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

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Message-ID: <536CF7B3.4040705@zytor.com>
Date:	Fri, 09 May 2014 08:43:47 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>, tglx@...utronix.de,
	mingo@...hat.com
CC:	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 08:11 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?

	-hpa


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