From: Steven Rostedt (VMware) <rostedt@goodmis.org>
Date: Fri, 30 Aug 2019 20:30:01 +0000 (-0400)
Subject: ftrace: Check for empty hash and comment the race with registering probes
X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=372e0d01da71c84dcecf7028598a33813b0d5256;p=linux.git

ftrace: Check for empty hash and comment the race with registering probes

The race between adding a function probe and reading the probes that exist
is very subtle. It needs a comment. Also, the issue can also happen if the
probe has has the EMPTY_HASH as its func_hash.

Cc: stable@vger.kernel.org
Fixes: 7b60f3d876156 ("ftrace: Dynamically create the probe ftrace_ops for the trace_array")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 80beed2cf0da0..6200a6fe10e3b 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3096,7 +3096,11 @@ t_probe_next(struct seq_file *m, loff_t *pos)
 
 	hash = iter->probe->ops.func_hash->filter_hash;
 
-	if (!hash)
+	/*
+	 * A probe being registered may temporarily have an empty hash
+	 * and it's at the end of the func_probes list.
+	 */
+	if (!hash || hash == EMPTY_HASH)
 		return NULL;
 
 	size = 1 << hash->size_bits;
@@ -4324,6 +4328,10 @@ register_ftrace_function_probe(char *glob, struct trace_array *tr,
 
 	mutex_unlock(&ftrace_lock);
 
+	/*
+	 * Note, there's a small window here that the func_hash->filter_hash
+	 * may be NULL or empty. Need to be carefule when reading the loop.
+	 */
 	mutex_lock(&probe->ops.func_hash->regex_lock);
 
 	orig_hash = &probe->ops.func_hash->filter_hash;