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Date:   Mon, 5 Nov 2018 15:24:53 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>,
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        "jlayton@kernel.org" <jlayton@kernel.org>,
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        "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>,
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        "jhogan@kernel.org" <jhogan@kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "arnd@arndb.de" <arnd@arndb.de>,
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        "mpe@ellerman.id.au" <mpe@ellerman.id.au>,
        "benh@kernel.crashing.org" <benh@kernel.crashing.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>, dvyukov@google.com,
        willy@infradead.org
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed
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On Fri, Nov 02, 2018 at 07:19:15PM +0300, Andrey Ryabinin wrote:
> UBSAN warns about signed overflows despite -fno-strict-overflow if gcc version is < 8.
> I have learned recently that UBSAN in GCC 8 ignores signed overflows if -fno-strict-overflow of fwrapv is used.
> 
> $ cat signed_overflow.c 
> #include <stdio.h>
> 
> __attribute__((noinline))
> int foo(int a, int b)
> {
>         return a+b;

 s/+/<</

> }
> 
> int main(void)
> {
>         int a = 0x7fffffff;
>         int b = 2;
>         printf("%d\n", foo(a,b));
>         return 0;
> }

It also seem to affect 'shift':

peterz@hirez:~/tmp$ gcc -fsanitize=signed-integer-overflow,shift overflow.c ; ./a.out
overflow.c:6:11: runtime error: left shift of 2147483647 by 2 places cannot be represented in type 'int'
-4
peterz@hirez:~/tmp$ gcc -fsanitize=signed-integer-overflow,shift -fwrapv overflow.c ; ./a.out
-4