[Xen-devel] [PATCH net 3/4 v2] xen-netback: Fix releasing header slot on error path

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

This patch makes this function aware that the first frag and the header might
share the same ring slot. That could happen if the first slot is bigger than
PKT_PROT_LEN. Due to this the error path might release that slot twice or never,
depending on the error scenario.
xenvif_idx_release is also removed from xenvif_idx_unmap, and called separately.

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
---
v2: remove stray blank line

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 8cbf60d..6fff911 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1039,6 +1039,8 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
         */
        struct skb_shared_info *first_shinfo = NULL;
        int nr_frags = shinfo->nr_frags;
+       const bool sharedslot = nr_frags &&
+                               frag_get_pending_idx(&shinfo->frags[0]) == 
pending_idx;
        int i, err;
 
        /* Check status of header. */
@@ -1051,7 +1053,10 @@ static int xenvif_tx_check_gop(struct xenvif_queue 
*queue,
                                   (*gopp_copy)->status,
                                   pending_idx,
                                   (*gopp_copy)->source.u.ref);
-               xenvif_idx_release(queue, pending_idx, XEN_NETIF_RSP_ERROR);
+               /* The first frag might still have this slot mapped */
+               if (!sharedslot)
+                       xenvif_idx_release(queue, pending_idx,
+                                          XEN_NETIF_RSP_ERROR);
        }
 
 check_frags:
@@ -1068,8 +1073,19 @@ check_frags:
                                                pending_idx,
                                                gop_map->handle);
                        /* Had a previous error? Invalidate this fragment. */
-                       if (unlikely(err))
+                       if (unlikely(err)) {
                                xenvif_idx_unmap(queue, pending_idx);
+                               /* If the mapping of the first frag was OK, but
+                                * the header's copy failed, and they are
+                                * sharing a slot, send an error
+                                */
+                               if (i == 0 && sharedslot)
+                                       xenvif_idx_release(queue, pending_idx,
+                                                          XEN_NETIF_RSP_ERROR);
+                               else
+                                       xenvif_idx_release(queue, pending_idx,
+                                                          XEN_NETIF_RSP_OKAY);
+                       }
                        continue;
                }
 
@@ -1081,15 +1097,27 @@ check_frags:
                                   gop_map->status,
                                   pending_idx,
                                   gop_map->ref);
+
                xenvif_idx_release(queue, pending_idx, XEN_NETIF_RSP_ERROR);
 
                /* Not the first error? Preceding frags already invalidated. */
                if (err)
                        continue;
-               /* First error: invalidate preceding fragments. */
+
+               /* First error: if the header haven't shared a slot with the
+                * first frag, release it as well.
+                */
+               if (!sharedslot)
+                       xenvif_idx_release(queue,
+                                          XENVIF_TX_CB(skb)->pending_idx,
+                                          XEN_NETIF_RSP_OKAY);
+
+               /* Invalidate preceding fragments of this skb. */
                for (j = 0; j < i; j++) {
                        pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
                        xenvif_idx_unmap(queue, pending_idx);
+                       xenvif_idx_release(queue, pending_idx,
+                                          XEN_NETIF_RSP_OKAY);
                }
 
                /* And if we found the error while checking the frag_list, unmap
@@ -1099,6 +1127,8 @@ check_frags:
                        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);
+                               xenvif_idx_release(queue, pending_idx,
+                                                  XEN_NETIF_RSP_OKAY);
                        }
                }
 
@@ -1834,8 +1864,6 @@ void xenvif_idx_unmap(struct xenvif_queue *queue, u16 
pending_idx)
                           tx_unmap_op.status);
                BUG();
        }
-
-       xenvif_idx_release(queue, pending_idx, XEN_NETIF_RSP_OKAY);
 }
 
 static inline int rx_work_todo(struct xenvif_queue *queue)

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