[PATCH v4] lib/vsprintf: defer filling siphash key on RT

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

On RT, we can't call get_random_bytes() from inside of the raw locks
that callers of vsprintf might take, because get_random_bytes() takes
normal spinlocks. So on those RT systems, defer the siphash key
generation to a worker.

Also, avoid using a static_branch, as this isn't the fast path.
Using static_branch_likely() to signal that ptr_key has been filled is a
bit much given that it is not a fast path.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Sebastian - feel free to take this and tweak it as needed. Sending this
mostly as something illustrative of what the "simpler" thing would be
that I had in mind. -Jason

 lib/vsprintf.c | 35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3c1853a9d1c0..5a67f6f65ddc 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -750,37 +750,42 @@ static int __init debug_boot_weak_hash_enable(char *str)
 }
 early_param("debug_boot_weak_hash", debug_boot_weak_hash_enable);
 
-static DEFINE_STATIC_KEY_FALSE(filled_random_ptr_key);
+static bool filled_ptr_key __read_mostly;
+static siphash_key_t ptr_key __read_mostly;
 
-static void enable_ptr_key_workfn(struct work_struct *work)
+static void fill_ptr_key_workfn(struct work_struct *work)
 {
-	static_branch_enable(&filled_random_ptr_key);
+	if (READ_ONCE(filled_ptr_key))
+		return;
+	get_random_bytes(&ptr_key, sizeof(ptr_key));
+	/* Pairs with smp_rmb() before reading ptr_key. */
+	smp_wmb();
+	WRITE_ONCE(filled_ptr_key, true);
 }
 
 /* Maps a pointer to a 32 bit unique identifier. */
 static inline int __ptr_to_hashval(const void *ptr, unsigned long *hashval_out)
 {
-	static siphash_key_t ptr_key __read_mostly;
 	unsigned long hashval;
 
-	if (!static_branch_likely(&filled_random_ptr_key)) {
-		static bool filled = false;
+	if (!READ_ONCE(filled_ptr_key)) {
 		static DEFINE_SPINLOCK(filling);
-		static DECLARE_WORK(enable_ptr_key_work, enable_ptr_key_workfn);
 		unsigned long flags;
 
-		if (!system_unbound_wq || !rng_is_initialized() ||
-		    !spin_trylock_irqsave(&filling, flags))
+		if (IS_ENABLED(CONFIG_PREEMPT_RT) && rng_is_initialized()) {
+			static DECLARE_WORK(fill_ptr_key_work, fill_ptr_key_workfn);
+			queue_work(system_unbound_wq, &fill_ptr_key_work);
 			return -EAGAIN;
-
-		if (!filled) {
-			get_random_bytes(&ptr_key, sizeof(ptr_key));
-			queue_work(system_unbound_wq, &enable_ptr_key_work);
-			filled = true;
 		}
+
+		if (!rng_is_initialized() || !spin_trylock_irqsave(&filling, flags))
+			return -EAGAIN;
+
+		fill_ptr_key_workfn(NULL);
 		spin_unlock_irqrestore(&filling, flags);
 	}
-
+	/* Pairs with smp_wmb() after writing ptr_key. */
+	smp_rmb();
 
 #ifdef CONFIG_64BIT
 	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
-- 
2.35.1