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Date: Wed, 26 Feb 2003 14:40:27 -0500 (EST)
From: "Richard B. Johnson" <root@chaos.analogic.com>
Reply-To: root@chaos.analogic.com
To: Daniel Jacobowitz <dan@debian.org>
cc: jt@hpl.hp.com, Albert Cahalan <albert@users.sourceforge.net>,
       linux-kernel@vger.kernel.org, Jouni Malinen <jkmaline@cc.hut.fi>
Subject: Re: Invalid compilation without -fno-strict-aliasing
In-Reply-To: <20030226192255.GA20127@nevyn.them.org>
Message-ID: <Pine.LNX.3.95.1030226143644.5091B-100000@chaos>
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On Wed, 26 Feb 2003, Daniel Jacobowitz wrote:

> On Wed, Feb 26, 2003 at 01:23:19PM -0500, Richard B. Johnson wrote:
> > On Wed, 26 Feb 2003, Jean Tourrilhes wrote:
> > 
> > > On Tue, Feb 25, 2003 at 11:33:09PM -0500, Albert Cahalan wrote:
> > > > Jean Tourrilhes writes:
> > > > 
> > > > > It looks like a compiler bug to me...
> > > > > Some users have complained that when the following
> > > > > code is compiled without the -fno-strict-aliasing,
> > > > > the order of the write and memcpy is inverted (which
> > > > > mean a bogus len is mem-copied into the stream).
> > > > > Code (from linux/include/net/iw_handler.h) :
> > > > > --------------------------------------------
> > > > > static inline char *
> > > > > iwe_stream_add_event(char * stream,  /* Stream of events */
> > > > >        char * ends,  /* End of stream */
> > > > >        struct iw_event *iwe, /* Payload */
> > > > >        int event_len) /* Real size of payload */
> > > > > {
> > > > >  /* Check if it's possible */
> > > > >  if((stream + event_len) < ends) {
> > > > >   iwe->len = event_len;
> > > > >   memcpy(stream, (char *) iwe, event_len);
> > > > >   stream += event_len;
> > > > >  } return stream;
> > > > > }
> > > > > --------------------------------------------
> > > > > IMHO, the compiler should have enough context to
> > > > > know that the reordering is dangerous. Any suggestion
> > > > > to make this simple code more bullet proof is welcomed.
> > > > >
> > > > > Have fun...
> > > > 
> > > > Since (char*) is special, I agree that it's a bug.
> > > > In any case, a warning sure would be nice!
> > > > 
> > > > Now for the fun. Pass iwe->len into this
> > > > macro before the memcpy, and all should be well.
> > > > 
> > > > #define FORCE_TO_MEM(x) asm volatile(""::"r"(&(x)))
> > > > 
> > > > Like this:
> > > > 
> > > >    iwe->len = event_len;
> > > >    FORCE_TO_MEM(iwe->len);
> > > >    memcpy(stream, (char *) iwe, event_len);
> > > 
> > > 	I'll try that, that sounds absolutely clever (but I only
> > > understand half of it).
> > > 	Thanks a lot !
> > > 
> > > 	Jean
> > > -
> > 
> > This does absoultely nothing with egcs-2.91.66. I also modified
> > 
> >  #define FORCE_TO_MEM(x) asm volatile(""::"r"(&(x)))
> >                                            |________ this to "memory"
> > 
> > and it still does nothing. The result of gcc -O2 -S -o xxx xxx.c just
> > shows:
> > 
> > #AP
> > #NOAP
> > 
> > With no code in-between.
> > 
> > I also changed it to:
> >  #define FORCE_TO_MEM(x) __asm__ __volatile__(""::"r"(&(x)))
> > to no avail.
> > 
> > What's up?
> 
> That's "working".  Does it prevent the reordering?

It was supposed to force x, which may be cached in a register,
to be written to memory __now__. It doesn't seem to do anything.
I think FORCE_TO_MEM() needs to claim that it uses most all the
registers. That will make sure that any register values get
written to their final memory locations.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.


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