[PATCH v3 bpf-next 2/4] bpf: Introduce bpf_cgroup_read_xattr to read xattr of cgroup's node
From: Song Liu <song@kernel.org>
Date: 2025-06-23 06:39:13
Also in:
bpf, linux-fsdevel, lkml
Subsystem:
bpf [general] (safe dynamic programs and tools), filesystems (vfs and infrastructure), the rest · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Alexander Viro, Christian Brauner, Linus Torvalds
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> Acked-by: Tejun Heo <tj@kernel.org> --- fs/bpf_fs_kfuncs.c | 34 ++++++++++++++++++++++++++++++++++ kernel/bpf/helpers.c | 3 +++ 2 files changed, 37 insertions(+)
diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index 08412532db1b..1e36a12b88f7 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c@@ -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)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b71e428ad936..9ff1b4090289 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c@@ -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 = {
--
2.47.1