From: Nick Desaulniers <ndesaulniers@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
kernel-team <kernel-team@android.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Linus Torvalds <torvalds@linux-foundation.org>,
Segher Boessenkool <segher@kernel.crashing.org>,
Christian Borntraeger <borntraeger@de.ibm.com>,
Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
Arnd Bergmann <arnd@arndb.de>,
Peter Oberparleiter <oberpar@linux.ibm.com>,
Masahiro Yamada <masahiroy@kernel.org>,
Pablo Neira Ayuso <pablo@netfilter.org>,
Jozsef Kadlecsik <kadlec@netfilter.org>,
Florian Westphal <fw@strlen.de>,
"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v2 02/10] netfilter: Avoid assigning 'const' pointer to non-const pointer
Date: Fri, 24 Jan 2020 09:20:14 -0800 [thread overview]
Message-ID: <CAKwvOdmTOoTXCGN9NaO5_+sqDsK364=oCiVO_D5=btj1GsJrnw@mail.gmail.com> (raw)
In-Reply-To: <20200124082443.GY14914@hirez.programming.kicks-ass.net>
On Fri, Jan 24, 2020 at 12:24 AM Peter Zijlstra <peterz@infradead.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
next prev parent reply other threads:[~2020-01-24 17:20 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-23 15:33 [PATCH v2 00/10] Rework READ_ONCE() to improve codegen Will Deacon
2020-01-23 15:33 ` [PATCH v2 01/10] compiler/gcc: Emit build-time warning for GCC prior to version 4.8 Will Deacon
2020-01-23 15:33 ` [PATCH v2 02/10] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-01-23 19:07 ` Nick Desaulniers
2020-01-24 8:24 ` Peter Zijlstra
2020-01-24 17:20 ` Nick Desaulniers [this message]
2020-01-27 12:04 ` David Laight
2020-01-24 17:36 ` Linus Torvalds
2020-01-24 22:00 ` Peter Zijlstra
2020-01-27 12:21 ` David Laight
2020-01-23 15:33 ` [PATCH v2 03/10] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-01-23 15:33 ` [PATCH v2 04/10] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-01-23 15:33 ` [PATCH v2 05/10] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-01-25 8:27 ` Peter Zijlstra
2020-01-29 10:49 ` Peter Zijlstra
2020-01-23 15:33 ` [PATCH v2 06/10] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-01-23 15:33 ` [PATCH v2 07/10] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-01-23 15:33 ` [PATCH v2 08/10] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-01-23 15:33 ` [PATCH v2 09/10] compiler/gcc: Raise minimum GCC version for kernel builds to 4.8 Will Deacon
2020-01-23 18:36 ` Nick Desaulniers
2020-01-24 8:26 ` Peter Zijlstra
2020-01-24 17:05 ` Nick Desaulniers
2020-01-24 23:29 ` Peter Zijlstra
2020-01-25 10:34 ` Michael Ellerman
2020-01-23 15:33 ` [PATCH v2 10/10] gcov: Remove old GCC 3.4 support Will Deacon
2020-01-23 18:51 ` Nick Desaulniers
2020-01-28 14:56 ` Peter Oberparleiter
2020-01-23 17:07 ` [PATCH v2 00/10] Rework READ_ONCE() to improve codegen David Laight
2020-01-23 17:16 ` Will Deacon
2020-01-23 17:32 ` David Laight
2020-01-23 18:45 ` Nick Desaulniers
2020-01-23 19:01 ` Arvind Sankar
2020-01-24 10:11 ` David Laight
2020-01-26 1:10 ` Qais Yousef
2020-01-27 7:26 ` Arnd Bergmann
2020-01-23 17:59 ` Linus Torvalds
2020-01-24 8:33 ` Peter Zijlstra
2020-01-24 10:41 ` Peter Zijlstra
2020-02-10 9:50 ` Masahiro Yamada
2020-02-10 9:59 ` Will Deacon
2020-01-31 10:20 ` David Howells
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