net: add and use skb_unclone_keeptruesize() helper

While commit 097b9146 ("net: fix up truesize of cloned
skb in skb_prepare_for_shift()") fixed immediate issues found
when KFENCE was enabled/tested, there are still similar issues,
when tcp_trim_head() hits KFENCE while the master skb
is cloned.

This happens under heavy networking TX workloads,
when the TX completion might be delayed after incoming ACK.

This patch fixes the WARNING in sk_stream_kill_queues
when sk->sk_mem_queued/sk->sk_forward_alloc are not zero.

Fixes: d3fb45f3 ("mm, kfence: insert KFENCE hooks for SLAB")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/r/20211102004555.1359210-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -1675,6 +1675,22 @@ static inline int skb_unclone(struct sk_buff *skb, gfp_t pri)
 	return 0;
 }
 
+/* This variant of skb_unclone() makes sure skb->truesize is not changed */
+static inline int skb_unclone_keeptruesize(struct sk_buff *skb, gfp_t pri)
+{
+	might_sleep_if(gfpflags_allow_blocking(pri));
+
+	if (skb_cloned(skb)) {
+		unsigned int save = skb->truesize;
+		int res;
+
+		res = pskb_expand_head(skb, 0, 0, pri);
+		skb->truesize = save;
+		return res;
+	}
+	return 0;
+}
+
 /**
  *	skb_header_cloned - is the header a clone
  *	@skb: buffer to check

net/core/skbuff.c

@@ -3449,19 +3449,7 @@ EXPORT_SYMBOL(skb_split);
  */
 static int skb_prepare_for_shift(struct sk_buff *skb)
 {
-	int ret = 0;
-
-	if (skb_cloned(skb)) {
-		/* Save and restore truesize: pskb_expand_head() may reallocate
-		 * memory where ksize(kmalloc(S)) != ksize(kmalloc(S)), but we
-		 * cannot change truesize at this point.
-		 */
-		unsigned int save_truesize = skb->truesize;
-
-		ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
-		skb->truesize = save_truesize;
-	}
-	return ret;
+	return skb_unclone_keeptruesize(skb, GFP_ATOMIC);
 }
 
 /**

net/ipv4/tcp_output.c

@@ -1559,7 +1559,7 @@ int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
 		return -ENOMEM;
 	}
 
-	if (skb_unclone(skb, gfp))
+	if (skb_unclone_keeptruesize(skb, gfp))
 		return -ENOMEM;
 
 	/* Get a new skb... force flag on. */
@@ -1667,7 +1667,7 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 {
 	u32 delta_truesize;
 
-	if (skb_unclone(skb, GFP_ATOMIC))
+	if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
 		return -ENOMEM;
 
 	delta_truesize = __pskb_trim_head(skb, len);
@@ -3166,7 +3166,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 				 cur_mss, GFP_ATOMIC))
 			return -ENOMEM; /* We'll try again later. */
 	} else {
-		if (skb_unclone(skb, GFP_ATOMIC))
+		if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
 			return -ENOMEM;
 
 		diff = tcp_skb_pcount(skb);