From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed
Date: Mon, 5 Nov 2018 15:24:53 +0100
Message-ID: <20181105142453.GA22449@hirez.programming.kicks-ass.net>
References: <1541015538-11382-1-git-send-email-linux@roeck-us.net>
 <20181031213240.zhh7dfcm47ucuyfl@pburton-laptop>
 <20181031220253.GA15505@roeck-us.net>
 <20181031233235.qbedw3pinxcuk7me@pburton-laptop>
 <4e2438a23d2edf03368950a72ec058d1d299c32e.camel@hammerspace.com>
 <20181101131846.biyilr2msonljmij@lakrids.cambridge.arm.com>
 <20181101145926.GE3178@hirez.programming.kicks-ass.net>
 <f38e272f7a96e983549e4281aa9fd02833a4277a.camel@hammerspace.com>
 <20181101163212.GF3159@hirez.programming.kicks-ass.net>
 <5a846924-e642-d9d1-4e0e-810bd4d01c26@virtuozzo.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: 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>,
        "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>,
        "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@kerne
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
Content-Disposition: inline
In-Reply-To: <5a846924-e642-d9d1-4e0e-810bd4d01c26@virtuozzo.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

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