Date: Wed, 2 Nov 2016 13:11:58 +0100
From: Markus Trippelsdorf <markus@trippelsdorf.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Biener <rguenther@suse.de>,
        "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Vegard Nossum <vegard.nossum@oracle.com>, Jiri Slaby <jslaby@suse.cz>,
        linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>, stable@vger.kernel.org,
        Ming Lei <ming.lei@canonical.com>,
        Steven Rostedt <srostedt@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Cesar Eduardo Barros <cesarb@cesarb.eti.br>,
        Michael Matz <matz@suse.de>, David Miller <davem@davemloft.net>,
        Guenter Roeck <linux@roeck-us.net>,
        Fengguang Wu <fengguang.wu@intel.com>, Borislav Petkov <bp@alien8.de>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>, Kees Cook <keescook@chromium.org>,
        ArnaldoCarva@x4
Subject: Re: [PATCH 01/12] extarray: define helpers for arrays defined in
 linker scripts
Message-ID: <20161102121158.GA11638@x4>
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On 2016.10.19 at 12:25 +0200, Peter Zijlstra wrote:
> On Wed, Oct 19, 2016 at 11:33:41AM +0200, Richard Biener wrote:
> > On Wed, 19 Oct 2016, Peter Zijlstra wrote:
> 
> > > This is also an entirely different class of optimizations than the whole
> > > pointer arithmetic is only valid inside an object thing.
> > 
> > Yes, it is not related to that.  I've opened 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78035 to track an
> > inconsistency in that new optimization.
> > 
> > > The kernel very much relies on unbounded pointer arithmetic, including
> > > overflow. Sure, C language says its UB, but we know our memory layout,
> > > and it would be very helpful if we could define it.
> > 
> > It's well-defined and correctly handled if you do the arithmetic
> > in uintptr_t.  No need for knobs.
> 
> So why not extend that to the pointers themselves and be done with it?
> 
> In any case, so you're saying our:
> 
> #define RELOC_HIDE(ptr, off)						\
> ({									\
> 	unsigned long __ptr;						\
> 	__asm__ ("" : "=r"(__ptr) : "0"(ptr));				\
> 	(typeof(ptr)) (__ptr + (off));					\
> })
> 
> could be written like:
> 
> #define RELOC_HIDE(ptr, off)			\
> ({						\
> 	uintptr_t __ptr = (ptr);		\
> 	(typeof(ptr)) (__ptr + (off));		\
> })
> 
> Without laundering it through inline asm?
> 
> Is there any advantage to doing so?
> 
> But this still means we need to be aware of this and use these macros to
> launder our pointers.
> 
> Which gets us back to the issue that started this whole thread. We have
> code that now gets miscompiled, silently.
> 
> That is a bad situation. So we need to either avoid the miscompilation,
> or make it verbose.

FYI this issue was fixed on gcc trunk by:
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=76bc343a2f1aa540e3f5c60e542586bb1ca0e032

-- 
Markus