linux-kernel - Re: [patch 13/13] tomoyo: Use sensible time interface

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Date:	Wed, 11 Jun 2014 17:28:44 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Kentaro Takeda <takedakn@...data.co.jp>,
	linux-security-module@...r.kernel.org
Subject: Re: [patch 13/13] tomoyo: Use sensible time interface

On Wed, Jun 11, 2014 at 5:22 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 11 Jun 2014, John Stultz wrote:
>
>> On Wed, Jun 11, 2014 at 4:59 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > There is no point in calling gettimeofday if only the seconds part of
>> > the timespec is used. Use get_seconds() instead. It's not only the
>> > proper interface it's also faster.
>>
>> My only caution here is you only get tick-granular time here. So if
>> the second rolled over after the last tick, you'd get the previous
>> second when you call get_seconds(). This can cause some surprising
>> effects if the get_seconds() return value is mixed with clocksource
>> granular gettimeofday() calls.
>
> If the whole thing only cares about the seconds value, then where is
> the problem?
>
> Even if you call gettimeofday() then you still can observe this
>
> gettimeofday(ts)
>         ts.tv_sec = 99
>         ts.tv_nsec = 999999999
>
> So if you readout the related value ONE nanosecond later, then this
> value will have
>         ts.tv_sec = 100
>         ts.tv_nsec = 0
>
> So what's the point? The tomoyo code chose to take seconds granular
> time stamps for whatever reasons. So it should be able to deal with
> that, right?

No, the problem I'm warning about is if they were using gettimeofday()
elsewhere in relation to those timestamps, they could see something
like:

do_gettimeofday()  { 99, 888....}
get_seconds()   { 99 }
do_gettimeofday()  { 99, 999....}
get_seconds()   { 99 }
do_gettimeofday()  { 100, 000....}
get_seconds()   { 99 }
do_gettimeofday()  { 100, 011....}
get_seconds()   { 100 }

This is the same problem people come across occasionally if they call
gettimeofday, then create a file and fret that the file's timestamp
seems to be before the gettimefoday call, and its all due to comparing
timestamps with different granularities.

I'm not saying its a problem in this case, but I'm just throwing up
some additional caution since the change you're making isn't
completely equivalent.

thanks
-john
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