linux-kernel - Re: [PATCH 3/4] blk-cgroup: Add NULL check of pd_alloc_fn in blkcg_activate

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Date:   Mon, 10 Oct 2022 10:29:17 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Kemeng Shi <shikemeng@...wei.com>
Cc:     axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] blk-cgroup: Add NULL check of pd_alloc_fn in
 blkcg_activate_policy

On Mon, Oct 10, 2022 at 10:38:58AM +0800, Kemeng Shi wrote:
> Function blkcg_policy_register only make sure pd_alloc_fn and pd_free_fn in
> pairs, so pd_alloc_fn could be NULL in registered blkcg_policy. Check NULL
> before use for pd_alloc_fn in blkcg_activate_policy to avoid protential
> NULL dereference.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...wei.com>
> ---
>  block/blk-cgroup.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 463c568d3e86..fc083c35dc42 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -1404,6 +1404,9 @@ int blkcg_activate_policy(struct request_queue *q,
>  	if (blkcg_policy_enabled(q, pol))
>  		return 0;
>  
> +	if (pol->pd_alloc_fn == NULL)
> +		return -EINVAL;

This isn't the only place this function is called, so the above won't
achieve much. Given that this is rather trivially noticeable and all the
current users do implement pd_alloc_fn, I'm not sure we need to update this
now.

Thanks.

-- 
tejun