[PATCH bpf v2 2/2] rethook: Reject getting a rethook if RCU is not watching

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Since the rethook_recycle() will involve the call_rcu() for reclaiming
the rethook_instance, the rethook must be set up at the RCU available
context (non idle). This rethook_recycle() in the rethook trampoline
handler is inevitable, thus the RCU available check must be done before
setting the rethook trampoline.

This adds a rcu_is_watching() check in the rethook_try_get() so that
it will return NULL if it is called when !rcu_is_watching().

Fixes: 54ecbe6f1ed5 ("rethook: Add a generic return hook")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 kernel/trace/rethook.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
index b56833700d23..c69d82273ce7 100644
--- a/kernel/trace/rethook.c
+++ b/kernel/trace/rethook.c
@@ -154,6 +154,15 @@ struct rethook_node *rethook_try_get(struct rethook *rh)
 	if (unlikely(!handler))
 		return NULL;
 
+	/*
+	 * This expects the caller will set up a rethook on a function entry.
+	 * When the function returns, the rethook will eventually be reclaimed
+	 * or released in the rethook_recycle() with call_rcu().
+	 * This means the caller must be run in the RCU-availabe context.
+	 */
+	if (unlikely(!rcu_is_watching()))
+		return NULL;
+
 	fn = freelist_try_get(&rh->pool);
 	if (!fn)
 		return NULL;