fuse: callback for invalidating cached children dentries for a directory

This patch solves a long standing bug.
"([bug torvalds#15](libfuse/libfuse#15)): if the
`default_permissions` mount option is not used, the results of the
first permission check performed by the file system for a directory
entry will be re-used for subsequent accesses as long as the inode of
the accessed entry is present in the kernel cache - even if the
permissions have since changed, and even if the subsequent access is
made by a different user.
This bug needs to be fixed in the Linux kernel and has been known
since 2006 but unfortunately no fix has been applied yet. If you
depend on correct permission handling for FUSE file systems, the only
workaround is to use `default_permissions` (which does not currently
support ACLs), or to completely disable caching of directory entry
attributes. Alternatively, the severity of the bug can be somewhat
reduced by not using the `allow_other` mount option."

This patch introduce a callback which the user space implementation can use
to invalidate the cached entries of a parent directory, for example when
the execute permissions are revoked and force real lookup.

Signed-off-by: Ashish Sangwan <ashishsangwan2@gmail.com>

fs/fuse/dev.c

@@ -1490,6 +1490,31 @@ static int fuse_notify_inval_inode(struct fuse_conn *fc, unsigned int size,
 	return err;
 }
 
+static int
+fuse_notify_inval_dircache_entries(struct fuse_conn *fc, unsigned int size,
+				   struct fuse_copy_state *cs)
+{
+	struct fuse_notify_inval_dircache_entries_out outarg;
+	int err = -EINVAL;
+
+	if (size < sizeof(outarg))
+		goto err;
+	err = fuse_copy_one(cs, &outarg, sizeof(outarg));
+	if (err)
+		goto err;
+	fuse_copy_finish(cs);
+	down_read(&fc->killsb);
+	err = -ENOENT;
+	if (fc->sb)
+		err = fuse_reverse_inval_dircache_entries(fc->sb,
+							  outarg.parent);
+	up_read(&fc->killsb);
+	return err;
+err:
+	fuse_copy_finish(cs);
+	return err;
+}
+
 static int fuse_notify_inval_entry(struct fuse_conn *fc, unsigned int size,
 				   struct fuse_copy_state *cs)
 {
@@ -1815,6 +1840,9 @@ static int fuse_notify(struct fuse_conn *fc, enum fuse_notify_code code,
 	case FUSE_NOTIFY_DELETE:
 		return fuse_notify_delete(fc, size, cs);
 
+	case FUSE_NOTIFY_INVAL_DIRCACHE_ENTRIES:
+		return fuse_notify_inval_dircache_entries(fc, size, cs);
+
 	default:
 		fuse_copy_finish(cs);
 		return -EINVAL;

fs/fuse/dir.c

@@ -932,6 +932,38 @@ int fuse_update_attributes(struct inode *inode, struct kstat *stat,
 	return err;
 }
 
+int fuse_reverse_inval_dircache_entries(struct super_block *sb,
+					u64 parent_nodeid)
+{
+	int err = -ENOTDIR;
+	struct inode *parent;
+	struct dentry *dir;
+	struct dentry *child;
+
+	parent = ilookup5(sb, parent_nodeid, fuse_inode_eq, &parent_nodeid);
+	if (!parent)
+		return -ENOENT;
+
+	inode_lock(parent);
+	if (!S_ISDIR(parent->i_mode))
+		goto unlock;
+
+	err = -ENOENT;
+	dir = d_find_alias(parent);
+	if (!dir)
+		goto unlock;
+	err = 0;
+	spin_lock(&dir->d_lock);
+	list_for_each_entry(child, &dir->d_subdirs, d_child)
+		fuse_invalidate_entry_cache(child);
+	spin_unlock(&dir->d_lock);
+	dput(dir);
+ unlock:
+	inode_unlock(parent);
+	iput(parent);
+	return err;
+}
+
 int fuse_reverse_inval_entry(struct super_block *sb, u64 parent_nodeid,
 			     u64 child_nodeid, struct qstr *name)
 {

fs/fuse/fuse_i.h

@@ -924,6 +924,9 @@ int fuse_reverse_inval_inode(struct super_block *sb, u64 nodeid,
 int fuse_reverse_inval_entry(struct super_block *sb, u64 parent_nodeid,
 			     u64 child_nodeid, struct qstr *name);
 
+int fuse_reverse_inval_dircache_entries(struct super_block *sb,
+					u64 parent_nodeid);
+
 int fuse_do_open(struct fuse_conn *fc, u64 nodeid, struct file *file,
 		 bool isdir);
 

include/uapi/linux/fuse.h

@@ -374,6 +374,7 @@ enum fuse_notify_code {
 	FUSE_NOTIFY_STORE = 4,
 	FUSE_NOTIFY_RETRIEVE = 5,
 	FUSE_NOTIFY_DELETE = 6,
+	FUSE_NOTIFY_INVAL_DIRCACHE_ENTRIES = 7,
 	FUSE_NOTIFY_CODE_MAX,
 };
 
@@ -715,6 +716,10 @@ struct fuse_direntplus {
 #define FUSE_DIRENTPLUS_SIZE(d) \
 	FUSE_DIRENT_ALIGN(FUSE_NAME_OFFSET_DIRENTPLUS + (d)->dirent.namelen)
 
+struct fuse_notify_inval_dircache_entries_out {
+	uint64_t	parent;
+};
+
 struct fuse_notify_inval_inode_out {
 	uint64_t	ino;
 	int64_t		off;