kernfs: Add option to enable user xattrs

User extended attributes are useful as metadata storage for kernfs
consumers like cgroups. Especially in the case of cgroups, it is useful
to have a central metadata store that multiple processes/services can
use to coordinate actions.

A concrete example is for userspace out of memory killers. We want to
let delegated cgroup subtree owners (running as non-root) to be able to
say "please avoid killing this cgroup". This is especially important for
desktop linux as delegated subtrees owners are less likely to run as
root.

This patch introduces a new flag, KERNFS_ROOT_SUPPORT_USER_XATTR, that
lets kernfs consumers enable user xattr support. An initial limit of 128
entries or 128KB -- whichever is hit first -- is placed per cgroup
because xattrs come from kernel memory and we don't want to let
unprivileged users accidentally eat up too much kernel memory.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Acked-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index dded2e5a9f42..89f6a4214a70 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -37,8 +37,10 @@ enum kernfs_node_type {
 	KERNFS_LINK		= 0x0004,
 };
 
-#define KERNFS_TYPE_MASK	0x000f
-#define KERNFS_FLAG_MASK	~KERNFS_TYPE_MASK
+#define KERNFS_TYPE_MASK		0x000f
+#define KERNFS_FLAG_MASK		~KERNFS_TYPE_MASK
+#define KERNFS_MAX_USER_XATTRS		128
+#define KERNFS_USER_XATTR_SIZE_LIMIT	(128 << 10)
 
 enum kernfs_node_flag {
 	KERNFS_ACTIVATED	= 0x0010,
@@ -78,6 +80,11 @@ enum kernfs_root_flag {
 	 * fhandle to access nodes of the fs.
 	 */
 	KERNFS_ROOT_SUPPORT_EXPORTOP		= 0x0004,
+
+	/*
+	 * Support user xattrs to be written to nodes rooted at this root.
+	 */
+	KERNFS_ROOT_SUPPORT_USER_XATTR		= 0x0008,
 };
 
 /* type-specific structures for kernfs_node union members */