Re: [Qemu-devel] [PATCH v2 5/7] nbd-client: Short-circuit 0-length operations

[Vladimir Sementsov-Ogievskiy]

09.11.2017 00:57, Eric Blake wrote:
The NBD spec was recently clarified to state that clients should
not send 0-length requests to the server, as the server behavior
is undefined [1].  We know that qemu-nbd's behavior is a successful
no-op (once it has filtered for read-only exports), but other NBD
implementations might return an error.  To avoid any questionable
server implementations, it is better to just short-circuit such
requests on the client side (we are relying on the block layer to
already filter out requests such as invalid offset, write to a
read-only volume, and so forth).
[1] https://github.com/NetworkBlockDevice/nbd/commit/ee926037

Signed-off-by: Eric Blake <ebl...@redhat.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>

---
  block/nbd-client.c | 11 ++++++++++-
  1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/block/nbd-client.c b/block/nbd-client.c
index daa4392531..0a675d0fab 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -674,6 +674,9 @@ int nbd_client_co_preadv(BlockDriverState *bs, uint64_t 
offset,
      assert(bytes <= NBD_MAX_BUFFER_SIZE);
      assert(!flags);

+    if (!bytes) {
+        return 0;
+    }
      ret = nbd_co_send_request(bs, &request, NULL);
      if (ret < 0) {
          return ret;
@@ -705,6 +708,9 @@ int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t 
offset,

      assert(bytes <= NBD_MAX_BUFFER_SIZE);

+    if (!bytes) {
+        return 0;
+    }

we don't do this check before flags manipulation to do not miss possible 
asserts...

      return nbd_co_request(bs, &request, qiov);
  }

@@ -731,6 +737,9 @@ int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, 
int64_t offset,
          request.flags |= NBD_CMD_FLAG_NO_HOLE;
      }

+    if (!bytes) {
+        return 0;
+    }
      return nbd_co_request(bs, &request, NULL);
  }

@@ -759,7 +768,7 @@ int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t 
offset, int bytes)
      };

      assert(!(client->info.flags & NBD_FLAG_READ_ONLY));
-    if (!(client->info.flags & NBD_FLAG_SEND_TRIM)) {
+    if (!(client->info.flags & NBD_FLAG_SEND_TRIM) || !bytes) {
          return 0;
      }



--
Best regards,
Vladimir