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[Qemu-devel] [PULL 3/5] 9pfs: fix off-by-one error in PDU free list

From: Greg Kurz
Subject: [Qemu-devel] [PULL 3/5] 9pfs: fix off-by-one error in PDU free list
Date: Wed, 25 Jan 2017 14:48:00 +0100

The server can handle MAX_REQ - 1 PDUs at a time and the virtio-9p
device has a MAX_REQ sized virtqueue. If the client manages to fill
up the virtqueue, pdu_alloc() will fail and the request won't be
processed without any notice to the client (it actually causes the
linux 9p client to hang).

This has been there since the beginning (commit 9f10751365b2 "virtio-9p:
Add a virtio 9p device to qemu"), but it needs an agressive workload to
run in the guest to show up.

We actually allocate MAX_REQ PDUs and I see no reason not to link them
all into the free list, so let's fix the init loop.

Reported-by: Tuomas Tynkkynen <address@hidden>
Suggested-by: Al Viro <address@hidden>
Signed-off-by: Greg Kurz <address@hidden>
 hw/9pfs/9p.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 58310ca8d5a5..d2d028828294 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -3454,7 +3454,7 @@ int v9fs_device_realize_common(V9fsState *s, Error **errp)
     /* initialize pdu allocator */
-    for (i = 0; i < (MAX_REQ - 1); i++) {
+    for (i = 0; i < MAX_REQ; i++) {
         QLIST_INSERT_HEAD(&s->free_list, &s->pdus[i], next);
         s->pdus[i].s = s;
         s->pdus[i].idx = i;

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