tcp: fix tcp_trim_head()

commit f07d960df3 (tcp: avoid frag allocation for small frames)
breaked assumption in tcp stack that skb is either linear (skb->data_len
== 0), or fully fragged (skb->data_len == skb->len)

tcp_trim_head() made this assumption, we must fix it.

Thanks to Vijay for providing a very detailed explanation.

Reported-by: Vijay Subramanian <subramanian.vijay@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 5 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 50788d67bdb7..cf3068038942 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1093,6 +1093,13 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
 {
         int i, k, eat;
 
+        eat = min_t(int, len, skb_headlen(skb));
+        if (eat) {
+                __skb_pull(skb, eat);
+                len -= eat;
+                if (!len)
+                        return;
+        }
         eat = len;
         k = 0;
         for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
@@ -1124,11 +1131,7 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
         if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
                 return -ENOMEM;
 
-        /* If len == headlen, we avoid __skb_pull to preserve alignment. */
-        if (unlikely(len < skb_headlen(skb)))
-                __skb_pull(skb, len);
-        else
-                __pskb_trim_head(skb, len - skb_headlen(skb));
+        __pskb_trim_head(skb, len);
 
         TCP_SKB_CB(skb)->seq += len;
         skb->ip_summed = CHECKSUM_PARTIAL;