Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Wed, May 12, 2021 at 07:52:27PM -0700, Andrew Morton wrote:
> This explodes in mysterious ways.  The patch as I have it is appended,
> for reference.
> 
> gcc-10.3.0 allmodconfig.
> 
> This patch suppresses the error:
> 
> --- a/mm/kfence/kfence_test.c~a
> +++ a/mm/kfence/kfence_test.c
> @@ -318,13 +318,13 @@ static void test_out_of_bounds_read(stru
>  
>  	/* Test both sides. */
>  
> -	buf = test_alloc(test, size, GFP_KERNEL, ALLOCATE_LEFT);
> +	buf = test_alloc(test, 32, GFP_KERNEL, ALLOCATE_LEFT);
>  	expect.addr = buf - 1;
>  	READ_ONCE(*expect.addr);
>  	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
>  	test_free(buf);
>
> -	buf = test_alloc(test, size, GFP_KERNEL, ALLOCATE_RIGHT);
> +	buf = test_alloc(test, 32, GFP_KERNEL, ALLOCATE_RIGHT);
>  	expect.addr = buf + size;
>  	READ_ONCE(*expect.addr);
>  	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> @@ -519,11 +519,11 @@ static void test_free_bulk(struct kunit
>  		const size_t size = setup_test_cache(test, 8 + prandom_u32_max(300), 0,
>  						     (iter & 1) ? ctor_set_x : NULL);
>  		void *objects[] = {
> -			test_alloc(test, size, GFP_KERNEL, ALLOCATE_RIGHT),
> -			test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
> -			test_alloc(test, size, GFP_KERNEL, ALLOCATE_LEFT),
> -			test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
> -			test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
> +			test_alloc(test, 32, GFP_KERNEL, ALLOCATE_RIGHT),
> +			test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
> +			test_alloc(test, 32, GFP_KERNEL, ALLOCATE_LEFT),
> +			test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
> +			test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
>  		};
>  
>  		kmem_cache_free_bulk(test_cache, ARRAY_SIZE(objects), objects);
>
>
> Is gcc-10.3.0 simply confused?  test_out_of_bounds_read() is clearly
> calling kmalloc_index(32) which is OK.
>
> Anyway, I'll drop this patch for now so I can compile a kernel!
>

The error messages isn't so clear to me.
but one problem I can see is in kfence_test.c, there are many places that
are using size which is not constant.

in kmalloc if size is not constant, it calls dummy function __kmalloc
which does not make use of size.