fat: fix oops on corrupted vfat fs

a) don't bother with ->d_time for positives - we only check it for
   negatives anyway.

b) make sure to set it at unlink and rmdir time - at *that* point
   soon-to-be negative dentry matches then-current directory contents

c) don't go into renaming of old alias in vfat_lookup() unless it
   has the same parent (which it will, unless we are seeing corrupted
   image)

[hirofumi@mail.parknet.co.jp: make change minimum, don't call d_move() for dir]
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: <stable@vger.kernel.org>	[3.17.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fat/namei_vfat.c

@@ -736,7 +736,12 @@ static struct dentry *vfat_lookup(struct inode *dir, struct dentry *dentry,
 	}
 
 	alias = d_find_alias(inode);
-	if (alias && !vfat_d_anon_disconn(alias)) {
+	/*
+	 * Checking "alias->d_parent == dentry->d_parent" to make sure
+	 * FS is not corrupted (especially double linked dir).
+	 */
+	if (alias && alias->d_parent == dentry->d_parent &&
+	    !vfat_d_anon_disconn(alias)) {
 		/*
 		 * This inode has non anonymous-DCACHE_DISCONNECTED
 		 * dentry. This means, the user did ->lookup() by an
@@ -755,12 +760,9 @@ static struct dentry *vfat_lookup(struct inode *dir, struct dentry *dentry,
 
 out:
 	mutex_unlock(&MSDOS_SB(sb)->s_lock);
-	dentry->d_time = dentry->d_parent->d_inode->i_version;
-	dentry = d_splice_alias(inode, dentry);
-	if (dentry)
-		dentry->d_time = dentry->d_parent->d_inode->i_version;
-	return dentry;
-
+	if (!inode)
+		dentry->d_time = dir->i_version;
+	return d_splice_alias(inode, dentry);
 error:
 	mutex_unlock(&MSDOS_SB(sb)->s_lock);
 	return ERR_PTR(err);
@@ -793,7 +795,6 @@ static int vfat_create(struct inode *dir, struct dentry *dentry, umode_t mode,
 	inode->i_mtime = inode->i_atime = inode->i_ctime = ts;
 	/* timestamp is already written, so mark_inode_dirty() is unneeded. */
 
-	dentry->d_time = dentry->d_parent->d_inode->i_version;
 	d_instantiate(dentry, inode);
 out:
 	mutex_unlock(&MSDOS_SB(sb)->s_lock);
@@ -824,6 +825,7 @@ static int vfat_rmdir(struct inode *dir, struct dentry *dentry)
 	clear_nlink(inode);
 	inode->i_mtime = inode->i_atime = CURRENT_TIME_SEC;
 	fat_detach(inode);
+	dentry->d_time = dir->i_version;
 out:
 	mutex_unlock(&MSDOS_SB(sb)->s_lock);
 
@@ -849,6 +851,7 @@ static int vfat_unlink(struct inode *dir, struct dentry *dentry)
 	clear_nlink(inode);
 	inode->i_mtime = inode->i_atime = CURRENT_TIME_SEC;
 	fat_detach(inode);
+	dentry->d_time = dir->i_version;
 out:
 	mutex_unlock(&MSDOS_SB(sb)->s_lock);
 
@@ -889,7 +892,6 @@ static int vfat_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
 	inode->i_mtime = inode->i_atime = inode->i_ctime = ts;
 	/* timestamp is already written, so mark_inode_dirty() is unneeded. */
 
-	dentry->d_time = dentry->d_parent->d_inode->i_version;
 	d_instantiate(dentry, inode);
 
 	mutex_unlock(&MSDOS_SB(sb)->s_lock);