linux-kernel - Re: [PATCH 4/5] device_cgroup: make may

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Date:	Mon, 3 Dec 2012 09:44:14 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Aristeu Rozanski <aris@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Serge Hallyn <serge.hallyn@...onical.com>,
	cgroups@...r.kernel.org
Subject: Re: [PATCH 4/5] device_cgroup: make may_access() stronger

Hello,

On Tue, Nov 27, 2012 at 02:35:05PM -0500, Aristeu Rozanski wrote:
> @@ -375,22 +376,33 @@
>  			continue;
>  		if (refex->access & (~ex->access))
>  			continue;
> -		match = true;
> +		match = 1;
>  		break;
>  	}
>  
>  	/*
> -	 * In two cases we'll consider this new exception valid:
> -	 * - the dev cgroup has its default policy to allow + exception list:
> -	 *   the new exception should *not* match any of the exceptions
> -	 *   (behavior == DEVCG_DEFAULT_ALLOW, !match)
> -	 * - the dev cgroup has its default policy to deny + exception list:
> -	 *   the new exception *should* match the exceptions
> -	 *   (behavior == DEVCG_DEFAULT_DENY, match)
> +	 * The only three possibilities are:
> +	 * devcg->behavior == ALLOW, rule behavior == ALLOW
> +	 * devcg->behavior == ALLOW, rule behavior == DENY
> +	 * devcg->behavior == DENY, rule behavior == DENY
> +	 * the remaining
> +	 * devcg->behavior == DENY, rule behavior == ALLOW
> +	 * won't be possible by hierarchy
> +	 *
> +	 * Since we want to simplify the code, here're the possibilites to
> +	 * make easier to understand:
> +	 *
> +	 * devcg behavior   rule behavior  match  result
> +	 * allow            allow          1      0
> +	 * allow            allow          0      1
> +	 * allow            deny           1      0
> +	 * allow            deny           0      1
> +	 * deny             deny           1      1
> +	 * deny             deny           0      0
>  	 */
> -	if ((dev_cgroup->behavior == DEVCG_DEFAULT_DENY) == match)
> -		return 1;
> -	return 0;
> +	if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW)
> +		return !match;
> +	return match;

I kinda dislike this.  This isn't a performanc critical path where we
must try our best to shave off a few condition checks.  There's no
reason to encode the test like this.  Please just spell the conditions
out in code rather than trying to build a magic series of equality
tests which somehow ends up spewing out the correct results.

Thanks.

-- 
tejun
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