netdev - Re: [RFC PATCH] lib: Introduce generic __cmpxchg

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Date:   Mon, 5 Nov 2018 15:24:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ralf@...ux-mips.org" <ralf@...ux-mips.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "bfields@...ldses.org" <bfields@...ldses.org>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        "linux@...ck-us.net" <linux@...ck-us.net>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "paul.burton@...s.com" <paul.burton@...s.com>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "jhogan@...nel.org" <jhogan@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "arnd@...db.de" <arnd@...db.de>,
        "paulus@...ba.org" <paulus@...ba.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>, dvyukov@...gle.com,
        willy@...radead.org
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed

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@...ez:~/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@...ez:~/tmp$ gcc -fsanitize=signed-integer-overflow,shift -fwrapv overflow.c ; ./a.out
-4