netdev - RE: [net-next 05/14] i40e: fix up 32 bit timespec references

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Date:   Mon, 25 Feb 2019 19:20:10 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Arnd Bergmann <arnd@...db.de>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
CC:     David Miller <davem@...emloft.net>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        Networking <netdev@...r.kernel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "jogreene@...hat.com" <jogreene@...hat.com>
Subject: RE: [net-next 05/14] i40e: fix up 32 bit timespec references

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On
> Behalf Of Arnd Bergmann
> Sent: Monday, February 25, 2019 6:46 AM
> To: Kirsher, Jeffrey T <jeffrey.t.kirsher@...el.com>
> Cc: David Miller <davem@...emloft.net>; Brandeburg, Jesse
> <jesse.brandeburg@...el.com>; Networking <netdev@...r.kernel.org>;
> nhorman@...hat.com; sassmann@...hat.com; jogreene@...hat.com
> Subject: Re: [net-next 05/14] i40e: fix up 32 bit timespec references
> 
> On Wed, Jul 26, 2017 at 12:33 PM Jeff Kirsher
> <jeffrey.t.kirsher@...el.com> wrote:
> >
> > From: Jesse Brandeburg <jesse.brandeburg@...el.com>
> >
> > As it turns out there was only a small set of errors
> > on 32 bit, and we just needed to be using the right calls
> > for dealing with timespec64 variables.
> 
> I just stumbled over code added by this older patch, and can't make sense
> of the commit description here. Was this an attempt to fix a bug, or
> just a cleanup?
> 
> >
> > -       then = ns_to_timespec64(delta);
> >         mutex_lock(&pf->tmreg_lock);
> >
> >         i40e_ptp_read(pf, &now);
> > -       now = timespec64_add(now, then);
> > +       timespec64_add_ns(&now, delta);
> >         i40e_ptp_write(pf, (const struct timespec64 *)&now);
> 
> The problem I noticed here is that 'delta' is a user provided 64-bit
> number from clock_adjtime(), and timespec64_add_ns() performs uses
> a repeated addition instead of a div/mod pair. When the number
> is large, we may end up adding a single second 8 billion times,
> which may take a while even on a fast CPU.
> 

It looked like the timespec64_add_ns does a div/mod pair...? Or am I mis-reading how the function is implemented? Quite probably.

Either way, the code is incorrect, because timespec64_add_ns doesn't actually work with signed values. A negative delta actually ends up resulting in a significant positive addition. Woops!

> Should the commit 0ac30ce43323 ("i40e: fix up 32 bit timespec
> references") just be reverted?
> 

Yea, let's revert it.

Thanks,
Jake

>       Arnd