Unverified Commit 535b070f authored by Song Liu's avatar Song Liu Committed by Christian Brauner
Browse files

bpf: Introduce bpf_cgroup_read_xattr to read xattr of cgroup's node 



BPF programs, such as LSM and sched_ext, would benefit from tags on
cgroups. One common practice to apply such tags is to set xattrs on
cgroupfs folders.

Introduce kfunc bpf_cgroup_read_xattr, which allows reading cgroup's
xattr.

Note that, we already have bpf_get_[file|dentry]_xattr. However, these
two APIs are not ideal for reading cgroupfs xattrs, because:

  1) These two APIs only works in sleepable contexts;
  2) There is no kfunc that matches current cgroup to cgroupfs dentry.

bpf_cgroup_read_xattr is generic and can be useful for many program
types. It is also safe, because it requires trusted or rcu protected
argument (KF_RCU). Therefore, we make it available to all program types.

Signed-off-by: default avatarSong Liu <song@kernel.org>
Link: https://lore.kernel.org/20250623063854.1896364-3-song@kernel.org


Acked-by: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent d1f4e902

fs/bpf_fs_kfuncs.c

+34 −0
Original line number Diff line number Diff line
@@ -9,6 +9,7 @@ 
#include <linux/fs.h>

#include <linux/fsnotify.h>

#include <linux/file.h>

#include <linux/kernfs.h>

#include <linux/mm.h>

#include <linux/xattr.h>
@@ -322,6 +323,39 @@ __bpf_kfunc int bpf_remove_dentry_xattr(struct dentry *dentry, const char *name_ 
	return ret;

}



#ifdef CONFIG_CGROUPS

/**

 * bpf_cgroup_read_xattr - read xattr of a cgroup's node in cgroupfs

 * @cgroup: cgroup to get xattr from

 * @name__str: name of the xattr

 * @value_p: output buffer of the xattr value

 *

 * Get xattr *name__str* of *cgroup* and store the output in *value_ptr*.

 *

 * For security reasons, only *name__str* with prefix "user." is allowed.

 *

 * Return: length of the xattr value on success, a negative value on error.

 */

__bpf_kfunc int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__str,

					struct bpf_dynptr *value_p)

{

	struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p;

	u32 value_len;

	void *value;



	/* Only allow reading "user.*" xattrs */

	if (strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN))

		return -EPERM;



	value_len = __bpf_dynptr_size(value_ptr);

	value = __bpf_dynptr_data_rw(value_ptr, value_len);

	if (!value)

		return -EINVAL;



	return kernfs_xattr_get(cgroup->kn, name__str, value, value_len);

}

#endif /* CONFIG_CGROUPS */



__bpf_kfunc_end_defs();



BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)

kernel/bpf/helpers.c

+3 −0
Original line number Diff line number Diff line
@@ -3397,6 +3397,9 @@ BTF_ID_FLAGS(func, bpf_iter_dmabuf_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPAB 
BTF_ID_FLAGS(func, bpf_iter_dmabuf_destroy, KF_ITER_DESTROY | KF_SLEEPABLE)

#endif

BTF_ID_FLAGS(func, __bpf_trap)

#ifdef CONFIG_CGROUPS

BTF_ID_FLAGS(func, bpf_cgroup_read_xattr, KF_RCU)

#endif

BTF_KFUNCS_END(common_btf_ids)



static const struct btf_kfunc_id_set common_kfunc_set = {