acl: don't depend on IOP_XATTR

All codepaths that don't want to implement POSIX ACLs should simply not
implement the associated inode operations instead of relying on
IOP_XATTR. That's the case for all filesystems today.

For vfs_listxattr() all filesystems that explicitly turn of xattrs for a
given inode all set inode->i_op to a dedicated set of inode operations
that doesn't implement ->listxattr().  We can remove the dependency of
vfs_listxattr() on IOP_XATTR.

Removing this dependency will allow us to decouple POSIX ACLs from
IOP_XATTR and they can still be listed even if no other xattr handlers
are implemented. Otherwise we would have to implement elaborate schemes
to raise IOP_XATTR even if sb->s_xattr is set to NULL.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

1 files changed, 24 insertions, 1 deletions

diff --git a/fs/xattr.c b/fs/xattr.c
index 95bb6b30ab13..fcf67d80d7f9 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -458,6 +458,28 @@ nolsm:
 }
 EXPORT_SYMBOL_GPL(vfs_getxattr);
 
+/**
+ * vfs_listxattr - retrieve \0 separated list of xattr names
+ * @dentry: the dentry from whose inode the xattr names are retrieved
+ * @list: buffer to store xattr names into
+ * @size: size of the buffer
+ *
+ * This function returns the names of all xattrs associated with the
+ * inode of @dentry.
+ *
+ * Note, for legacy reasons the vfs_listxattr() function lists POSIX
+ * ACLs as well. Since POSIX ACLs are decoupled from IOP_XATTR the
+ * vfs_listxattr() function doesn't check for this flag since a
+ * filesystem could implement POSIX ACLs without implementing any other
+ * xattrs.
+ *
+ * However, since all codepaths that remove IOP_XATTR also assign of
+ * inode operations that either don't implement or implement a stub
+ * ->listxattr() operation.
+ *
+ * Return: On success, the size of the buffer that was used. On error a
+ *         negative error code.
+ */
 ssize_t
 vfs_listxattr(struct dentry *dentry, char *list, size_t size)
 {
@@ -467,7 +489,8 @@ vfs_listxattr(struct dentry *dentry, char *list, size_t size)
 	error = security_inode_listxattr(dentry);
 	if (error)
 		return error;
-	if (inode->i_op->listxattr && (inode->i_opflags & IOP_XATTR)) {
+
+	if (inode->i_op->listxattr) {
 		error = inode->i_op->listxattr(dentry, list, size);
 	} else {
 		error = security_inode_listsecurity(inode, list, size);