[PATCH RFC v1 1/6] random: add back async readiness notifier

["Jason A. Donenfeld" <Jason@zx2c4.com>]

This is required by vsprint, because it can't do things synchronously
from hardirq context, and it will be useful for an EFI notifier as well.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c  | 30 ++++++++++++++++++++++++++++++
 include/linux/random.h |  1 +
 2 files changed, 31 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6b7aca683b81..b3cad16ec567 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -84,6 +84,8 @@ static DEFINE_STATIC_KEY_FALSE(crng_is_ready);
 /* Various types of waiters for crng_init->CRNG_READY transition. */
 static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 static struct fasync_struct *fasync;
+static DEFINE_SPINLOCK(random_ready_chain_lock);
+static RAW_NOTIFIER_HEAD(random_ready_chain);
 
 /* Control how we warn userspace. */
 static struct ratelimit_state urandom_warning =
@@ -140,6 +142,33 @@ int wait_for_random_bytes(void)
 }
 EXPORT_SYMBOL(wait_for_random_bytes);
 
+/*
+ * Add a callback function that will be invoked when the crng is initialised,
+ * or immediately if it already has been.
+ */
+int __cold notify_on_rng_initialized(struct notifier_block *nb)
+{
+	unsigned long flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&random_ready_chain_lock, flags);
+	if (crng_ready())
+		nb->notifier_call(nb, 0, NULL);
+	else
+		ret = raw_notifier_chain_register(&random_ready_chain, nb);
+	spin_unlock_irqrestore(&random_ready_chain_lock, flags);
+	return ret;
+}
+
+static void __cold process_random_ready_list(void)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&random_ready_chain_lock, flags);
+	raw_notifier_call_chain(&random_ready_chain, 0, NULL);
+	spin_unlock_irqrestore(&random_ready_chain_lock, flags);
+}
+
 #define warn_unseeded_randomness() \
 	if (IS_ENABLED(CONFIG_WARN_ALL_UNSEEDED_RANDOM) && !crng_ready()) \
 		printk_deferred(KERN_NOTICE "random: %s called from %pS with crng_init=%d\n", \
@@ -685,6 +714,7 @@ static void __cold _credit_init_bits(size_t bits)
 		crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
 		if (static_key_initialized)
 			execute_in_process_context(crng_set_ready, &set_ready);
+		process_random_ready_list();
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
 		pr_notice("crng init done\n");
diff --git a/include/linux/random.h b/include/linux/random.h
index acaa328fb34d..566ffc3ab80d 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -119,6 +119,7 @@ void __init random_init_early(const char *command_line);
 void __init random_init(void);
 bool rng_is_initialized(void);
 int wait_for_random_bytes(void);
+int notify_on_rng_initialized(struct notifier_block *nb);
 
 /* Calls wait_for_random_bytes() and then calls get_random_bytes(buf, nbytes).
  * Returns the result of the call to wait_for_random_bytes. */
-- 
2.38.1