linux-kernel - Re: [PATCH v2 02/10] netfilter: Avoid assigning 'const' pointer to non-const pointer

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Message-ID: <CAKwvOdmTOoTXCGN9NaO5_+sqDsK364=oCiVO_D5=btj1GsJrnw@mail.gmail.com>
Date:   Fri, 24 Jan 2020 09:20:14 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v2 02/10] netfilter: Avoid assigning 'const' pointer to
 non-const pointer

On Fri, Jan 24, 2020 at 12:24 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Jan 23, 2020 at 11:07:59AM -0800, Nick Desaulniers wrote:
>
> > Good thing it's the variable being modified was not declared const; I
> > get spooked when I see -Wdiscarded-qualifiers because of Section
> > 6.7.3.6 of the ISO C11 draft spec:
> >
> > ```
> > If an attempt is made to modify an object defined with a
> > const-qualified type through use
> > of an lvalue with non-const-qualified type, the behavior is undefined.
> > If an attempt is
> > made to refer to an object defined with a volatile-qualified type
> > through use of an lvalue
> > with non-volatile-qualified type, the behavior is undefined.133)
> >
> > 133) This applies to those objects that behave as if they were defined
> > with qualified types, even if they are
> > never actually defined as objects in the program (such as an object at
> > a memory-mapped input/output
> > address).
> > ```
> >
> > Which is about the modification of a const-declared variable (explicit
> > UB which Clang actively exploits),
>
> Just for curiosity's sake. What does clang actually do in that case?
>

https://bugs.llvm.org/show_bug.cgi?id=42763
I was playing around with this in godbolt but couldn't quickly come up
with a simple reproducer.  IIRC, I've fixed maybe 3 instances of this
recently in code though.
-- 
Thanks,
~Nick Desaulniers