tracing: Fix last_cmd_set() string management in histogram code

Using strnlen(dest, str, n) is confusing, as the size of dest must be
strlen(dest) + n + 1. Even more confusing, using sizeof(string constant)
gives you strlen(string constant) + 1 and not just strlen().

These two together made using strncat() with a constant string a bit off
in the calculations as we have:

	len = sizeof(HIST_PREFIX) + strlen(str) + 1;
	kfree(last_cmd);
	last_cmd = kzalloc(len, GFP_KERNEL);
	strcpy(last_cmd, HIST_PREFIX);
	len -= sizeof(HIST_PREFIX) + 1;
	strncat(last_cmd, str, len);

The above works if we s/sizeof/strlen/ with HIST_PREFIX (which is defined
as "hist:", but because sizeof(HIST_PREFIX) is equal to
strlen(HIST_PREFIX) + 1, we can drop the +1 in the code. But at least
comment that we are doing so.

Link: https://lore.kernel.org/all/202203082112.Iu7tvFl4-lkp@intel.com/

Fixes: 9f8e5ae ("tracing: Fix allocation of last_cmd in last_cmd_set()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: rostedt

kernel/trace/trace_events_hist.c

@@ -749,14 +749,16 @@ static void last_cmd_set(struct trace_event_file *file, char *str)
 	if (!str)
 		return;
 
-	len = sizeof(HIST_PREFIX) + strlen(str) + 1;
+	/* sizeof() contains the nul byte */
+	len = sizeof(HIST_PREFIX) + strlen(str);
 	kfree(last_cmd);
 	last_cmd = kzalloc(len, GFP_KERNEL);
 	if (!last_cmd)
 		return;
 
 	strcpy(last_cmd, HIST_PREFIX);
-	len -= sizeof(HIST_PREFIX) + 1;
+	/* Again, sizeof() contains the nul byte */
+	len -= sizeof(HIST_PREFIX);
 	strncat(last_cmd, str, len);
 
 	if (file) {