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From: Will Deacon <will@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	dja@axtens.net,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linuxppc-dev@lists.ozlabs.org,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	linux-arch <linux-arch@vger.kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
Date: Tue, 17 Dec 2019 17:07:19 +0000	[thread overview]
Message-ID: <20191217170719.GA869@willie-the-truck> (raw)
In-Reply-To: <CAHk-=wiMuHmWzQ7-CRQB6o+SHtA-u-Rp6VZwPcqDbjAaug80rQ@mail.gmail.com>

On Thu, Dec 12, 2019 at 12:49:52PM -0800, Linus Torvalds wrote:
> On Thu, Dec 12, 2019 at 11:34 AM Will Deacon <will@kernel.org> wrote:
> >
> > The root of my concern in all of this, and what started me looking at it in
> > the first place, is the interaction with 'typeof()'. Inheriting 'volatile'
> > for a pointer means that local variables in macros declared using typeof()
> > suddenly start generating *hideous* code, particularly when pointless stack
> > spills get stackprotector all excited.
> 
> Yeah, removing volatile can be a bit annoying.
> 
> For the particular case of the bitops, though, it's not an issue.
> Since you know the type there, you can just cast it.
> 
> And if we had the rule that READ_ONCE() was an arithmetic type, you could do
> 
>     typeof(0+(*p)) __var;
> 
> since you might as well get the integer promotion anyway (on the
> non-volatile result).
> 
> But that doesn't work with structures or unions, of course.
> 
> I'm not entirely sure we have READ_ONCE() with a struct. I do know we
> have it with 64-bit entities on 32-bit machines, but that's ok with
> the "0+" trick.

Other than the two trivial examples Arnd and I spotted, it looks like
we're in for some fun with the page-table types such as pud_t but that
/should/ be fixable with enough effort.

However, I'm really banging my head against the compiler trying to get
your trick above to work for pointer types when the pointed-to-type is
not defined. As a very cut down example (I pulled this back out of the
preprocessor and cleaned it up a bit):


struct dentry {
	struct inode *d_inode;
};

static inline struct inode *d_inode_rcu(struct dentry *dentry)
{
	return ({
		typeof(0 + dentry->d_inode) __x = (*(volatile typeof(dentry->d_inode) *)&(dentry->d_inode));
		(typeof(dentry->d_inode))__x;
	});
}


Trying to compile this results in:

  | In function 'd_inode_rcu':
  | error: invalid use of undefined type 'struct inode'

whereas it compiles fine if you remove the '0 +' from the first typeof.

What am I missing? Perhaps the compiler wants the size information of
'struct inode' before it will contemplate the arithmetic, but if so then
I don't think we can use this trick after all. Hmm.

Will

  parent reply	other threads:[~2019-12-17 17:07 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-06 12:46 [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops) Michael Ellerman
2019-12-06 13:16 ` Peter Zijlstra
2019-12-10  5:38   ` Michael Ellerman
2019-12-10 10:15     ` Peter Zijlstra
2019-12-11  0:29       ` Michael Ellerman
2019-12-12  5:42   ` READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)) Michael Ellerman
2019-12-12  8:01     ` Peter Zijlstra
2019-12-12 10:07       ` Will Deacon
2019-12-12 10:46         ` Peter Zijlstra
2019-12-12 17:04           ` Will Deacon
2019-12-12 17:16             ` Will Deacon
2019-12-12 17:41           ` Linus Torvalds
2019-12-12 17:50             ` Segher Boessenkool
2019-12-12 18:06             ` Will Deacon
2019-12-12 18:29               ` Christian Borntraeger
2019-12-12 18:43               ` Linus Torvalds
2019-12-12 19:34                 ` Will Deacon
2019-12-12 20:21                   ` Peter Zijlstra
2019-12-12 20:53                     ` Peter Zijlstra
2019-12-13 10:47                       ` Luc Van Oostenryck
2019-12-13 12:56                         ` Peter Zijlstra
2019-12-13 14:28                           ` Luc Van Oostenryck
2019-12-12 20:49                   ` Linus Torvalds
2019-12-13 13:17                     ` Arnd Bergmann
2019-12-13 21:32                       ` Arnd Bergmann
2019-12-13 22:01                         ` Linus Torvalds
2019-12-16 10:28                       ` Will Deacon
2019-12-16 11:47                         ` Peter Zijlstra
2019-12-16 12:06                         ` Arnd Bergmann
2019-12-17 17:07                     ` Will Deacon [this message]
2019-12-17 18:04                       ` Linus Torvalds
2019-12-17 18:05                         ` Linus Torvalds
2019-12-17 18:31                           ` Will Deacon
2019-12-17 18:32                         ` Linus Torvalds
2019-12-18 12:17                           ` Michael Ellerman
2019-12-19 12:11                           ` Will Deacon
2019-12-18 10:22                     ` Christian Borntraeger
2019-12-18 10:35                       ` Will Deacon
2019-12-13 12:07           ` Michael Ellerman
2019-12-13 13:53             ` Segher Boessenkool
2019-12-13 21:06               ` Michael Ellerman
2019-12-12 15:10     ` Segher Boessenkool
2019-12-06 22:15 ` [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops) pr-tracker-bot

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