linux-kernel - Re: [PATCH v2 bpf-next 2/5] bpf: Introduce bpf_cgroup_read

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Message-ID: <aFYcl8KQU9upkZ0f@slm.duckdns.org>
Date: Fri, 20 Jun 2025 16:44:39 -1000
From: Tejun Heo <tj@...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, brauner@...nel.org, jack@...e.cz,
	kpsingh@...nel.org, mattbobrowski@...gle.com, amir73il@...il.com,
	gregkh@...uxfoundation.org, daan.j.demeyer@...il.com
Subject: Re: [PATCH v2 bpf-next 2/5] bpf: Introduce bpf_cgroup_read_xattr to
 read xattr of cgroup's node

On Thu, Jun 19, 2025 at 03:01:11PM -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 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.
> 
> Signed-off-by: Song Liu <song@...nel.org>
...
> +__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;

Just out of curiosity, what security holes are there if we allow BPF
programs to read other xattrs? Given how priviledged BPF programs already
are, does this make meaningful difference?

>From cgroup POV:

 Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun