[CIFS] Ensure that cifs multiplex ids do not collide.

Signed-off-by: Steve French (sfrench@us.ibm.com)

1 files changed, 77 insertions, 7 deletions

diff --git a/fs/cifs/misc.c b/fs/cifs/misc.c
index 20ae4153f791..beeff8284169 100644
--- a/fs/cifs/misc.c
+++ b/fs/cifs/misc.c
@@ -34,8 +34,6 @@ extern mempool_t *cifs_sm_req_poolp;
 extern mempool_t *cifs_req_poolp;
 extern struct task_struct * oplockThread;
 
-static __u16 GlobalMid;         /* multiplex id - rotating counter */
-
 /* The xid serves as a useful identifier for each incoming vfs request, 
    in a similar way to the mid which is useful to track each sent smb, 
    and CurrentXid can also provide a running counter (although it 
@@ -51,6 +49,8 @@ _GetXid(void)
         GlobalTotalActiveXid++;
         if (GlobalTotalActiveXid > GlobalMaxActiveXid)
                 GlobalMaxActiveXid = GlobalTotalActiveXid;      /* keep high water mark for number of simultaneous vfs ops in our filesystem */
+        if(GlobalTotalActiveXid > 65000)
+                cFYI(1,("warning: more than 65000 requests active"));
         xid = GlobalCurrentXid++;
         spin_unlock(&GlobalMid_Lock);
         return xid;
@@ -218,6 +218,76 @@ cifs_small_buf_release(void *buf_to_free)
         return;
 }
 
+/* 
+        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.
+*/
+__u16 GetNextMid(struct TCP_Server_Info *server)
+{
+        __u16 mid = 0;
+        __u16 last_mid;
+        int   collision;  
+
+        if(server == NULL)
+                return mid;
+
+        spin_lock(&GlobalMid_Lock);
+        last_mid = server->CurrentMid; /* we do not want to loop forever */
+        server->CurrentMid++;
+        /* 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(server->CurrentMid != last_mid) {
+                struct list_head *tmp;
+                struct mid_q_entry *mid_entry;
+
+                collision = 0;
+                if(server->CurrentMid == 0)
+                        server->CurrentMid++;
+
+                list_for_each(tmp, &server->pending_mid_q) {
+                        mid_entry = list_entry(tmp, struct mid_q_entry, qhead);
+
+                        if ((mid_entry->mid == server->CurrentMid) &&
+                            (mid_entry->midState == MID_REQUEST_SUBMITTED)) {
+                                /* This mid is in use, try a different one */
+                                collision = 1;
+                                break;
+                        }
+                }
+                if(collision == 0) {
+                        mid = server->CurrentMid;
+                        break;
+                }
+                server->CurrentMid++;
+        }
+        spin_unlock(&GlobalMid_Lock);
+        return mid;
+}
+
+/* NB: MID can not be set if treeCon not passed in, in that
+   case it is responsbility of caller to set the mid */
 void
 header_assemble(struct smb_hdr *buffer, char smb_command /* command */ ,
                 const struct cifsTconInfo *treeCon, int word_count
@@ -233,7 +303,8 @@ header_assemble(struct smb_hdr *buffer, char smb_command /* command */ ,
             (2 * word_count) + sizeof (struct smb_hdr) -
             4 /*  RFC 1001 length field does not count */  +
             2 /* for bcc field itself */ ;
-        /* Note that this is the only network field that has to be converted to big endian and it is done just before we send it */
+        /* Note that this is the only network field that has to be converted
+           to big endian and it is done just before we send it */
 
         buffer->Protocol[0] = 0xFF;
         buffer->Protocol[1] = 'S';
@@ -245,8 +316,6 @@ header_assemble(struct smb_hdr *buffer, char smb_command /* command */ ,
         buffer->Pid = cpu_to_le16((__u16)current->tgid);
         buffer->PidHigh = cpu_to_le16((__u16)(current->tgid >> 16));
         spin_lock(&GlobalMid_Lock);
-        GlobalMid++;
-        buffer->Mid = GlobalMid;
         spin_unlock(&GlobalMid_Lock);
         if (treeCon) {
                 buffer->Tid = treeCon->tid;
@@ -256,8 +325,9 @@ header_assemble(struct smb_hdr *buffer, char smb_command /* command */ ,
                         if (treeCon->ses->capabilities & CAP_STATUS32) {
                                 buffer->Flags2 |= SMBFLG2_ERR_STATUS;
                         }
-
-                        buffer->Uid = treeCon->ses->Suid;       /* always in LE format */
+                        /* Uid is not converted */
+                        buffer->Uid = treeCon->ses->Suid;
+                        buffer->Mid = GetNextMid(treeCon->ses->server);
                         if(multiuser_mount != 0) {
                 /* For the multiuser case, there are few obvious technically  */
                 /* possible mechanisms to match the local linux user (uid)    */