rethook: Reject getting a rethook if RCU is not watching

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: 2e385b6f172e ("rethook: Add a generic return hook")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/165461827269.280167.7379263615545598958.stgit@devnote2

diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
index b56833700d23fb515a1b0e7a1f9a53e2e66d82c9..c69d82273ce782e101203e9f04639004a7e14db9 100644 (file)
--- 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;