linux-kernel - Re: Memory corruption due to word sharing

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Date:	Fri, 3 Feb 2012 10:37:22 +0100 (CET)
From:	Richard Guenther <rguenther@...e.de>
To:	DJ Delorie <dj@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-ia64@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	dsterba@...e.cz, ptesarik@...e.cz, gcc@....gnu.org
Subject: Re: Memory corruption due to word sharing

On Fri, 3 Feb 2012, DJ Delorie wrote:

> 
> Jan Kara <jack@...e.cz> writes:
> >   we've spotted the following mismatch between what kernel folks expect
> > from a compiler and what GCC really does, resulting in memory corruption on
> > some architectures. Consider the following structure:
> > struct x {
> >     long a;
> >     unsigned int b1;
> >     unsigned int b2:1;
> > };
> 
> If this structure were volatile, you could try
> -fstrict-volatile-bitfields, which forces GCC to use the C type to
> define the access width, instead of doing whatever it thinks is optimal.
> 
> Note: that flag is enabled by default for some targets already, most
> notably ARM.

Note that -fstrict-volatile-bitfields does not work for

volatile struct S {
    int i : 1;
    char c;
} s;
int main()
{
  s.i = 1;
  s.c = 2;
}

where it accesses s.i using SImode.  -fstrict-volatile-bitfields
falls foul of all the games bitfield layout plays and the
irrelevantness of the declared bitfield type (but maybe the
ARM ABI exactly specifies it that way).

So no, I would not recommend -fstrict-volatile-bitfields.

Richard.
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