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Date: Wed, 31 Oct 2012 21:15:30 +0200 From: "Michael S. Tsirkin" <mst@...hat.com> To: Ben Hutchings <bhutchings@...arflare.com> Cc: netdev@...r.kernel.org, Herbert Xu <herbert@...dor.hengli.com.au> Subject: Re: skb_linearize On Sun, Sep 16, 2012 at 04:07:12PM +0100, Ben Hutchings wrote: > On Sun, 2012-09-16 at 12:17 +0300, Michael S. Tsirkin wrote: > > I notice that dev_hard_start_xmit might invoke > > __skb_linearize e.g. if device does not support NETIF_F_SG. > > > > This in turn onvokes __pskb_pull_tail, and > > documentation of __pskb_pull_tail says: > > &sk_buff MUST have reference count of 1. > > > > I am guessing 'reference count' means users in this context, right? > > IIUC this is because it modifies skb in a way that > > isn't safe if anyone else is looking at the skb. > > > > > > However, I don't see what guarantees that reference > > count is 1 when dev_hard_start_xmit invokes > > linearize. In particular it calls dev_queue_xmit_nit > > which could queue packets on a network tap. > > > > Could someone help me understand please? > > Reference count here means references to struct sk_buff itself. The > header area and data fragments are allowed to be shared. > > dev_queue_xmit_nit() clones the skb for each tap, so the reference count > on the original skb remains 1. > > Ben. Interesting. But don't skb clones share the fragment list? Maybe I misunderstand? If they do it looks like the following race would be possible: - skb is cloned and queued e.g. at socket receive queue. dataref becomes 2. - On CPU 1, skb_copy_datagram_iovec is called on clone 1, is reads nr_frags and sees value > 1. - On CPU 2, __skb_linearize is now called on clone 2, it modified the skb so nr_frags is now 0, and does put_page for all frags > 1. - On CPU 1, skb_copy_datagram_iovec will now use the previously read nr_frags > 1 and access a fragment page that was already freed. What did I miss? > -- > Ben Hutchings, Staff Engineer, Solarflare > Not speaking for my employer; that's the marketing department's job. > They asked us to note that Solarflare product names are trademarked. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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