Re: [PATCH net-next v3 RFC] skb_expand_head() adjust skb->truesize incorrectly

On 8/31/21 10:38 PM, Eric Dumazet wrote:

On 8/31/21 7:34 AM, Vasily Averin wrote:

quoted

RFC because it have an extra changes:
new is_skb_wmem() helper can be called
 - either before pskb_expand_head(), to create skb clones 
    for skb with destructors that does not change sk->sk_wmem_alloc
 - or after pskb_expand_head(), to change owner in skb_set_owner_w()

In current patch I've added both these ways,
we need to keep one of them.

If nobody object I vote for 2nd way:

quoted

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f931176..3ce33f2 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -1804,30 +1804,47 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
 struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
 {

... skipped ...

quoted

-	return skb;
+	if (oskb) {
+		if (sk)
+			skb_set_owner_w(skb, sk);

Broken for non full sockets.
Calling skb_set_owner_w(skb, sk) for them is a bug.

I think you're wrong here.
It is 100% equivalent of old code, 
skb_set_owner_w() handles sk_fullsock(sk) inside and does not adjust sk->sk_wmem_alloc.
Please explain if I'm wrong.

quoted

+		consume_skb(oskb);
+	} else if (sk) {
+		delta = osize - skb_end_offset(skb);
+		if (!is_skb_wmem(skb))
+			skb_set_owner_w(skb, sk);

This would be broken for non full sockets.
Calling skb_set_owner_w(skb, sk) for them is a bug.

See my comment above.

quoted

+		skb->truesize += delta;
+		if (sk_fullsock(sk))
+			refcount_add(delta, &sk->sk_wmem_alloc);

quoted

+	}	return skb;

Strange line, will fix it.

quoted

 }
 EXPORT_SYMBOL(skb_expand_head);