linux-kernel - Re: [PATCH] time: Cast tv_nsec to u64 for proper shifting in update

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Date:	Fri, 09 May 2014 09:03:08 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	stefani@...bold.net, luto@...capital.net, konrad.wilk@...cle.com,
	stable@...r.kernel.org
Subject: Re: [PATCH] time: Cast tv_nsec to u64 for proper shifting in update_vsyscall()

On 05/09/2014 08:59 AM, Boris Ostrovsky wrote:
> On 05/09/2014 11:43 AM, H. Peter Anvin wrote:
>> On 05/09/2014 08:11 AM, Boris Ostrovsky wrote:
>>> With tk->wall_to_monotonic.tv_nsec being a 32-bit value on 32-bit
>>> systems, (tk->wall_to_monotonic.tv_nsec << tk->shift) in
>>> update_vsyscall()
>>> may lose upper bits or, worse, add them since compiler will do this:
>>>     (u64)(tk->wall_to_monotonic.tv_nsec << tk->shift)
>>> instead of
>>>     ((u64)tk->wall_to_monotonic.tv_nsec << tk->shift)
>>>
>>> So if, for example, tv_nsec is 0x800000 and shift is 8 we will end up
>>> with 0xffffffff80000000 instead of 0x80000000. And then we are stuck in
>>> the subsequent 'while' loop.
>>>
>>> We need explicit cast.
>>>
>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>>> Cc: stable@...r.kernel.org
>> This is needed in stable only for v3.14, right?
> 
> I suspect anything that has commit 650ea024 needs to be fixed. I see
> this code, for example, in 3.12 (it used to be in vsyscall_64.c).
> 

But you're talking about 32-bit platforms.  Vsyscalls aren't used on
x86-32 until 3.14.  Am I missing something?

	-hpa


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