From: David Laight <David.Laight@ACULAB.COM>
To: 'Nick Desaulniers' <ndesaulniers@google.com>,
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: Mon, 27 Jan 2020 12:04:37 +0000 [thread overview]
Message-ID: <ae9e908f4cfd4908a24a0e542731d31b@AcuMS.aculab.com> (raw)
In-Reply-To: <CAKwvOdmTOoTXCGN9NaO5_+sqDsK364=oCiVO_D5=btj1GsJrnw@mail.gmail.com>
From: Nick Desaulniers
> Sent: 24 January 2020 17:20
...
> > > 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.
Well some old systems had small integer constants at fixes addresses.
So 'const int one = 1;' would be a reference to the global constant.
An assignment like '*(int *)&one = 2;' would change the value of the
system-wide 'one' constant'.
Pretty much 'undefined'.
But no excuse for the compiler just discarding the code.
I suspect that the code to remove 'const' needs to 'launder' the value
through a suitable integer type.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2020-01-27 12:04 UTC|newest]
Thread overview: 42+ 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
2020-01-27 12:04 ` David Laight [this message]
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 9:50 ` Masahiro Yamada
2020-02-10 9:59 ` Will Deacon
2020-01-31 10:20 ` David Howells
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