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Re: [Xen-devel] Moving frags and SKBTX_DEV_ZEROCOPY skbs

On Wed, 2014-05-14 at 14:40 +0100, Zoltan Kiss wrote:
> Hi,
> Recently I've investigated issues around SKBTX_DEV_ZEROCOPY skbs where 
> the frags list were modified. I came across this function skb_shift(), 
> which moves frags between skbs. And there are a lot more of such kind, 
> skb_split or skb_try_coalesce, for example.
> It could be a dangerous thing if a frag is referenced from an skb which 
> doesn't have the original destructor_arg, and to avoid that 
> skb_orphan_frags should be called. Although probably these functions are 
> not normally touched in usual usecases, I think it would be useful to 
> review core skb functions proactively and add an skb_orphan_frags 
> everywhere where the frags could be referenced from other places.
> Any opinion about this?

For skb_shift(), it is currently used from tcp stack only, where
this SKBTX_DEV_ZEROCOPY thing is not used, so I do not think there is a
bug for the moment.

I already gave a patch for skb_try_coalesce() : For this one we do not
wan skb_orphan_frags() overhead. Its simply better in this case to

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 1b62343f5837..85995a14aafc 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3838,7 +3839,10 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff 
                return true;
-       if (skb_has_frag_list(to) || skb_has_frag_list(from))
+       if (skb_has_frag_list(to) ||
+           skb_has_frag_list(from) ||
+           (skb_shinfo(to)->tx_flags & SKBTX_DEV_ZEROCOPY) ||
+           (skb_shinfo(from)->tx_flags & SKBTX_DEV_ZEROCOPY))
                return false;
        if (skb_headlen(from) != 0) {

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