linux-kernel - RE: [PATCH v2] ubsan: don't handle misaligned address when support unaligned access

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Date:   Fri, 8 Dec 2017 11:14:34 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andrey Ryabinin' <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Ding Tianhong <dingtianhong@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        LinuxArm <linuxarm@...wei.com>
Subject: RE: [PATCH v2] ubsan: don't handle misaligned address when support
 unaligned access

From: Andrey Ryabinin
> Sent: 08 December 2017 10:49
...
> CONFIG_UBSAN_ALIGNMENT is already disabled by default for HAVE_EFFICIENT_UNALIGNED_ACCESS=y because it's noisy,
> but we still allow users to enable it if they want to.
> 
> I don't think we should completely forbid enabling it for HAVE_EFFICIENT_UNALIGNED_ACCESS=y.
> Unaligned access is still a bug in non-arch code and outside of sections like #ifdef HAVE_EFFICIENT_UNALIGNED_ACCESS .. #endif .

Don't think so.
Code that knows that unaligned accesses don't fault can set up pointers
that non-arch code dereferences.
Happens all the time in the networking stack.

...
> And one day, GCC might start doing optimizations based on this, e.g.:
> 
> 	u64 *ptr;
> 	...
> 	x = *ptr;
> 	...
> 	if (ptr & 7)  // Compiler can assume that this statement is always false, because 'ptr' was deferenced, so it must be aligned
> 		do_something();

Ugg - shoot the gcc developers :-)

	David