tracing: tprobe-events: Fix leakage of module refcount

When enabling the tracepoint at loading module, the target module
refcount is incremented by find_tracepoint_in_module(). But it is
unnecessary because the module is not unloaded while processing
module loading callbacks.
Moreover, the refcount is not decremented in that function.
To be clear the module refcount handling, move the try_module_get()
callsite to trace_fprobe_create_internal(), where it is actually
required.

Link: https://lore.kernel.org/all/174182761071.83274.18334217580449925882.stgit@devnote2/

Fixes: 57a7e6d ("tracing/fprobe: Support raw tracepoints on future loaded modules")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: stable@vger.kernel.org

Author: mhiramat

kernel/trace/trace_fprobe.c

@@ -920,13 +920,8 @@ static void __find_tracepoint_module_cb(struct tracepoint *tp, struct module *mo
 
 	if (!data->tpoint && !strcmp(data->tp_name, tp->name)) {
 		data->tpoint = tp;
-		if (!data->mod) {
+		if (!data->mod)
 			data->mod = mod;
-			if (!try_module_get(data->mod)) {
-				data->tpoint = NULL;
-				data->mod = NULL;
-			}
-		}
 	}
 }
 
@@ -938,13 +933,7 @@ static void __find_tracepoint_cb(struct tracepoint *tp, void *priv)
 		data->tpoint = tp;
 }
 
-/*
- * Find a tracepoint from kernel and module. If the tracepoint is in a module,
- * this increments the module refcount to prevent unloading until the
- * trace_fprobe is registered to the list. After registering the trace_fprobe
- * on the trace_fprobe list, the module refcount is decremented because
- * tracepoint_probe_module_cb will handle it.
- */
+/* Find a tracepoint from kernel and module. */
 static struct tracepoint *find_tracepoint(const char *tp_name,
 					  struct module **tp_mod)
 {
@@ -973,6 +962,7 @@ static void reenable_trace_fprobe(struct trace_fprobe *tf)
 	}
 }
 
+/* Find a tracepoint from specified module. */
 static struct tracepoint *find_tracepoint_in_module(struct module *mod,
 						    const char *tp_name)
 {
@@ -1179,6 +1169,11 @@ static int trace_fprobe_create_internal(int argc, const char *argv[],
 	if (is_tracepoint) {
 		ctx->flags |= TPARG_FL_TPOINT;
 		tpoint = find_tracepoint(symbol, &tp_mod);
+		/* lock module until register this tprobe. */
+		if (tp_mod && !try_module_get(tp_mod)) {
+			tpoint = NULL;
+			tp_mod = NULL;
+		}
 		if (tpoint) {
 			ctx->funcname = kallsyms_lookup(
 				(unsigned long)tpoint->probestub,