linux-kernel - Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count to counter

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Date:   Wed, 7 Oct 2020 13:33:43 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.org>, gregkh@...uxfoundation.org
Cc:     johannes@...solutions.net, rafael@...nel.org,
        linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count to
 counter_atomic32

On 10/7/20 12:15 PM, Kees Cook wrote:
> On Tue, Oct 06, 2020 at 02:44:35PM -0600, Shuah Khan wrote:
>> counter_atomic* is introduced to be used when a variable is used as
>> a simple counter and doesn't guard object lifetimes. This clearly
>> differentiates atomic_t usages that guard object lifetimes.
>>
>> counter_atomic* variables will wrap around to 0 when it overflows and
>> should not be used to guard resource lifetimes, device usage and
>> open counts that control state changes, and pm states.
>>
>> devcd_count is used to track dev_coredumpm device count and used in
>> device name string. It doesn't guard object lifetimes, device usage
>> counts, device open counts, and pm states. There is very little chance
>> of this counter overflowing. Convert it to use counter_atomic32.
>>
>> This conversion doesn't change the overflow wrap around behavior.
>>
>> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
> 
> I actually wonder if this should use refcount_t just because it is
> designed to be an alway-unique value. It is hard to imagine ever causing
> this to overflow, but why not let it be protected?
> 

This is one of the cases where devcd_count doesn't guard lifetimes,
however if it ever overflows, refcount_t is a better choice.

If we decide refcount_t is a better choice, I can drop this patch
and send refcount_t conversion patch instead.

Greg! Any thoughts on refcount_t for this being a better choice?


thanks,
-- Shuah