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Date:   Thu, 1 Nov 2018 22:45:29 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "ralf@linux-mips.org" <ralf@linux-mips.org>,
        "jlayton@kernel.org" <jlayton@kernel.org>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        "bfields@fieldses.org" <bfields@fieldses.org>,
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        "linux@roeck-us.net" <linux@roeck-us.net>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "will.deacon@arm.com" <will.deacon@arm.com>,
        "boqun.feng@gmail.com" <boqun.feng@gmail.com>,
        "paul.burton@mips.com" <paul.burton@mips.com>,
        "anna.schumaker@netapp.com" <anna.schumaker@netapp.com>,
        "jhogan@kernel.org" <jhogan@kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
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        "paulus@samba.org" <paulus@samba.org>,
        "mpe@ellerman.id.au" <mpe@ellerman.id.au>,
        "benh@kernel.crashing.org" <benh@kernel.crashing.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed
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On Thu, Nov 01, 2018 at 06:46:50PM +0100, Dmitry Vyukov wrote:
> If there is a warning that we don't want to see at all, then we can
> disable it. It supposed to be a useful tool, rather than a thing in
> itself that lives own life. We already I think removed 1 particularly
> noisy warning and made another optional via a config.

> But the thing with overflows is that, even if it's defined, it's not
> necessary the intended behavior. For example, take allocation size
> calculation done via unsigned size_t. If it overflows it does not help
> if C defines result or not, it still gives a user controlled write
> primitive. We've seen similar cases with timeout/deadline calculation
> in kernel, we really don't want it to just wrap modulo-2, right. Some
> user-space projects even test with unsigned overflow warnings or
> implicit truncation warnings, which are formally legal, but frequently
> bugs.

Sure; but then don't call it UB.

If we want to have an additional integer over/underflow checker (ideally
with a gcc plugin that has explicit annotations like __wrap to make it
go away) that is fine; and it can be done on unsigned and signed.