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random: use offstack cpumask when necessary 

The entropy generation function keeps a local cpu mask on the stack,
which can trigger warnings in configurations with a large number of
CPUs:

    drivers/char/random.c:1292:20: error: stack frame size (1288)
    exceeds limit (1280) in 'try_to_generate_entropy' [-Werror,-Wframe-larger-than]

Use the cpumask interface to dynamically allocate it in those
configurations.

Fixes: 1c21fe00ed ("random: spread out jitter callback to different CPUs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
This commit is contained in:
Arnd Bergmann 2025-06-10 11:27:08 +02:00 committed by Jason A. Donenfeld
parent 3c0c81de52
commit 5d49f1a5bd
1 changed files with 12 additions and 7 deletions
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19
drivers/char/random.c
View File

@ -1296,6 +1296,7 @@ static void __cold try_to_generate_entropy(void)
struct entropy_timer_state *stack = PTR_ALIGN((void *)stack_bytes, SMP_CACHE_BYTES);
unsigned int i, num_different = 0;
unsigned long last = random_get_entropy();
cpumask_var_t timer_cpus;
int cpu = -1;
for (i = 0; i < NUM_TRIAL_SAMPLES - 1; ++i) {
@ -1310,13 +1311,15 @@ static void __cold try_to_generate_entropy(void)
atomic_set(&stack->samples, 0);
timer_setup_on_stack(&stack->timer, entropy_timer, 0);
if (!alloc_cpumask_var(&timer_cpus, GFP_KERNEL))
goto out;
while (!crng_ready() && !signal_pending(current)) {
/*
* Check !timer_pending() and then ensure that any previous callback has finished
* executing by checking timer_delete_sync_try(), before queueing the next one.
*/
if (!timer_pending(&stack->timer) && timer_delete_sync_try(&stack->timer) >= 0) {
struct cpumask timer_cpus;
unsigned int num_cpus;
/*
@ -1326,19 +1329,19 @@ static void __cold try_to_generate_entropy(void)
preempt_disable();
/* Only schedule callbacks on timer CPUs that are online. */
cpumask_and(&timer_cpus, housekeeping_cpumask(HK_TYPE_TIMER), cpu_online_mask);
num_cpus = cpumask_weight(&timer_cpus);
cpumask_and(timer_cpus, housekeeping_cpumask(HK_TYPE_TIMER), cpu_online_mask);
num_cpus = cpumask_weight(timer_cpus);
/* In very bizarre case of misconfiguration, fallback to all online. */
if (unlikely(num_cpus == 0)) {
timer_cpus = *cpu_online_mask;
num_cpus = cpumask_weight(&timer_cpus);
*timer_cpus = *cpu_online_mask;
num_cpus = cpumask_weight(timer_cpus);
}
/* Basic CPU round-robin, which avoids the current CPU. */
do {
cpu = cpumask_next(cpu, &timer_cpus);
cpu = cpumask_next(cpu, timer_cpus);
if (cpu >= nr_cpu_ids)
cpu = cpumask_first(&timer_cpus);
cpu = cpumask_first(timer_cpus);
} while (cpu == smp_processor_id() && num_cpus > 1);
/* Expiring the timer at `jiffies` means it's the next tick. */
@ -1354,6 +1357,8 @@ static void __cold try_to_generate_entropy(void)
}
mix_pool_bytes(&stack->entropy, sizeof(stack->entropy));
free_cpumask_var(timer_cpus);
out:
timer_delete_sync(&stack->timer);
timer_destroy_on_stack(&stack->timer);
}