static_key: WARN on usage before jump_label_init was called

Usage of the static key primitives to toggle a branch must not be used
before jump_label_init() is called from init/main.c. jump_label_init
reorganizes and wires up the jump_entries so usage before that could
have unforeseen consequences.

Following primitives are now checked for correct use:
* static_key_slow_inc
* static_key_slow_dec
* static_key_slow_dec_deferred
* jump_label_rate_limit

The x86 architecture already checks this by testing if the default_nop
was already replaced with an optimal nop or with a branch instruction. It
will panic then. Other architectures don't check for this.

Because we need to relax this check for the x86 arch to allow code to
transition from default_nop to the enabled state and other architectures
did not check for this at all this patch introduces checking on the
static_key primitives in a non-arch dependent manner.

All checked functions are considered slow-path so the additional check
does no harm to performance.

The warnings are best observed with earlyprintk.

Based on a patch from Andi Kleen.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 297a9247a3b3..9019f15deab2 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -58,6 +58,7 @@ static void jump_label_update(struct static_key *key, int enable);
 
 void static_key_slow_inc(struct static_key *key)
 {
+	STATIC_KEY_CHECK_USE();
 	if (atomic_inc_not_zero(&key->enabled))
 		return;
 
@@ -103,12 +104,14 @@ static void jump_label_update_timeout(struct work_struct *work)
 
 void static_key_slow_dec(struct static_key *key)
 {
+	STATIC_KEY_CHECK_USE();
 	__static_key_slow_dec(key, 0, NULL);
 }
 EXPORT_SYMBOL_GPL(static_key_slow_dec);
 
 void static_key_slow_dec_deferred(struct static_key_deferred *key)
 {
+	STATIC_KEY_CHECK_USE();
 	__static_key_slow_dec(&key->key, key->timeout, &key->work);
 }
 EXPORT_SYMBOL_GPL(static_key_slow_dec_deferred);
@@ -116,6 +119,7 @@ EXPORT_SYMBOL_GPL(static_key_slow_dec_deferred);
 void jump_label_rate_limit(struct static_key_deferred *key,
 		unsigned long rl)
 {
+	STATIC_KEY_CHECK_USE();
 	key->timeout = rl;
 	INIT_DELAYED_WORK(&key->work, jump_label_update_timeout);
 }
@@ -212,6 +216,7 @@ void __init jump_label_init(void)
 		key->next = NULL;
 #endif
 	}
+	static_key_initialized = true;
 	jump_label_unlock();
 }