[Xen-devel] [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

Commit 4f0581d25827d5e864bcf07b05d73d0d12a20a5c introduced an error into
xenvif_count_skb_slots() for skbs with a linear area spanning a page
boundary. The alignment of skb->data needs to be taken into account, not
just the head length. This patch fixes the issue by dry-running the code
from xenvif_gop_skb() (and adjusting the comment above the function to note
that).

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Xi Xiong <xixiong@xxxxxxxxxx>
Cc: Matt Wilson <msw@xxxxxxxxxx>
Cc: Annie Li <annie.li@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxx>

---
 drivers/net/xen-netback/netback.c |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index d0b0feb..6f680f4 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -223,15 +223,28 @@ static bool start_new_rx_buffer(int offset, unsigned long 
size, int head)
 /*
  * Figure out how many ring slots we're going to need to send @skb to
  * the guest. This function is essentially a dry run of
- * xenvif_gop_frag_copy.
+ * xenvif_gop_skb.
  */
 unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
 {
+       unsigned char *data;
        unsigned int count;
        int i, copy_off;
        struct skb_cb_overlay *sco;
 
-       count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
+       count = 0;
+
+       data = skb->data;
+       while (data < skb_tail_pointer(skb)) {
+               unsigned int offset = offset_in_page(data);
+               unsigned int len = PAGE_SIZE - offset;
+
+               if (data + len > skb_tail_pointer(skb))
+                       len = skb_tail_pointer(skb) - data;
+
+               count++;
+               data += len;
+       }
 
        copy_off = skb_headlen(skb) % PAGE_SIZE;
 
-- 
1.7.10.4


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