linux-kernel - Re: [PATCH 8/8] selftests/proc: Assert clock_gettime(CLOCK

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Message-ID: <ZBmmtMlKXcf2+hnq@lothringen>
Date:   Tue, 21 Mar 2023 13:44:36 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mirsad Todorovac <mirsad.todorovac@....unizg.hr>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Wei Li <liwei391@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Yu Liao <liaoyu15@...wei.com>, Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 8/8] selftests/proc: Assert clock_gettime(CLOCK_BOOTTIME)
 VS /proc/uptime monotonicity

On Wed, Mar 08, 2023 at 04:59:41PM +0100, Mirsad Todorovac wrote:
> On 2/22/23 15:46, Frederic Weisbecker wrote:
> From what I see, you round the CLOCK_BOOTIME time to 1/100ths of a second.
> 
> A simple program that queries clock_getres() on system clocks gives this
> result:
> 
> clock_res [CLOCK_REALTIME] = 0.000000001s
> clock_res [CLOCK_REALTIME_COARSE] = 0.004000000s
> clock_res [CLOCK_MONOTONIC] = 0.000000001s
> clock_res [CLOCK_MONOTONIC_COARSE] = 0.004000000s
> clock_res [CLOCK_MONOTONIC_RAW] = 0.000000001s
> clock_res [CLOCK_BOOTTIME] = 0.000000001s
> clock_res [CLOCK_PROCESS_CPUTIME_ID] = 0.000000001s
> clock_res [CLOCK_THREAD_CPUTIME_ID] = 0.000000001s
> 
> A number of programs may depend i.e. on CLOCK_REALTIME or CLOCK_BOOTIME to give
> different result each nanosecond.
> 
> I came across this when generating nonces for HMACs according to recommendations
> from RFC 4086 "Randomness Requirements for Security".
> 
> If the value of CLOCK_BOOTTIME or CLOCK_REALTIME is incremented not in what
> clock_getres() gives, but at best in 1/100th of second instead, that would seriously
> weaken our security (for as you know, in many cryptographic uses nonces need not
> be random, but MUST NOT ever repeat nor go backwards).
> 
> Could we modify the test for this assumption, or is the assumption wrong?
> 
> Here the test for CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID
> increasing monotonically with guaranteed increased value of nanoseconds
> would also seem good.
> 
> Maybe this is already covered in another test case, but it seems that all
> clocks should be guaranteed to be monotonically increasing, and increased
> at least by one nanosecond with each syscall, or many algorithms would break.
> 
> In other words, CLOCK_BOOTTIME should be tested to increase monotonically in
> the resolution given by clock_getres (CLOCK_BOOTTIME, &tv_res), not in 1/100ths
> of second (IMHO).

Maybe but verifying a clock against its own resolution is another testcase. Here the
point is to verify that CLOCK_BOOTTIME is monotonic against /proc/uptime, and
since /proc/uptime has an 1/100 second resolution, rounding clock_gettime(CLOCK_BOOTTIME)
result down to that is the best we can do.

Thanks.