Wei Liu
2013-Jul-10 12:53 UTC
Re: [PATCH] xen-netfront: pull on receive skb may need to happen earlier
On Wed, Jul 10, 2013 at 01:50:44PM +0100, Ian Campbell wrote:> On Wed, 2013-07-10 at 11:46 +0100, Jan Beulich wrote: > > >>> On 10.07.13 at 12:04, Wei Liu <wei.liu2@citrix.com> wrote: > > > Jan, looking at the commit log, the overrun issue in > > > xennet_get_responses was not introduced by __pskb_pull_tail. The call to > > > xennet_fill_frags has always been in the same place. > > > > I''m convinced it was: Prior to that commit, if the first response slot > > contained up to RX_COPY_THRESHOLD bytes, it got entirely > > consumed into the linear portion of the SKB, leaving the number of > > fragments available for filling at MAX_SKB_FRAGS. Said commit > > dropped the early copying, leaving the fragment count at 1 > > unconditionally, and now accumulates all of the response slots into > > fragments, only pulling after all of them got filled in. It neglected to > > realize - due to the count now always being 1 at the beginning - that > > this can lead to MAX_SKB_FRAGS + 1 frags getting filled, corrupting > > memory. > > That argument makes sense to me. > > Is it possible to hit a scenario where we need to pull more than > RX_COPY_THRESHOLD in order to fit all of the data in MAX_SKB_FRAGS ? > > > Ian - I have to admit that I''m slightly irritated by you so far not > > having participated at all in sorting out the fix for this bug that a > > change of yours introduced. > > Sorry I''ve been travelling and not following closely enough to realise > this was related to something I''d done. > > Does this relate somehow to the patch Annie has sent out recently too? >No. That''s not related. Wei.> Ian.