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[Qemu-devel] [PATCH 63/99] nbd/client: Relax handling of large NBD_CMD_B

From: Michael Roth
Subject: [Qemu-devel] [PATCH 63/99] nbd/client: Relax handling of large NBD_CMD_BLOCK_STATUS reply
Date: Mon, 23 Jul 2018 15:17:12 -0500

From: Eric Blake <address@hidden>

The NBD spec is proposing a relaxation of NBD_CMD_BLOCK_STATUS
where a server may have the final extent per context give a
length beyond the original request, if it can easily prove that
subsequent bytes have the same status, on the grounds that a
client can take advantage of this information for fewer block
status requests.  Since qemu 2.12 as a client always sends
NBD_CMD_FLAG_REQ_ONE, and rejects a server that sends extra
length, the upstream NBD spec will probably limit this behavior
to clients that don't request REQ_ONE semantics; but it doesn't
hurt to relax qemu to always be permissive of this server
behavior, even if it continues to use REQ_ONE.

CC: address@hidden
Signed-off-by: Eric Blake <address@hidden>
Message-Id: <address@hidden>
Reviewed-by:  Vladimir Sementsov-Ogievskiy <address@hidden>
(cherry picked from commit acfd8f7a5f92e703d2d046cbe3d510008a697194)
Signed-off-by: Michael Roth <address@hidden>
 block/nbd-client.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/block/nbd-client.c b/block/nbd-client.c
index e7caf49fbb..8d69eaaa32 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -259,14 +259,18 @@ static int nbd_parse_blockstatus_payload(NBDClientSession 
     if (extent->length == 0 ||
         (client->info.min_block && !QEMU_IS_ALIGNED(extent->length,
-                                                    client->info.min_block)) ||
-        extent->length > orig_length)
-    {
+                                                    client->info.min_block))) {
         error_setg(errp, "Protocol error: server sent status chunk with "
                    "invalid length");
         return -EINVAL;
+    /* The server is allowed to send us extra information on the final
+     * extent; just clamp it to the length we requested. */
+    if (extent->length > orig_length) {
+        extent->length = orig_length;
+    }
     return 0;

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