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Subject[PATCH v3 3/4] kernfs: Add option to enable user xattrs
Date
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>
---
fs/kernfs/inode.c | 89 +++++++++++++++++++++++++++++++++++++
fs/kernfs/kernfs-internal.h | 2 +
include/linux/kernfs.h | 11 ++++-
3 files changed, 100 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index 5f10ae95fbfa..fc2469a20fed 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -53,6 +53,8 @@ static struct kernfs_iattrs *__kernfs_iattrs(struct kernfs_node *kn, int alloc)
kn->iattr->ia_ctime = kn->iattr->ia_atime;

simple_xattrs_init(&kn->iattr->xattrs);
+ atomic_set(&kn->iattr->nr_user_xattrs, 0);
+ atomic_set(&kn->iattr->user_xattr_size, 0);
out_unlock:
ret = kn->iattr;
mutex_unlock(&iattr_mutex);
@@ -327,6 +329,86 @@ static int kernfs_vfs_xattr_set(const struct xattr_handler *handler,
return kernfs_xattr_set(kn, name, value, size, flags);
}

+static int kernfs_vfs_user_xattr_add(struct kernfs_node *kn,
+ const char *full_name,
+ struct simple_xattrs *xattrs,
+ const void *value, size_t size, int flags)
+{
+ atomic_t *sz = &kn->iattr->user_xattr_size;
+ atomic_t *nr = &kn->iattr->nr_user_xattrs;
+ ssize_t removed_size;
+ int ret;
+
+ if (atomic_inc_return(nr) > KERNFS_MAX_USER_XATTRS) {
+ ret = -ENOSPC;
+ goto dec_count_out;
+ }
+
+ if (atomic_add_return(size, sz) > KERNFS_USER_XATTR_SIZE_LIMIT) {
+ ret = -ENOSPC;
+ goto dec_size_out;
+ }
+
+ ret = simple_xattr_set(xattrs, full_name, value, size, flags,
+ &removed_size);
+
+ if (!ret && removed_size >= 0)
+ size = removed_size;
+ else if (!ret)
+ return 0;
+dec_size_out:
+ atomic_sub(size, sz);
+dec_count_out:
+ atomic_dec(nr);
+ return ret;
+}
+
+static int kernfs_vfs_user_xattr_rm(struct kernfs_node *kn,
+ const char *full_name,
+ struct simple_xattrs *xattrs,
+ const void *value, size_t size, int flags)
+{
+ atomic_t *sz = &kn->iattr->user_xattr_size;
+ atomic_t *nr = &kn->iattr->nr_user_xattrs;
+ ssize_t removed_size;
+ int ret;
+
+ ret = simple_xattr_set(xattrs, full_name, value, size, flags,
+ &removed_size);
+
+ if (removed_size >= 0) {
+ atomic_sub(removed_size, sz);
+ atomic_dec(nr);
+ }
+
+ return ret;
+}
+
+static int kernfs_vfs_user_xattr_set(const struct xattr_handler *handler,
+ struct dentry *unused, struct inode *inode,
+ const char *suffix, const void *value,
+ size_t size, int flags)
+{
+ const char *full_name = xattr_full_name(handler, suffix);
+ struct kernfs_node *kn = inode->i_private;
+ struct kernfs_iattrs *attrs;
+
+ if (!(kernfs_root(kn)->flags & KERNFS_ROOT_SUPPORT_USER_XATTR))
+ return -EOPNOTSUPP;
+
+ attrs = kernfs_iattrs(kn);
+ if (!attrs)
+ return -ENOMEM;
+
+ if (value)
+ return kernfs_vfs_user_xattr_add(kn, full_name, &attrs->xattrs,
+ value, size, flags);
+ else
+ return kernfs_vfs_user_xattr_rm(kn, full_name, &attrs->xattrs,
+ value, size, flags);
+
+}
+
static const struct xattr_handler kernfs_trusted_xattr_handler = {
.prefix = XATTR_TRUSTED_PREFIX,
.get = kernfs_vfs_xattr_get,
@@ -339,8 +421,15 @@ static const struct xattr_handler kernfs_security_xattr_handler = {
.set = kernfs_vfs_xattr_set,
};

+static const struct xattr_handler kernfs_user_xattr_handler = {
+ .prefix = XATTR_USER_PREFIX,
+ .get = kernfs_vfs_xattr_get,
+ .set = kernfs_vfs_user_xattr_set,
+};
+
const struct xattr_handler *kernfs_xattr_handlers[] = {
&kernfs_trusted_xattr_handler,
&kernfs_security_xattr_handler,
+ &kernfs_user_xattr_handler,
NULL
};
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 2f3c51d55261..7ee97ef59184 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -26,6 +26,8 @@ struct kernfs_iattrs {
struct timespec64 ia_ctime;

struct simple_xattrs xattrs;
+ atomic_t nr_user_xattrs;
+ atomic_t user_xattr_size;
};

/* +1 to avoid triggering overflow warning when negating it */
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 */
--
2.21.1
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