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

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Date:   Sat, 8 Sep 2018 01:55:54 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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,
        linux-sparse@...r.kernel.org
Subject: Re: [PATCH] perf: enum overflow in uapi/linux/perf_event.h

On Fri, Sep 07, 2018 at 08:43:59PM +0200, Luc Van Oostenryck wrote:
> On Fri, Sep 07, 2018 at 04:15:33PM +0200, Christophe LEROY wrote:
> > Le 07/09/2018 à 15:58, Peter Zijlstra a écrit :
> > > 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.
> 
> Sparse is a bit weird about the exact underlying type used for enums.
> 
> > Ah yes, it seems that GCC is happy. So sparse should be fixed instead ?
> 
> I'll investigate (I suppose the same is given on x86-32).


It's definitively a bug in sparse. A relatively nasty one and which
open a can of worms. Fortunately, I had already looked at these
problems in May, I just didn't had the time to push the patches.

-- Luc