CIFS: Move get_next_mid to ops struct

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Pavel Shilovsky <pshilovsky@samba.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 89 insertions, 0 deletions

diff --git a/fs/cifs/smb1ops.c b/fs/cifs/smb1ops.c
index d9d615fbed3f..6dec38f5522d 100644
--- a/fs/cifs/smb1ops.c
+++ b/fs/cifs/smb1ops.c
@@ -125,6 +125,94 @@ cifs_get_credits_field(struct TCP_Server_Info *server)
         return &server->credits;
 }
 
+/*
+ * Find a free multiplex id (SMB mid). Otherwise there could be
+ * mid collisions which might cause problems, demultiplexing the
+ * wrong response to this request. Multiplex ids could collide if
+ * one of a series requests takes much longer than the others, or
+ * if a very large number of long lived requests (byte range
+ * locks or FindNotify requests) are pending. No more than
+ * 64K-1 requests can be outstanding at one time. If no
+ * mids are available, return zero. A future optimization
+ * could make the combination of mids and uid the key we use
+ * to demultiplex on (rather than mid alone).
+ * In addition to the above check, the cifs demultiplex
+ * code already used the command code as a secondary
+ * check of the frame and if signing is negotiated the
+ * response would be discarded if the mid were the same
+ * but the signature was wrong. Since the mid is not put in the
+ * pending queue until later (when it is about to be dispatched)
+ * we do have to limit the number of outstanding requests
+ * to somewhat less than 64K-1 although it is hard to imagine
+ * so many threads being in the vfs at one time.
+ */
+static __u64
+cifs_get_next_mid(struct TCP_Server_Info *server)
+{
+        __u64 mid = 0;
+        __u16 last_mid, cur_mid;
+        bool collision;
+
+        spin_lock(&GlobalMid_Lock);
+
+        /* mid is 16 bit only for CIFS/SMB */
+        cur_mid = (__u16)((server->CurrentMid) & 0xffff);
+        /* we do not want to loop forever */
+        last_mid = cur_mid;
+        cur_mid++;
+
+        /*
+         * This nested loop looks more expensive than it is.
+         * In practice the list of pending requests is short,
+         * fewer than 50, and the mids are likely to be unique
+         * on the first pass through the loop unless some request
+         * takes longer than the 64 thousand requests before it
+         * (and it would also have to have been a request that
+         * did not time out).
+         */
+        while (cur_mid != last_mid) {
+                struct mid_q_entry *mid_entry;
+                unsigned int num_mids;
+
+                collision = false;
+                if (cur_mid == 0)
+                        cur_mid++;
+
+                num_mids = 0;
+                list_for_each_entry(mid_entry, &server->pending_mid_q, qhead) {
+                        ++num_mids;
+                        if (mid_entry->mid == cur_mid &&
+                            mid_entry->mid_state == MID_REQUEST_SUBMITTED) {
+                                /* This mid is in use, try a different one */
+                                collision = true;
+                                break;
+                        }
+                }
+
+                /*
+                 * if we have more than 32k mids in the list, then something
+                 * is very wrong. Possibly a local user is trying to DoS the
+                 * box by issuing long-running calls and SIGKILL'ing them. If
+                 * we get to 2^16 mids then we're in big trouble as this
+                 * function could loop forever.
+                 *
+                 * Go ahead and assign out the mid in this situation, but force
+                 * an eventual reconnect to clean out the pending_mid_q.
+                 */
+                if (num_mids > 32768)
+                        server->tcpStatus = CifsNeedReconnect;
+
+                if (!collision) {
+                        mid = (__u64)cur_mid;
+                        server->CurrentMid = mid;
+                        break;
+                }
+                cur_mid++;
+        }
+        spin_unlock(&GlobalMid_Lock);
+        return mid;
+}
+
 struct smb_version_operations smb1_operations = {
         .send_cancel = send_nt_cancel,
         .compare_fids = cifs_compare_fids,
@@ -133,6 +221,7 @@ struct smb_version_operations smb1_operations = {
         .add_credits = cifs_add_credits,
         .set_credits = cifs_set_credits,
         .get_credits_field = cifs_get_credits_field,
+        .get_next_mid = cifs_get_next_mid,
         .read_data_offset = cifs_read_data_offset,
         .read_data_length = cifs_read_data_length,
         .map_error = map_smb_to_linux_error,