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

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Message-ID: <202010071114.EE9A2A47C@keescook>
Date:   Wed, 7 Oct 2020 11:15:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     johannes@...solutions.net, gregkh@...uxfoundation.org,
        rafael@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count
 to counter_atomic32

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?

-- 
Kees Cook