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

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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: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/20250623063854.1896364-3-song@kernel.org
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Song Liu 2025-06-22 23:38:52 -07:00 • committed by Christian Brauner
parent d1f4e90260
commit 535b070f4a
No known key found for this signature in database
GPG key ID: 91C61BC06578DCA2
2 changed files with 37 additions and 0 deletions
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34
fs/bpf_fs_kfuncs.c
View file

@ -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)

3
kernel/bpf/helpers.c
View file

@ -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 = {