eCryptfs: fix lookup error for special files

When ecryptfs_lookup() is called against special files, eCryptfs generates
the following errors because it tries to treat them like regular eCryptfs
files.

Error opening lower file for lower_dentry [0xffff810233a6f150], lower_mnt [0xffff810235bb4c80], and flags [0x8000]
Error opening lower_file to read header region
Error attempting to read the [user.ecryptfs] xattr from the lower file; return value = [-95]
Valid metadata not found in header region or xattr region; treating file as unencrypted

For instance, the problem can be reproduced by the steps below.

  # mkdir /root/crypt /mnt/crypt
  # mount -t ecryptfs /root/crypt /mnt/crypt
  # mknod /mnt/crypt/c0 c 0 0
  # umount /mnt/crypt
  # mount -t ecryptfs /root/crypt /mnt/crypt
  # ls -l /mnt/crypt

This patch fixes it by adding a check similar to directories and
symlinks.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Acked-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ecryptfs/inode.c

@@ -357,6 +357,10 @@ static struct dentry *ecryptfs_lookup(struct inode *dir, struct dentry *dentry,
 		ecryptfs_printk(KERN_DEBUG, "Is a special file; returning\n");
 		goto out;
 	}
+	if (special_file(lower_inode->i_mode)) {
+		ecryptfs_printk(KERN_DEBUG, "Is a special file; returning\n");
+		goto out;
+	}
 	if (!nd) {
 		ecryptfs_printk(KERN_DEBUG, "We have a NULL nd, just leave"
 				"as we *think* we are about to unlink\n");