linux-kernel - RE: [tip: timers/urgent] time: Handle negative seconds correctly in timespec64_to

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Message-ID: <87zgsmesj8.ffs@tglx>
Date:   Wed, 08 Sep 2021 22:11:55 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>
Cc:     Lukas Hannen <lukas.hannen@...nsource.tttech-industrial.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: RE: [tip: timers/urgent] time: Handle negative seconds correctly in
 timespec64_to_ns()

David,

On Wed, Sep 08 2021 at 16:01, David Laight wrote:
>> +	if (ts->tv_sec <= KTIME_SEC_MIN)
>> +		return KTIME_MIN;
>> +
>>  	return ((s64) ts->tv_sec * NSEC_PER_SEC) + ts->tv_nsec;
>>  }
>
> Adding tv_nsec can still overflow -  even if tv_nsec is bounded to +/- 1 second.
> This is no more 'garbage in' => 'garbage out' than the code without the
> multiply under/overflow check.

In kernel timespecs are always normalized:  0 < tv_nsec < 1e9 - 1

Let's do the math:

  KTIME_SEC_MAX = KTIME_MAX / NSEC_PER_SEC

  The overflow prevention does:

    if PSVAL >= KTIME_SEC_MAX:
       return KTIME_MAX

  so the largest positive seconds value which passes the above is:

    PSMAX = KTIME_SEC_MAX - 1

  ergo:

    PSMAX * NSEC_PER_SEC + (NSEC_PER_SEC - 1) < KTIME_SEC_MAX < KTIME_MAX

I leave the proof for negative values as an excercise for the reader.

Thanks,

        tglx
---
"Math is hard, let's go shopping!" - John Stultz