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); do the short-circuit as late as possible to still benefit from protections from assertions that the block layer is not violating our assumptions.
[1] https://github.com/NetworkBlockDevice/nbd/commit/ee926037 Signed-off-by: Eric Blake <ebl...@redhat.com> Message-Id: <20171108215703.9295-6-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; + } 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; } -- 2.13.6
