[PATCH] skip data conversion in compat_sys_mount when data_page is NULL

OpenVZ Linux kernel team has found a problem with mounting in compat mode.

Simple command "mount -t smbfs ..." on Fedora Core 5 distro in 32-bit mode
leads to oops:

  Unable to handle kernel NULL pointer dereference at 0000000000000000 RIP: compat_sys_mount+0xd6/0x290
  Process mount (pid: 14656, veid=300, threadinfo ffff810034d30000, task ffff810034c86bc0)
  Call Trace: ia32_sysret+0x0/0xa

The problem is that data_page pointer can be NULL, so we should skip data
conversion in this case.

Signed-off-by: Andrey Mirkin <amirkin@openvz.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/compat.c

@@ -871,7 +871,7 @@ asmlinkage long compat_sys_mount(char __user * dev_name, char __user * dir_name,
 
 	retval = -EINVAL;
 
-	if (type_page) {
+	if (type_page && data_page) {
 		if (!strcmp((char *)type_page, SMBFS_NAME)) {
 			do_smb_super_data_conv((void *)data_page);
 		} else if (!strcmp((char *)type_page, NCPFS_NAME)) {