skb_try_coalesce and fraglists (bug?)

[Erik Hugne <erik.hugne@ericsson.com>]

skb_try_coalesce should bail out for a number of reasons, if the target skb is
cloned, doesn't have enough room, or if either source or target skb have
fraglists.

However, it seems that the skb_has_frag_list check is done too late, and a small
skb may be copied into the tailroom of the head, even if it has a fraglist.

Wouldn't this be more correct?

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 56db472..8d02140 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4056,6 +4056,9 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
        if (skb_cloned(to))
                return false;
 
+       if (skb_has_frag_list(to) || skb_has_frag_list(from))
+               return false;
+
        if (len <= skb_tailroom(to)) {
                if (len)
                        BUG_ON(skb_copy_bits(from, 0, skb_put(to, len), len));
@@ -4063,9 +4066,6 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
                return true;
        }
 
-       if (skb_has_frag_list(to) || skb_has_frag_list(from))
-               return false;
-
        if (skb_headlen(from) != 0) {
                struct page *page;
                unsigned int offset;