[PATCH v3 1/6] printk: honor the max_reason field in kmsg_dumper

[Kees Cook]

From: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>

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"
which gets ignored unless always_kmsg_dump is passed as kernel parameter.

Allow clients to decide max_reason, and keep the current behavior when
max_reason is not set.

Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20200505154510.93506-2-pasha.tatashin@xxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/kmsg_dump.h |  1 +
 kernel/printk/printk.c    | 15 +++++++++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/linux/kmsg_dump.h b/include/linux/kmsg_dump.h
index 2e7a1e032c71..cfc042066be7 100644
--- a/include/linux/kmsg_dump.h
+++ b/include/linux/kmsg_dump.h
@@ -28,6 +28,7 @@ enum kmsg_dump_reason {
 	KMSG_DUMP_RESTART,
 	KMSG_DUMP_HALT,
 	KMSG_DUMP_POWEROFF,
+	KMSG_DUMP_MAX
 };
 
 /**
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9a9b6156270b..1aab69a8a2bf 100644
--- a/kernel/printk/printk.c
+++ b/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 cur_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 (cur_reason == KMSG_DUMP_UNDEF) {
+			cur_reason = always_kmsg_dump ? KMSG_DUMP_MAX :
+							KMSG_DUMP_OOPS;
+		}
+		if (reason > cur_reason)
 			continue;
 
 		/* initialize iterator with data about the stored records */
-- 
2.20.1