Re: [PATCH 1/2] mm: krealloc: consider spare memory for __GFP_ZERO

[Vlastimil Babka <vbabka@suse.cz>]

On 7/30/24 9:42 PM, Danilo Krummrich wrote:
> As long as krealloc() is called with __GFP_ZERO consistently, starting
> with the initial memory allocation, __GFP_ZERO should be fully honored.
> 
> However, if for an existing allocation krealloc() is called with a
> decreased size, it is not ensured that the spare portion the allocation
> is zeroed. Thus, if krealloc() is subsequently called with a larger size
> again, __GFP_ZERO can't be fully honored, since we don't know the
> previous size, but only the bucket size.
> 
> Example:
> 
> 	buf = kzalloc(64, GFP_KERNEL);
> 	memset(buf, 0xff, 64);
> 
> 	buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);
> 
> 	/* After this call the last 16 bytes are still 0xff. */
> 	buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO);
> 
> Fix this, by explicitly setting spare memory to zero, when shrinking an
> allocation with __GFP_ZERO flag set or init_on_alloc enabled.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  mm/slab_common.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 40b582a014b8..cff602cedf8e 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
>  
>  	/* If the object still fits, repoison it precisely. */
>  	if (ks >= new_size) {
> +		/* Zero out spare memory. */
> +		if (want_init_on_alloc(flags)) {
> +			kasan_disable_current();
> +			memset((void *)p + new_size, 0, ks - new_size);
> +			kasan_enable_current();

If we do kasan_krealloc() first, shouldn't the memset then be legal
afterwards without the disable/enable dance?

> +		}
> +
>  		p = kasan_krealloc((void *)p, new_size, flags);
>  		return (void *)p;
>  	}
> 
> base-commit: 7c3dd6d99f2df6a9d7944ee8505b195ba51c9b68