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zx2c4torvalds
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random: cap jitter samples per bit to factor of HZ
Currently the jitter mechanism will require two timer ticks per iteration, and it requires N iterations per bit. This N is determined with a small measurement, and if it's too big, it won't waste time with jitter entropy because it'd take too long or not have sufficient entropy anyway. With the current max N of 32, there are large timeouts on systems with a small CONFIG_HZ. Rather than set that maximum to 32, instead choose a factor of CONFIG_HZ. In this case, 1/30 seems to yield sane values for different configurations of CONFIG_HZ. Reported-by: Vladimir Murzin <vladimir.murzin@arm.com> Fixes: 78c768e ("random: vary jitter iterations based on cycle counter speed") Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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drivers/char/random.c

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@@ -1174,7 +1174,7 @@ static void __cold entropy_timer(struct timer_list *timer)
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*/
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static void __cold try_to_generate_entropy(void)
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{
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enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = 32 };
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enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 };
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struct entropy_timer_state stack;
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unsigned int i, num_different = 0;
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unsigned long last = random_get_entropy();

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