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

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Message-ID: <2b78febd-ff00-0ed1-33fa-8575541a72dc@virtuozzo.com>
Date:   Fri, 8 Dec 2017 14:45:20 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     David Laight <David.Laight@...LAB.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

On 12/08/2017 02:14 PM, David Laight wrote:
> 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.
> 

Ok, *could* be a bug.


> ...
>> 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 :-)
 
The gcc developers must have good self-preservation instinct, that's why they usually provide switch off for optimizations like this.