[TCP]: Avoid skb_pull if possible when trimming head (f2911969) · Commits · 戴 / test

Trimming the head of an skb by calling skb_pull can cause the packet to become unaligned if the length pulled is odd. Since the length is entirely arbitrary for a FIN packet carrying data, this is actually quite common. Unaligned data is not the end of the world, but we should avoid it if it's easily done. In this case it is trivial. Since we're discarding all of the head data it doesn't matter whether we move skb->data forward or back. However, it is still possible to have unaligned skb->data in general. So network drivers should be prepared to handle it instead of crashing. This patch also adds an unlikely marking on len < headlen since partial ACKs on head data are extremely rare in the wild. As the return value of __pskb_trim_head is no longer ever NULL that has been removed. Signed-off-by:

Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au> Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

+5 −7

Original line number	Diff line number	Diff line
		@@ -642,7 +642,7 @@ int tcp_fragment(struct sock sk, struct sk_buff skb, u32 len, unsigned int mss
		* eventually). The difference is that pulled data not copied, but
		* immediately discarded.
		*/
		static unsigned char __pskb_trim_head(struct sk_buff skb, int len)
		static void __pskb_trim_head(struct sk_buff *skb, int len)
		{
		int i, k, eat;

		@@ -667,7 +667,6 @@ static unsigned char __pskb_trim_head(struct sk_buff skb, int len)
		skb->tail = skb->data;
		skb->data_len -= len;
		skb->len = skb->data_len;
		return skb->tail;
		}

		int tcp_trim_head(struct sock sk, struct sk_buff skb, u32 len)
		@@ -676,12 +675,11 @@ int tcp_trim_head(struct sock sk, struct sk_buff skb, u32 len)
		pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
		return -ENOMEM;

		if (len <= skb_headlen(skb)) {
		/* If len == headlen, we avoid __skb_pull to preserve alignment. */
		if (unlikely(len < skb_headlen(skb)))
		__skb_pull(skb, len);
		} else {
		if (__pskb_trim_head(skb, len-skb_headlen(skb)) == NULL)
		return -ENOMEM;
		}
		else
		__pskb_trim_head(skb, len - skb_headlen(skb));

		TCP_SKB_CB(skb)->seq += len;
		skb->ip_summed = CHECKSUM_HW;

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