linux-kernel - Re: [RFC PATCH 01/11] counters: Introduce counter and counter

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Date:   Wed, 23 Sep 2020 12:35:17 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     corbet@....net, keescook@...omium.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 01/11] counters: Introduce counter and counter_atomic

On Tue, Sep 22, 2020 at 07:43:30PM -0600, Shuah Khan wrote:
> Introduce Simple atomic and non-atomic counters.
> 
> There are a number of atomic_t usages in the kernel where atomic_t api
> is used strictly for counting and not for managing object lifetime. In
> some cases, atomic_t might not even be needed.
> 
> The purpose of these counters is twofold: 1. clearly differentiate
> atomic_t counters from atomic_t usages that guard object lifetimes,
> hence prone to overflow and underflow errors. It allows tools that scan
> for underflow and overflow on atomic_t usages to detect overflow and
> underflows to scan just the cases that are prone to errors. 2. provides
> non-atomic counters for cases where atomic isn't necessary.
> 
> Simple atomic and non-atomic counters api provides interfaces for simple
> atomic and non-atomic counters that just count, and don't guard resource
> lifetimes. Counters 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.
> 
> Using counter_atomic to guard lifetimes could lead to use-after free
> when it overflows and undefined behavior when used to manage state
> changes and device usage/open states.
> 
> Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
> ---
>  Documentation/core-api/counters.rst | 158 +++++++++++++
>  MAINTAINERS                         |   7 +
>  include/linux/counters.h            | 343 ++++++++++++++++++++++++++++
>  lib/Kconfig                         |  10 +
>  lib/Makefile                        |   1 +
>  lib/test_counters.c                 | 283 +++++++++++++++++++++++

Tests for new apis, nice!

Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>