linux-kernel - Re: [PATCH] mips: avoid explicit UB in assignment of mips_io_port

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Message-ID: <20190824141213.svzmdr3pxdaqssuj@pburton-laptop>
Date:   Sat, 24 Aug 2019 14:12:18 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        Hassan Naveed <hnaveed@...ecomp.com>,
        Stephen Kitt <steve@....org>,
        Serge Semin <fancer.lancer@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "regehr@...utah.edu" <regehr@...utah.edu>,
        Philip Reames <listmail@...lipreames.com>,
        Alexander Potapenko <glider@...gle.com>,
        Alistair Delva <adelva@...gle.com>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: Re: [PATCH] mips: avoid explicit UB in assignment of
 mips_io_port_base

Hi Nick,

On Fri, Aug 23, 2019 at 10:16:04AM -0700, Nick Desaulniers wrote:
> On Tue, Aug 20, 2019 at 10:15 AM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> > Hi Paul,
> > Bumping this thread; we'd really like to be able to boot test another
> > ISA in our CI.  This lone patch is affecting our ability to boot.  Can
> > you please pick it up?
> > https://lore.kernel.org/lkml/20190729211014.39333-1-ndesaulniers@google.com/
> 
> Hi Paul,
> Following up with this link that explains the undefined behavior issue more:
> https://wiki.sei.cmu.edu/confluence/display/c/EXP05-C.+Do+not+cast+away+a+const+qualification
> Please reconsider accepting this patch.

Sorry, it's been a crazy few months & I'm currently away awaiting my
father's funeral so I'm working through a backlog & catching up on
things.

It will be a shame to lose the optimization opportunities const offers
us, but it is an ugly hack & so I'm OK with applying this. It's likely
to affect older machines more than newer ones (which tend to use less or
no I/O port access) so I'm not too worried about the impact, but if we
find it matters we can always try the fixmap approach I suggested
previously.

Thanks,
    Paul