tracing: Have process_string() also allow arrays

JIRA: https://issues.redhat.com/browse/RHEL-86682

commit afc6717
Author: Steven Rostedt <rostedt@goodmis.org>
Date:   Tue Dec 31 00:06:46 2024 -0500

    tracing: Have process_string() also allow arrays

    In order to catch a common bug where a TRACE_EVENT() TP_fast_assign()
    assigns an address of an allocated string to the ring buffer and then
    references it in TP_printk(), which can be executed hours later when the
    string is free, the function test_event_printk() runs on all events as
    they are registered to make sure there's no unwanted dereferencing.

    It calls process_string() to handle cases in TP_printk() format that has
    "%s". It returns whether or not the string is safe. But it can have some
    false positives.

    For instance, xe_bo_move() has:

     TP_printk("move_lacks_source:%s, migrate object %p [size %zu] from %s to %s device_id:%s",
                __entry->move_lacks_source ? "yes" : "no", __entry->bo, __entry->size,
                xe_mem_type_to_name[__entry->old_placement],
                xe_mem_type_to_name[__entry->new_placement], __get_str(device_id))

    Where the "%s" references into xe_mem_type_to_name[]. This is an array of
    pointers that should be safe for the event to access. Instead of flagging
    this as a bad reference, if a reference points to an array, where the
    record field is the index, consider it safe.

    Link: https://lore.kernel.org/all/9dee19b6185d325d0e6fa5f7cbba81d007d99166.camel@sapience.com/

    Cc: stable@vger.kernel.org
    Cc: Masami Hiramatsu <mhiramat@kernel.org>
    Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Link: https://lore.kernel.org/20241231000646.324fb5f7@gandalf.local.home
    Fixes: 65a25d9 ("tracing: Add "%s" check in test_event_printk()")
    Reported-by: Genes Lists <lists@sapience.com>
    Tested-by: Gene C <arch@sapience.com>
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/trace/trace_events.c

@@ -364,6 +364,18 @@ static bool process_string(const char *fmt, int len, struct trace_event_call *ca
 		s = r + 1;
 	} while (s < e);
 
+	/*
+	 * Check for arrays. If the argument has: foo[REC->val]
+	 * then it is very likely that foo is an array of strings
+	 * that are safe to use.
+	 */
+	r = strstr(s, "[");
+	if (r && r < e) {
+		r = strstr(r, "REC->");
+		if (r && r < e)
+			return true;
+	}
+
 	/*
 	 * If there's any strings in the argument consider this arg OK as it
 	 * could be: REC->field ? "foo" : "bar" and we don't want to get into