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>

include/block/nbd.h

@@ -90,11 +90,11 @@ ssize_t nbd_wr_syncv(QIOChannel *ioc,
                      size_t niov,
                      size_t length,
                      bool do_read);
-int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint32_t *flags,
+int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint16_t *flags,
                           QCryptoTLSCreds *tlscreds, const char *hostname,
                           QIOChannel **outioc,
                           off_t *size, Error **errp);
-int nbd_init(int fd, QIOChannelSocket *sioc, uint32_t flags, off_t size);
+int nbd_init(int fd, QIOChannelSocket *sioc, uint16_t flags, off_t size);
 ssize_t nbd_send_request(QIOChannel *ioc, struct nbd_request *request);
 ssize_t nbd_receive_reply(QIOChannel *ioc, struct nbd_reply *reply);
 int nbd_client(int fd);
@@ -104,7 +104,7 @@ typedef struct NBDExport NBDExport;
 typedef struct NBDClient NBDClient;
 
 NBDExport *nbd_export_new(BlockBackend *blk, off_t dev_offset, off_t size,
-                          uint32_t nbdflags, void (*close)(NBDExport *),
+                          uint16_t nbdflags, void (*close)(NBDExport *),
                           Error **errp);
 void nbd_export_close(NBDExport *exp);
 void nbd_export_get(NBDExport *exp);