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Re: [PATCH] x86/refcount: Implement fast refcount_t handling

From Kees Cook <keescook@chromium.org>
Newsgroups linux.kernel
Subject Re: [PATCH] x86/refcount: Implement fast refcount_t handling
Date 2017-04-24 22:20 +0200
Message-ID <tzSCK-37e-5@gated-at.bofh.it> (permalink)
References <tyOUy-2RV-13@gated-at.bofh.it> <tzHHk-4GC-11@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On Mon, Apr 24, 2017 at 1:32 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote:
>> This patch ports the x86-specific atomic overflow handling from PaX's
>> PAX_REFCOUNT to the upstream refcount_t API. This is an updated version
>> from PaX that eliminates the saturation race condition by resetting the
>> atomic counter back to the INT_MAX saturation value on both overflow and
>> underflow. To win a race, a system would have to have INT_MAX threads
>> simultaneously overflow before the saturation handler runs.
>
> And is this impossible? Highly unlikely I'll grant you, but absolutely
> impossible?

I'll adjust the language. "Highly unlikely" is still better than
"trivially doable with a single thread". :)

> Also, you forgot nr_cpus in your bound. Afaict the worst case here is
> O(nr_tasks + 3*nr_cpus).
>
> Because PaX does it, is not a correctness argument. And this really
> wants one.

Sure, I didn't mean to imply anything other than a demonstration of
what PaX is doing (and that it's better than not having it). If we can
improve it, that's great.

-Kees

-- 
Kees Cook
Pixel Security

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Thread

[PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-22 00:10 +0200
  Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 10:40 +0200
    Re: [kernel-hardening] Re: [PATCH] x86/refcount: Implement fast  refcount_t handling Jann Horn <jannh@google.com> - 2017-04-24 11:00 +0200
      Re: [kernel-hardening] Re: [PATCH] x86/refcount: Implement fast  refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 11:30 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-24 13:10 +0200
      Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 13:20 +0200
        Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-24 15:20 +0200
          Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 15:40 +0200
            Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-24 17:20 +0200
              Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:50 +0200
                Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-25 00:10 +0200
                Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-25 00:40 +0200
                Re: [kernel-hardening] Re: [PATCH] x86/refcount: Implement fast  refcount_t handling Rik van Riel <riel@redhat.com> - 2017-04-25 03:20 +0200
                Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-25 11:10 +0200
                Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-25 13:30 +0200
                Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-25 18:40 +0200
      Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:40 +0200
        Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-25 13:30 +0200
          Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-25 18:50 +0200
            Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-26 04:20 +0200
              Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-26 06:50 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:20 +0200
  Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 12:50 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:20 +0200
  Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 13:00 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:30 +0200
  Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-25 12:30 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-25 13:30 +0200

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