linux-kernel - Re: [PATCH stable] x86/tsc: Prevent result truncation on 32 bit

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Message-ID: <20180912184100.GC10317@kroah.com>
Date:   Wed, 12 Sep 2018 20:41:00 +0200
From:   Greg KH <greg@...ah.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH stable] x86/tsc: Prevent result truncation on 32 bit

On Wed, Sep 12, 2018 at 02:42:08PM +0200, Thomas Gleixner wrote:
> Subject: x86/tsc: Prevent result truncation on 32 bit
> From: Chuanhua Lei <chuanhua.lei@...ux.intel.com>
> Date: Thu Sep 6 18:03:23 2018 +0800
> 
> From: Chuanhua Lei <chuanhua.lei@...ux.intel.com>
> 
> Commit 17f6bac2249356c795339e03a0742cd79be3cab8 upstream.
> 
> Loops per jiffy is calculated by multiplying tsc_khz with 1e3 and then
> dividing it by HZ.
>     
> Both tsc_khz and the temporary variable holding the multiplication result
> are of type unsigned long, so on 32bit the result is truncated to the lower
> 32bit.
>     
> Use u64 as type for the temporary variable and cast tsc_khz to it before
> multiplying.
>     
> [ tglx: Backport to stable. Due to massive code changes is the upstream
>   	commit not applicable anymore. The issue has gone unnoticed in
>   	kernels pre 4.19 because the bogus LPJ value gets fixed up in a
>   	later stage of early boot, but it still might cause subtle and hard
>   	to debug issues between these two points. ]

Thanks for the backport, now queued up.

greg k-h