linux-kernel - Re: [PATCH bpf-next 2/4] bpf: Introduce bpf_kernfs_read

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Message-ID: <20250619-danksagung-besoldung-72cd313928b8@brauner>
Date: Thu, 19 Jun 2025 10:49:56 +0200
From: Christian Brauner <brauner@...nel.org>
To: Song Liu <song@...nel.org>
Cc: bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, kernel-team@...a.com, 
	andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, daniel@...earbox.net, 
	martin.lau@...ux.dev, viro@...iv.linux.org.uk, jack@...e.cz, kpsingh@...nel.org, 
	mattbobrowski@...gle.com, amir73il@...il.com, gregkh@...uxfoundation.org, tj@...nel.org, 
	daan.j.demeyer@...il.com
Subject: Re: [PATCH bpf-next 2/4] bpf: Introduce bpf_kernfs_read_xattr to
 read xattr of kernfs nodes

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@...nel.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.