nbd: Limit nbdflags to 16 bits

Rather than asserting that nbdflags is within range, just give
it the correct type to begin with :)  nbdflags corresponds to
the per-export portion of NBD Protocol "transmission flags", which
is 16 bits in response to NBD_OPT_EXPORT_NAME and NBD_OPT_GO.

Furthermore, upstream NBD has never passed the global flags to
the kernel via ioctl(NBD_SET_FLAGS) (the ioctl was first
introduced in NBD 2.9.22; then a latent bug in NBD 3.1 actually
tried to OR the global flags with the transmission flags, with
the disaster that the addition of NBD_FLAG_NO_ZEROES in 3.9
caused all earlier NBD 3.x clients to treat every export as
read-only; NBD 3.10 and later intentionally clip things to 16
bits to pass only transmission flags).  Qemu should follow suit,
since the current two global flags (NBD_FLAG_FIXED_NEWSTYLE
and NBD_FLAG_NO_ZEROES) have no impact on the kernel's behavior
during transmission.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>

Message-Id: <1469129688-22848-3-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/qemu-nbd.c b/qemu-nbd.c
index 321f02b..e3571c2 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -251,7 +251,7 @@ static void *nbd_client_thread(void *arg)
 {
     char *device = arg;
     off_t size;
-    uint32_t nbdflags;
+    uint16_t nbdflags;
     QIOChannelSocket *sioc;
     int fd;
     int ret;
@@ -465,7 +465,7 @@ int main(int argc, char **argv)
     BlockBackend *blk;
     BlockDriverState *bs;
     off_t dev_offset = 0;
-    uint32_t nbdflags = 0;
+    uint16_t nbdflags = 0;
     bool disconnect = false;
     const char *bindto = "0.0.0.0";
     const char *port = NULL;