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    SubjectRe: [PATCH net 1/4] xen-netback: Fix handling frag_list on grant op error path
    On 18/07/14 16:24, Wei Liu wrote:
    > On Thu, Jul 17, 2014 at 08:09:49PM +0100, Zoltan Kiss wrote:
    >> The error handling for skb's with frag_list was completely wrong, it caused
    >> double unmap attempts to happen if the error was on the first skb. Move it to
    >> the right place in the loop.
    >>
    >> Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
    >> Reported-by: Armin Zentai <armin.zentai@ezit.hu>
    >> Cc: netdev@vger.kernel.org
    >> Cc: linux-kernel@vger.kernel.org
    >> Cc: xen-devel@lists.xenproject.org
    >> ---
    >> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
    >> index 1844a47..604ff71 100644
    >> --- a/drivers/net/xen-netback/netback.c
    >> +++ b/drivers/net/xen-netback/netback.c
    >> @@ -1030,10 +1030,16 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
    >> {
    >> struct gnttab_map_grant_ref *gop_map = *gopp_map;
    >> u16 pending_idx = XENVIF_TX_CB(skb)->pending_idx;
    >> + /* This points to the shinfo of the actually checked skb, which could be
    >> + * either the first or the one on the frag_list
    >> + */
    >
    > I think "checked skb" should be "skb being checked". Feel free to
    > disagree as I'm not native English speaker. :-/
    >
    >> struct skb_shared_info *shinfo = skb_shinfo(skb);
    >> + /* If this is non-NULL, we are currently checking the frag_list skb, and
    >> + * this points to the shinfo of the first one
    >> + */
    >> + struct skb_shared_info *first_shinfo = NULL;
    >> int nr_frags = shinfo->nr_frags;
    >> int i, err;
    >> - struct sk_buff *first_skb = NULL;
    >>
    >> /* Check status of header. */
    >> err = (*gopp_copy)->status;
    >> @@ -1086,31 +1092,28 @@ check_frags:
    >> xenvif_idx_unmap(queue, pending_idx);
    >> }
    >>
    >> + /* And if we found the error while checking the frag_list, unmap
    >> + * the first skb's frags
    >> + */
    >> + if (first_shinfo) {
    >> + for (j = 0; j < first_shinfo->nr_frags; j++) {
    >> + pending_idx = frag_get_pending_idx(&first_shinfo->frags[j]);
    >> + xenvif_idx_unmap(queue, pending_idx);
    >> + }
    >> + }
    >> +
    >> /* Remember the error: invalidate all subsequent fragments. */
    >> err = newerr;
    >> }
    >>
    >> - if (skb_has_frag_list(skb)) {
    >> - first_skb = skb;
    >> - skb = shinfo->frag_list;
    >> - shinfo = skb_shinfo(skb);
    >> + if (skb_has_frag_list(skb) && !first_shinfo) {
    >
    > Will it ever come to the point that we have another skb in this skb's
    > frag list? Is there any reason prevents you from looping over the
    > (possible) subsequent skbs? I guess if the error is deep in the list
    > it's a bit hard to bookkeep...
    >
    >> + first_shinfo = skb_shinfo(skb);
    >> + shinfo = skb_shinfo(skb_shinfo(skb)->frag_list);
    >
    > In that case I would suggest you add
    > BUG_ON(skb_has_frag_list(skb_shinfo(skb)->frag_list)). I think having
    > more nested frag_list should be a bug in current design.

    There are already 3 things which prevents this
    - in count_requests we drop the packet if it has more than
    XEN_NETBK_LEGACY_SLOTS_MAX slots
    - in get_requests there is a BUG_ON(frag_overflow > MAX_SKB_FRAGS),
    which shouldn't really due to the prev point
    - in the same funciont we create a frag_list skb exactly once


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