Fix Jitter time measurement on Apple platforms #722
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On Apple platforms, Jitter RNG implementation uses `mach_absolute_time`
function to get the current value of the clock. However, this function
deals in "tick" units, not in time units. On Intel based Apple
platforms 1 tick = 1 ns, which is good enough timer resolution for
Jitter. On M1 macbooks 1 tick ~ 41.67 ns. This seems to be a problem
for Jitter since it causes frequent failures to produce enough entropy
on M1 macbooks. Another potential cause for the issue might be that
`mach_absolute_time` clock doesn't increment while they system is
asleep. I am not sure what is the definition of "asleep" on M1 macs,
how it correlates to performance and efficiency cores and different
power saving states, etc. So to be on the safe side we change:
- Instead of `mach_absolute_time` we call `clock_gettime_nsec_np`
function, as suggested by Apple's documentation [1],
- Instead of using `CLOCK_UPTIME_RAW` that corresponds to
`mach_absolut_time`, we use `CLOCK_MONOTONIC_RAW` that
corresponds to `mach_continuous_time` whose clock increments
even while the system is asleep [2].
[1] https://developer.apple.com/documentation/kernel/1462446-mach_absolute_time
[2] https://developer.apple.com/documentation/kernel/1646199-mach_continuous_time
Taffer
reviewed
Dec 12, 2022
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Taffer
previously approved these changes
Dec 14, 2022
torben-hansen
approved these changes
Dec 14, 2022
Taffer
approved these changes
Dec 14, 2022
Taffer
pushed a commit
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Dec 14, 2022
On Apple platforms, Jitter RNG implementation uses `mach_absolute_time` function to get the current value of the clock. However, this function deals in "tick" units, not in time units. On Intel based Apple platforms 1 tick = 1 ns, which is good enough timer resolution for Jitter. On M1 macbooks 1 tick ~ 41.67 ns. This seems to be a problem for Jitter since it causes frequent failures to produce enough entropy on M1 macbooks. Another potential cause for the issue might be that `mach_absolute_time` clock doesn't increment while they system is asleep. I am not sure what is the definition of "asleep" on M1 macs, how it correlates to performance and efficiency cores and different power saving states, etc. The upstream Jitter RNG repository has recently added support for using the system counter on `aarch64` which seems like way to go for all 64-bit Arm platforms. So this change: - Adds the system counter calls on `aarch64` platforms, - Removes the `__MACH__` special case.
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Issues:
CryptoAlg-1441
Description of changes:
On Apple platforms, Jitter RNG implementation uses
mach_absolute_timefunction to get the current value of the clock. However, this function
deals in "tick" units, not in time units. On Intel based Apple
platforms 1 tick = 1 ns, which is good enough timer resolution for
Jitter. On M1 macbooks 1 tick ~ 41.67 ns. This seems to be a problem
for Jitter since it causes frequent failures to produce enough entropy
on M1 macbooks. Another potential cause for the issue might be that
mach_absolute_timeclock doesn't increment while they system isasleep. I am not sure what is the definition of "asleep" on M1 macs,
how it correlates to performance and efficiency cores and different
power saving states, etc. The upstream Jitter RNG repository has
recently added support for using the system counter on
aarch64which seems like way to go for all 64-bit Arm platforms.
So this change:
aarch64platforms,__MACH__special case.Call-outs:
Point out areas that need special attention or support during the review process. Discuss architecture or design changes.
Testing:
How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and
the ISC license.