[CIFS] work around bug in Samba server handling for posix open

Samba server (version 3.3.1 and earlier, and 3.2.8 and earlier) incorrectly
required the O_CREAT flag on posix open (even when a file was not being
created).  This disables posix open (create is still ok) after the first
attempt returns EINVAL (and logs an error, once, recommending that they
update their server).
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/CHANGES

@@ -11,6 +11,8 @@ to better ensure that we wait for server to write all of the data to
 server disk (not just write it over the network).  Add new mount
 parameter to allow user to disable sending the (slow) SMB flush on
 fsync if desired (fsync still flushes all cached write data to the server).
+Posix file open support added (turned off after one attempt if server
+fails to support it properly, as with Samba server versions prior to 3.3.2)
 
 Version 1.56
 ------------

fs/cifs/cifsglob.h

@@ -299,6 +299,7 @@ struct cifsTconInfo {
 	bool unix_ext:1;  /* if false disable Linux extensions to CIFS protocol
 				for this mount even if server would support */
 	bool local_lease:1; /* check leases (only) on local system not remote */
+	bool broken_posix_open; /* e.g. Samba server versions < 3.3.2, 3.2.9 */
 	bool need_reconnect:1; /* connection reset, tid now invalid */
 	/* BB add field for back pointer to sb struct(s)? */
 };

fs/cifs/file.c

@@ -328,7 +328,8 @@ int cifs_open(struct inode *inode, struct file *file)
 	else
 		oplock = 0;
 
-	if (tcon->unix_ext && (tcon->ses->capabilities & CAP_UNIX) &&
+	if (!tcon->broken_posix_open && tcon->unix_ext &&
+	    (tcon->ses->capabilities & CAP_UNIX) &&
 	    (CIFS_UNIX_POSIX_PATH_OPS_CAP &
 			le64_to_cpu(tcon->fsUnixInfo.Capability))) {
 		int oflags = (int) cifs_posix_convert_flags(file->f_flags);
@@ -344,11 +345,20 @@ int cifs_open(struct inode *inode, struct file *file)
 			cifs_posix_open_inode_helper(inode, file, pCifsInode,
 						     pCifsFile, oplock, netfid);
 			goto out;
+		} else if ((rc == -EINVAL) || (rc == -EOPNOTSUPP)) {
+			if (tcon->ses->serverNOS)
+				cERROR(1, ("server %s of type %s returned"
+					   " unexpected error on SMB posix open"
+					   ", disabling posix open support."
+					   " Check if server update available.",
+					   tcon->ses->serverName,
+					   tcon->ses->serverNOS));
+			tcon->broken_posix_open = true;
 		} else if ((rc != -EIO) && (rc != -EREMOTE) &&
 			 (rc != -EOPNOTSUPP)) /* path not found or net err */
 			goto out;
-		/* fallthrough to retry open the old way on operation
-		   not supported or DFS errors */
+		/* else fallthrough to retry open the old way on network i/o
+		   or DFS errors */
 	}
 
 	desiredAccess = cifs_convert_flags(file->f_flags);