[PATCH bpf-next 2/4] bpf: Introduce bpf_kernfs_read_xattr to read xattr of kernfs nodes

[Christian Brauner]

On Wed, Jun 18, 2025 at 04:37:37PM -0700, Song Liu wrote:
> 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 files and folders.
> 
> Introduce kfunc bpf_kernfs_read_xattr, which allows reading kernfs
> xattr under RCU read lock.
> 
> 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.
> 
> Signed-off-by: Song Liu <song at kernel.org>
> ---
>  fs/bpf_fs_kfuncs.c | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
> index 08412532db1b..7576dbc9b340 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 +323,37 @@ __bpf_kfunc int bpf_remove_dentry_xattr(struct dentry *dentry, const char *name_
>  	return ret;
>  }
>  
> +/**
> + * bpf_kernfs_read_xattr - get xattr of a kernfs node
> + * @kn: kernfs_node to get xattr from
> + * @name__str: name of the xattr
> + * @value_p: output buffer of the xattr value
> + *
> + * Get xattr *name__str* of *kn* 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_kernfs_read_xattr(struct kernfs_node *kn, const char *name__str,
> +				      struct bpf_dynptr *value_p)

Please pass struct cgroup, then go from struct cgroup to kernfs node.