printk: honor the max_reason field in kmsg_dumper

kmsg_dump() allows to dump kmesg buffer for various system events: oops,
panic, reboot, etc. It provides an interface to register a callback
call for clients, and in that callback interface there is a field
"max_reason", but it was getting ignored when set to any "reason"
higher than KMSG_DUMP_OOPS unless "always_kmsg_dump" was passed as
kernel parameter.

Allow clients to actually control their "max_reason", and keep the
current behavior when "max_reason" is not set.
Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/lkml/20200515184434.8470-3-keescook@chromium.org/Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/kmsg_dump.h

@@ -26,6 +26,7 @@ enum kmsg_dump_reason {
 	KMSG_DUMP_OOPS,
 	KMSG_DUMP_EMERG,
 	KMSG_DUMP_SHUTDOWN,
+	KMSG_DUMP_MAX
 };
 
 /**

kernel/printk/printk.c

@@ -3157,12 +3157,19 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 	struct kmsg_dumper *dumper;
 	unsigned long flags;
 
-	if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)
-		return;
-
 	rcu_read_lock();
 	list_for_each_entry_rcu(dumper, &dump_list, list) {
-		if (dumper->max_reason && reason > dumper->max_reason)
+		enum kmsg_dump_reason max_reason = dumper->max_reason;
+
+		/*
+		 * If client has not provided a specific max_reason, default
+		 * to KMSG_DUMP_OOPS, unless always_kmsg_dump was set.
+		 */
+		if (max_reason == KMSG_DUMP_UNDEF) {
+			max_reason = always_kmsg_dump ? KMSG_DUMP_MAX :
+							KMSG_DUMP_OOPS;
+		}
+		if (reason > max_reason)
 			continue;
 
 		/* initialize iterator with data about the stored records */