Bug 490887

OpenVZ linux kernel team has found that mounting CIFS subdirectory (allowed by POSIX extensions enabled by default) which is a symlink on the CIFS server leads to the kernel panic.

2.6.18-128.1.1

How reproducible:
* assume server HOSTNAME shares the directory A via CIFS
* assume A has a symlink S inside
* following commands will crash the client node:
  # mount -t cifs //HOSTNAME/A/S /mnt
  # cd /mnt

1) This crash can be avoided by disabling POSIX extensions before mounting:
  # modprobe cifs
  # echo 0 > /proc/fs/cifs/LinuxExtensionsEnabled

2) Another workaround:
   CIFS POSIX extensions can be disabled for single mount by undocumnted option "nounix":
  # mount -t cifs //HOSTNAME/A/S /mnt -o nounix

3) The long-term workaround might be in disabling by default POSIX extensions in kernel:
diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 4c1c4f3..0005d16 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -63,7 +63,7 @@ int cifsERROR = 1;
 int traceSMB = 0;
 unsigned int oplockEnabled = 1;
 unsigned int experimEnabled = 0;
-unsigned int linuxExtEnabled = 1;
+unsigned int linuxExtEnabled = 0;
 unsigned int lookupCacheEnabled = 1;
 unsigned int multiuser_mount = 0;
 unsigned int extended_security = CIFSSEC_DEF;

Hope this helps.