Re: [PATCH] skbuff: Reallocate to ksize() in __build_skb_around()

[Kees Cook]

On December 6, 2022 5:55:57 PM PST, Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>On Tue,  6 Dec 2022 15:17:14 -0800 Kees Cook wrote:
>> -	unsigned int size = frag_size ? : ksize(data);
>> +	unsigned int size = frag_size;
>> +
>> +	/* When frag_size == 0, the buffer came from kmalloc, so we
>> +	 * must find its true allocation size (and grow it to match).
>> +	 */
>> +	if (unlikely(size == 0)) {
>> +		void *resized;
>> +
>> +		size = ksize(data);
>> +		/* krealloc() will immediate return "data" when
>> +		 * "ksize(data)" is requested: it is the existing upper
>> +		 * bounds. As a result, GFP_ATOMIC will be ignored.
>> +		 */
>> +		resized = krealloc(data, size, GFP_ATOMIC);
>> +		if (WARN_ON(resized != data))
>> +			data = resized;
>> +	}
>>  
>
>Aammgh. build_skb(0) is plain silly, AFAIK. The performance hit of
>using kmalloc()'ed heads is large because GRO can't free the metadata.
>So we end up carrying per-MTU skbs across to the application and then
>freeing them one by one. With pages we just aggregate up to 64k of data
>in a single skb.

This isn't changed by this patch, though? The users of kmalloc+build_skb are pre-existing.

>I can only grep out 3 cases of build_skb(.. 0), could we instead
>convert them into a new build_skb_slab(), and handle all the silliness
>in such a new helper? That'd be a win both for the memory safety and one
>fewer branch for the fast path.

When I went through callers, it was many more than 3. Regardless, I don't see the point: my patch has no more branches than the original code (in fact, it may actually be faster because I made the initial assignment unconditional, and zero-test-after-assign is almost free, where as before it tested before the assign. And now it's marked as unlikely to keep it out-of-line.

>I think it's worth doing, so LMK if you're okay to do this extra work,
>otherwise I can help (unless e.g. Eric tells me I'm wrong..).

I had been changing callers to round up (e.g. bnx2), but it seemed like centralizing this makes more sense. I don't think a different helper will clean this up.

-Kees


-- 
Kees Cook