smbfs: fix calculation of kernel_recvmsg size parameter in smb_receive()

smb_receive calls kernel_recvmsg with a size that's the minimum of the
amount of buffer space in the kvec passed in or req->rq_rlen (which
represents the length of the response).  This does not take into account
any data that was read in a request earlier pass through smb_receive.

If the first pass through smb_receive receives some but not all of the
response, then the next pass can call kernel_recvmsg with a size field
that's too big.  kernel_recvmsg can overrun into the next response,
throwing off the alignment and making it unrecognizable.

This causes messages like this to pop up in the ring buffer:

smb_get_length: Invalid NBT packet, code=69

as well as other errors indicating that the response is unrecognizable.
Typically this is seen on a smbfs mount under heavy I/O.

This patch changes the code to use (req->rq_rlen - req->rq_bytes_recvd)
instead instead of just req->rq_rlen, since that should represent the
amount of unread data in the response.

I think this is correct, but an ACK or NACK from someone more familiar
with this code would be appreciated...

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/smbfs/sock.c

@@ -329,9 +329,8 @@ smb_receive(struct smb_sb_info *server, struct smb_request *req)
 	msg.msg_control = NULL;
 
 	/* Dont repeat bytes and count available bufferspace */
-	rlen = smb_move_iov(&p, &num, iov, req->rq_bytes_recvd);
-	if (req->rq_rlen < rlen)
-		rlen = req->rq_rlen;
+	rlen = min_t(int, smb_move_iov(&p, &num, iov, req->rq_bytes_recvd),
+			(req->rq_rlen - req->rq_bytes_recvd));
 
 	result = kernel_recvmsg(sock, &msg, p, num, rlen, flags);