linux-kernel - Re: [PATCH] perf: enum overflow in uapi/linux/perf

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Date:   Fri, 7 Sep 2018 16:13:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] perf: enum overflow in uapi/linux/perf_event.h

On Fri, Sep 07, 2018 at 03:58:17PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 07, 2018 at 01:50:18PM +0000, Christophe Leroy wrote:
> > 
> > 
> > On 09/07/2018 01:42 PM, Peter Zijlstra wrote:
> > > On Fri, Sep 07, 2018 at 01:27:19PM +0000, Christophe Leroy wrote:
> > > > On PPC32, enums are 32 bits, so __PERF_SAMPLE_CALLCHAIN_EARLY is
> > > > out of scope. The following sparse warning is encountered:
> > > > 
> > > >    CHECK   arch/powerpc/kernel/process.c
> > > > ./include/uapi/linux/perf_event.h:147:56: warning: cast truncates bits from constant value (8000000000000000 becomes 0)
> > > 
> > > Urgh... what compiler is that? I've not seen anything like that from the
> > > build bots.
> > > 
> > 
> > [root@...6082vm linux-powerpc]# sparse --version
> > 0.5.2
> > 
> > [root@...6082vm linux-powerpc]# ppc-linux-gcc --version
> > ppc-linux-gcc (GCC) 5.4.0
> 
> Ah, that's a sparse warning. But does your GCC agree? The thing is,
> sparse uses the C enum spec, but I suspect GCC uses the C++ enum spec
> and it all works fine.

What does the below proglet print on your PPC32 box? I suspect the
output will be:

400000000000

and all is well.

---
#include <stdio.h>

enum ponies {
	big = 1ULL<<46,
};

int main(int argc, char **argv)
{
	unsigned long long val = big;
	printf("%Lx\n", val);
	return 0;
}