Re: [PATCH v3][next] skbuff: Proactively round up to kmalloc bucket size

[Paolo Abeni <pabeni@redhat.com>]

Hello,

On Tue, 2022-10-18 at 02:33 -0700, Kees Cook wrote:
> Instead of discovering the kmalloc bucket size _after_ allocation, round
> up proactively so the allocation is explicitly made for the full size,
> allowing the compiler to correctly reason about the resulting size of
> the buffer through the existing __alloc_size() hint.
> 
> This will allow for kernels built with CONFIG_UBSAN_BOUNDS or the
> coming dynamic bounds checking under CONFIG_FORTIFY_SOURCE to gain
> back the __alloc_size() hints that were temporarily reverted in commit
> 93dd04ab0b2b ("slab: remove __alloc_size attribute from __kmalloc_track_caller")
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: netdev@vger.kernel.org
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v3: refactor again to pass allocation size more cleanly to callers
> v2: https://lore.kernel.org/lkml/20220923202822.2667581-4-keescook@chromium.org/
> ---
>  net/core/skbuff.c | 41 ++++++++++++++++++++++-------------------
>  1 file changed, 22 insertions(+), 19 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 1d9719e72f9d..3ea1032d03ec 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -425,11 +425,12 @@ EXPORT_SYMBOL(napi_build_skb);
>   * memory is free
>   */
>  static void *kmalloc_reserve(size_t size, gfp_t flags, int node,
> -			     bool *pfmemalloc)
> +			     bool *pfmemalloc, size_t *alloc_size)
>  {
>  	void *obj;
>  	bool ret_pfmemalloc = false;
>  
> +	size = kmalloc_size_roundup(size);
>  	/*
>  	 * Try a regular allocation, when that fails and we're not entitled
>  	 * to the reserves, fail.
> @@ -448,6 +449,7 @@ static void *kmalloc_reserve(size_t size, gfp_t flags, int node,
>  	if (pfmemalloc)
>  		*pfmemalloc = ret_pfmemalloc;
>  
> +	*alloc_size = size;
>  	return obj;
>  }
>  
> @@ -479,7 +481,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  {
>  	struct kmem_cache *cache;
>  	struct sk_buff *skb;
> -	unsigned int osize;
> +	size_t alloc_size;
>  	bool pfmemalloc;
>  	u8 *data;
>  
> @@ -506,15 +508,15 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  	 */
>  	size = SKB_DATA_ALIGN(size);
>  	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> -	data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc);
> -	if (unlikely(!data))
> -		goto nodata;

I'm sorry for not noticing the above in the previous iteration, but I
think this revision will produce worse code than the V1, as
kmalloc_reserve() now pollutes an additional register.

Why did you prefer adding an additional parameter to kmalloc_reserve()?
I think computing the alloc_size in the caller is even more readable.

Additionally, as a matter of personal preference, I would not introduce
an additional variable for alloc_size, just:

	// ...
	size = kmalloc_size_roundup(size);
	data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc);

The rationale is smaller diff, and consistent style with the existing
code where 'size' is already adjusted multiple times icrementally.

Cheers,

Paolo