[Xen-devel] [PATCH for stable 3.15 1/4] xen-netback: Fix handling frag_list on grant op error path

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

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@xxxxxxxxxx>
Reported-by: Armin Zentai <armin.zentai@xxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 7367208..d7ae93d 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1007,10 +1007,16 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
 {
        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
+        */
        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;
@@ -1063,31 +1069,28 @@ check_frags:
                        xenvif_idx_unmap(vif, 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(vif, 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) {
+               first_shinfo = skb_shinfo(skb);
+               shinfo = skb_shinfo(skb_shinfo(skb)->frag_list);
                nr_frags = shinfo->nr_frags;
 
                goto check_frags;
        }
 
-       /* There was a mapping error in the frag_list skb. We have to unmap
-        * the first skb's frags
-        */
-       if (first_skb && err) {
-               int j;
-               shinfo = skb_shinfo(first_skb);
-               for (j = 0; j < shinfo->nr_frags; j++) {
-                       pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
-                       xenvif_idx_unmap(vif, pending_idx);
-               }
-       }
-
        *gopp_map = gop_map;
        return err;
 }

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