linux-kernel - Re: [PATCH v5 1/3] refcount: Create unchecked atomic

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Date:   Thu, 1 Jun 2017 07:43:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Reshetova, Elena" <elena.reshetova@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "arozansk@...hat.com" <arozansk@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 1/3] refcount: Create unchecked atomic_t implementation

On Wed, May 31, 2017 at 4:09 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, May 31, 2017 at 10:45:09AM +0000, Reshetova, Elena wrote:
>
>> > +static inline __must_check bool refcount_add_not_zero(unsigned int i,
>> > +
>> >           refcount_t *r)
>> > +{
>> > +   return atomic_add_return(i, &r->refs) != 0;
>> > +}
>>
>> Maybe atomic_add_unless(&r->refs, i, 0) in order to be consistent with the below inc_not_zero implementation?
>
> Yes, atomic_add_return() is strictly incorrect here since the add is
> unconditional.
>
>> > +static inline __must_check bool refcount_sub_and_test(unsigned int i,
>> > +
>> >           refcount_t *r)
>> > +{
>> > +   return atomic_sub_return(i, &r->refs) == 0;
>> > +}
>>
>> Any reason for not using atomic_sub_and_test() here?
>
>> > +static inline __must_check bool refcount_dec_and_test(refcount_t *r)
>> > +{
>> > +   return atomic_dec_return(&r->refs) == 0;
>> > +}
>>
>> Same here: atomic_dec_and_test()?
>
> Both those are better because they return condition codes generated from
> the operand itself.

Ah yes, thanks to both of you for the corrections. I'll send a new version...

-Kees

-- 
Kees Cook
Pixel Security