[RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed

[Peter Zijlstra]

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