fix races between __d_instantiate() and checks of dentry flags

in non-lazy walk we need to be careful about dentry switching from
negative to positive - both ->d_flags and ->d_inode are updated,
and in some places we might see only one store.  The cases where
dentry has been obtained by dcache lookup with ->i_mutex held on
parent are safe - ->d_lock and ->i_mutex provide all the barriers
we need.  However, there are several places where we run into
trouble:
	* do_last() fetches ->d_inode, then checks ->d_flags and
assumes that inode won't be NULL unless d_is_negative() is true.
Race with e.g. creat() - we might have fetched the old value of
->d_inode (still NULL) and new value of ->d_flags (already not
DCACHE_MISS_TYPE).  Lin Ming has observed and reported the resulting
oops.
	* a bunch of places checks ->d_inode for being non-NULL,
then checks ->d_flags for "is it a symlink".  Race with symlink(2)
in case if our CPU sees ->d_inode update first - we see non-NULL
there, but ->d_flags still contains DCACHE_MISS_TYPE instead of
DCACHE_SYMLINK_TYPE.  Result: false negative on "should we follow
link here?", with subsequent unpleasantness.

Cc: stable@vger.kernel.org # 3.13 and 3.14 need that one
Reported-and-tested-by: Lin Ming <minggr@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -1647,8 +1647,7 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
 	unsigned add_flags = d_flags_for_inode(inode);
 
 	spin_lock(&dentry->d_lock);
-	dentry->d_flags &= ~DCACHE_ENTRY_TYPE;
-	dentry->d_flags |= add_flags;
+	__d_set_type(dentry, add_flags);
 	if (inode)
 		hlist_add_head(&dentry->d_alias, &inode->i_dentry);
 	dentry->d_inode = inode;

fs/namei.c

@@ -1542,7 +1542,7 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
 		inode = path->dentry->d_inode;
 	}
 	err = -ENOENT;
-	if (!inode)
+	if (!inode || d_is_negative(path->dentry))
 		goto out_path_put;
 
 	if (should_follow_link(path->dentry, follow)) {
@@ -2249,7 +2249,7 @@ mountpoint_last(struct nameidata *nd, struct path *path)
 	mutex_unlock(&dir->d_inode->i_mutex);
 
 done:
-	if (!dentry->d_inode) {
+	if (!dentry->d_inode || d_is_negative(dentry)) {
 		error = -ENOENT;
 		dput(dentry);
 		goto out;
@@ -2994,7 +2994,7 @@ static int do_last(struct nameidata *nd, struct path *path,
 finish_lookup:
 	/* we _can_ be in RCU mode here */
 	error = -ENOENT;
-	if (d_is_negative(path->dentry)) {
+	if (!inode || d_is_negative(path->dentry)) {
 		path_to_nameidata(path, nd);
 		goto out;
 	}