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Message-ID: <b60f19c9-1e40-d621-dae5-f42bf99d6e90@redhat.com>
Date:   Mon, 12 Dec 2022 17:16:38 -0500
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Subject: Re: [PATCH-block v2 1/3] bdi, blk-cgroup: Fix potential UAF of blkcg
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To:     Tejun Heo <tj@kernel.org>
Cc:     Jens Axboe <axboe@kernel.dk>, Josef Bacik <josef@toxicpanda.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Yi Zhang <yi.zhang@redhat.com>
References: <20221211222058.2946830-1-longman@redhat.com>
 <20221211222058.2946830-2-longman@redhat.com>
 <Y5enmzQM7BIiEv9n@slm.duckdns.org>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <Y5enmzQM7BIiEv9n@slm.duckdns.org>
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On 12/12/22 17:13, Tejun Heo wrote:
> On Sun, Dec 11, 2022 at 05:20:56PM -0500, Waiman Long wrote:
>>   static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>>   {
>> +	/*
>> +	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
>> +	 * references gone.
>> +	 */
>> +	if (WARN_ON_ONCE(!css_tryget(&blkcg->css)))
>> +		return;
> Wouldn't it make more sense to use percpu_ref_is_zero()? It's not like the
> obtained extra reference does anything, right?

Yes, that makes sense. Will incorporate the change in the next version.

Thanks,
Longman