[PATCH bpf-next 1/4] kernfs: Add __kernfs_xattr_get for RCU protected access

[Song Liu]

Existing kernfs_xattr_get() locks iattr_mutex, so it cannot be used in
RCU critical sections. Introduce __kernfs_xattr_get(), which reads xattr
under RCU read lock. This can be used by BPF programs to access cgroupfs
xattrs.

Signed-off-by: Song Liu <song at kernel.org>
---
 fs/kernfs/inode.c      | 14 ++++++++++++++
 include/linux/kernfs.h |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index b83054da68b3..0ca231d2012c 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -302,6 +302,20 @@ int kernfs_xattr_get(struct kernfs_node *kn, const char *name,
 	return simple_xattr_get(&attrs->xattrs, name, value, size);
 }
 
+int __kernfs_xattr_get(struct kernfs_node *kn, const char *name,
+		       void *value, size_t size)
+{
+	struct kernfs_iattrs *attrs;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	attrs = rcu_dereference(kn->iattr);
+	if (!attrs)
+		return -ENODATA;
+
+	return simple_xattr_get(&attrs->xattrs, name, value, size);
+}
+
 int kernfs_xattr_set(struct kernfs_node *kn, const char *name,
 		     const void *value, size_t size, int flags)
 {
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index b5a5f32fdfd1..8536ffc5c9f1 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -456,6 +456,8 @@ void kernfs_notify(struct kernfs_node *kn);
 
 int kernfs_xattr_get(struct kernfs_node *kn, const char *name,
 		     void *value, size_t size);
+int __kernfs_xattr_get(struct kernfs_node *kn, const char *name,
+		       void *value, size_t size);
 int kernfs_xattr_set(struct kernfs_node *kn, const char *name,
 		     const void *value, size_t size, int flags);
 
-- 
2.47.1