1] in clang, I am seeing an instance of this pop up in
> mm/kfence/kfence_test.c on linux-6.6.y and linux-6.1.y:
> 
>   mm/kfence/kfence_test.c:723:8: error: allocation of insufficient size '0' for type 'char' with size '1' [-Werror,-Walloc-size]
>     723 |         buf = krealloc(buf, 0, GFP_KERNEL); /* Free. */
>         |               ^
> 
> I do not see this in linux-6.12.y or newer but I wonder if that is just
> because the memory allocation profiling adds some indirection that makes
> it harder for clang to perform this analysis?
> 
> Should this warning just be silenced for this translation unit or is
> there some other fix that could be done here?

I mean, it's defined behaviour:

        if (unlikely(!new_size)) {
                kfree(p);
                return ZERO_SIZE_PTR;
        }

so we have to have a test which checks that it works.