linux-kernel - Re: [RFC PATCH v3 1/3] bpf: cgroup: Introduce helper cgroup_bpf_current

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Date:   Thu, 14 Dec 2023 09:17:08 +0100
From:   Michael Weiß <michael.weiss@...ec.fraunhofer.de>
To:     Yonghong Song <yonghong.song@...ux.dev>,
        Christian Brauner <brauner@...nel.org>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Paul Moore <paul@...l-moore.com>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Amir Goldstein <amir73il@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, <bpf@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <gyroidos@...ec.fraunhofer.de>,
        Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Subject: Re: [RFC PATCH v3 1/3] bpf: cgroup: Introduce helper
 cgroup_bpf_current_enabled()

On 13.12.23 17:59, Yonghong Song wrote:
> 
> On 12/13/23 6:38 AM, Michael Weiß wrote:
>> This helper can be used to check if a cgroup-bpf specific program is
>> active for the current task.
>>
>> Signed-off-by: Michael Weiß <michael.weiss@...ec.fraunhofer.de>
>> Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
>> ---
>>   include/linux/bpf-cgroup.h |  2 ++
>>   kernel/bpf/cgroup.c        | 14 ++++++++++++++
>>   2 files changed, 16 insertions(+)
>>
>> diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
>> index a789266feac3..7cb49bde09ff 100644
>> --- a/include/linux/bpf-cgroup.h
>> +++ b/include/linux/bpf-cgroup.h
>> @@ -191,6 +191,8 @@ static inline bool cgroup_bpf_sock_enabled(struct sock *sk,
>>   	return array != &bpf_empty_prog_array.hdr;
>>   }
>>   
>> +bool cgroup_bpf_current_enabled(enum cgroup_bpf_attach_type type);
>> +
>>   /* Wrappers for __cgroup_bpf_run_filter_skb() guarded by cgroup_bpf_enabled. */
>>   #define BPF_CGROUP_RUN_PROG_INET_INGRESS(sk, skb)			      \
>>   ({									      \
>> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
>> index 491d20038cbe..9007165abe8c 100644
>> --- a/kernel/bpf/cgroup.c
>> +++ b/kernel/bpf/cgroup.c
>> @@ -24,6 +24,20 @@
>>   DEFINE_STATIC_KEY_ARRAY_FALSE(cgroup_bpf_enabled_key, MAX_CGROUP_BPF_ATTACH_TYPE);
>>   EXPORT_SYMBOL(cgroup_bpf_enabled_key);
>>   
>> +bool cgroup_bpf_current_enabled(enum cgroup_bpf_attach_type type)
>> +{
>> +	struct cgroup *cgrp;
>> +	struct bpf_prog_array *array;
>> +
>> +	rcu_read_lock();
>> +	cgrp = task_dfl_cgroup(current);
>> +	rcu_read_unlock();
>> +
>> +	array = rcu_access_pointer(cgrp->bpf.effective[type]);
> 
> This seems wrong here. The cgrp could become invalid once leaving
> rcu critical section.

You are right, maybe we where to opportunistic here. We just wanted
to hold the lock as short as possible.

> 
>> +	return array != &bpf_empty_prog_array.hdr;
> 
> I guess you need include 'array' usage as well in the rcu cs.
> So overall should look like:
> 
> 	rcu_read_lock();
> 	cgrp = task_dfl_cgroup(current);
> 	array = rcu_access_pointer(cgrp->bpf.effective[type]);

Looks reasonable, but that we are in the cs now I would change this to
rcu_dereference() then.

> 	bpf_prog_exists = array != &bpf_empty_prog_array.hdr;
> 	rcu_read_unlock();
> 
> 	return bpf_prog_exists;
> 
>> +}
>> +EXPORT_SYMBOL(cgroup_bpf_current_enabled);
>> +
>>   /* __always_inline is necessary to prevent indirect call through run_prog
>>    * function pointer.
>>    */