fuse: readdirplus: fix RCU walk

Doing dput(parent) is not valid in RCU walk mode.  In RCU mode it would
probably be okay to update the parent flags, but it's actually not
necessary most of the time...

So only set the FUSE_I_ADVISE_RDPLUS flag on the parent when the entry was
recently initialized by READDIRPLUS.

This is achieved by setting FUSE_I_INIT_RDPLUS on entries added by
READDIRPLUS and only dropping out of RCU mode if this flag is set.
FUSE_I_INIT_RDPLUS is cleared once the FUSE_I_ADVISE_RDPLUS flag is set in
the parent.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: stable@vger.kernel.org

fs/fuse/dir.c

@@ -182,6 +182,7 @@ static int fuse_dentry_revalidate(struct dentry *entry, unsigned int flags)
 	struct inode *inode;
 	struct dentry *parent;
 	struct fuse_conn *fc;
+	struct fuse_inode *fi;
 	int ret;
 
 	inode = ACCESS_ONCE(entry->d_inode);
@@ -228,7 +229,7 @@ static int fuse_dentry_revalidate(struct dentry *entry, unsigned int flags)
 		if (!err && !outarg.nodeid)
 			err = -ENOENT;
 		if (!err) {
-			struct fuse_inode *fi = get_fuse_inode(inode);
+			fi = get_fuse_inode(inode);
 			if (outarg.nodeid != get_node_id(inode)) {
 				fuse_queue_forget(fc, forget, outarg.nodeid, 1);
 				goto invalid;
@@ -246,8 +247,11 @@ static int fuse_dentry_revalidate(struct dentry *entry, unsigned int flags)
 				       attr_version);
 		fuse_change_entry_timeout(entry, &outarg);
 	} else if (inode) {
-		fc = get_fuse_conn(inode);
-		if (fc->readdirplus_auto) {
+		fi = get_fuse_inode(inode);
+		if (flags & LOOKUP_RCU) {
+			if (test_bit(FUSE_I_INIT_RDPLUS, &fi->state))
+				return -ECHILD;
+		} else if (test_and_clear_bit(FUSE_I_INIT_RDPLUS, &fi->state)) {
 			parent = dget_parent(entry);
 			fuse_advise_use_readdirplus(parent->d_inode);
 			dput(parent);
@@ -1292,6 +1296,8 @@ static int fuse_direntplus_link(struct file *file,
 	}
 
 found:
+	if (fc->readdirplus_auto)
+		set_bit(FUSE_I_INIT_RDPLUS, &get_fuse_inode(inode)->state);
 	fuse_change_entry_timeout(dentry, o);
 
 	err = 0;

fs/fuse/fuse_i.h

@@ -115,6 +115,8 @@ struct fuse_inode {
 enum {
 	/** Advise readdirplus  */
 	FUSE_I_ADVISE_RDPLUS,
+	/** Initialized with readdirplus */
+	FUSE_I_INIT_RDPLUS,
 	/** An operation changing file size is in progress  */
 	FUSE_I_SIZE_UNSTABLE,
 };