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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: 78c768e619fb ("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>

authored by

Jason A. Donenfeld and committed by
Linus Torvalds 829d680e 51a6fa07

+1 -1
+1 -1
drivers/char/random.c
··· 1174 1174 */ 1175 1175 static void __cold try_to_generate_entropy(void) 1176 1176 { 1177 - enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = 32 }; 1177 + enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 }; 1178 1178 struct entropy_timer_state stack; 1179 1179 unsigned int i, num_different = 0; 1180 1180 unsigned long last = random_get_entropy();