[PATCH v4] panic: add an option to replay all the printk message in buffer

[Feng Tang]

Currently on panic, kernel will lower the loglevel and print out
pending printk msg only with console_flush_on_panic().

Add an option for users to configure the "panic_print" to replay
all dmesg in buffer, some of which they may have never seen due
to the loglevel setting, which will help panic debugging .

Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
Cc: Aaro Koskinen <aaro.koskinen@xxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
---
Changelog:

  v4:
    - keep the original console_flush_on_panic() inside
      panic(), as suggested by Petr Mladek

  v3:
    - Solve the compile issue when CONFIG_PRINTK=n, found by
      Andrew Morton
    - Add enum for parameter of console_flush_on_panic()
    - move the printk replay code to console_flush_on_panic()

  v2:
    - Add a new func in printk.c dedicated for the replaying, as
      suggested by Petr Mladek
    - Combine the 2 patches in v1 into one suggested by both Petr
      and Sergey

 Documentation/admin-guide/kernel-parameters.txt | 1 +
 arch/powerpc/kernel/traps.c                     | 2 +-
 include/linux/console.h                         | 7 ++++++-
 kernel/panic.c                                  | 6 +++++-
 kernel/printk/printk.c                          | 8 +++++++-
 5 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2b8ee90..7b15c94 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3135,6 +3135,7 @@
 			bit 2: print timer info
 			bit 3: print locks info if CONFIG_LOCKDEP is on
 			bit 4: print ftrace buffer
+			bit 5: print all printk messages in buffer
 
 	panic_on_warn	panic() instead of WARN().  Useful to cause kdump
 			on a WARN().
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 1fd45a8..0fb785e 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -179,7 +179,7 @@ extern void panic_flush_kmsg_end(void)
 	kmsg_dump(KMSG_DUMP_PANIC);
 	bust_spinlocks(0);
 	debug_locks_off();
-	console_flush_on_panic();
+	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
 }
 
 static unsigned long oops_begin(struct pt_regs *regs)
diff --git a/include/linux/console.h b/include/linux/console.h
index ec9bdb3..d09951d 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -166,6 +166,11 @@ struct console {
 extern int console_set_on_cmdline;
 extern struct console *early_console;
 
+enum con_flush_mode {
+	CONSOLE_FLUSH_PENDING,
+	CONSOLE_REPLAY_ALL,
+};
+
 extern int add_preferred_console(char *name, int idx, char *options);
 extern void register_console(struct console *);
 extern int unregister_console(struct console *);
@@ -175,7 +180,7 @@ extern int console_trylock(void);
 extern void console_unlock(void);
 extern void console_conditional_schedule(void);
 extern void console_unblank(void);
-extern void console_flush_on_panic(void);
+extern void console_flush_on_panic(enum con_flush_mode mode);
 extern struct tty_driver *console_device(int *);
 extern void console_stop(struct console *);
 extern void console_start(struct console *);
diff --git a/kernel/panic.c b/kernel/panic.c
index 0ae0d73..f795feb 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -51,6 +51,7 @@ EXPORT_SYMBOL_GPL(panic_timeout);
 #define PANIC_PRINT_TIMER_INFO		0x00000004
 #define PANIC_PRINT_LOCK_INFO		0x00000008
 #define PANIC_PRINT_FTRACE_INFO		0x00000010
+#define PANIC_PRINT_ALL_PRINTK_MSG	0x00000020
 unsigned long panic_print;
 
 ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
@@ -134,6 +135,9 @@ EXPORT_SYMBOL(nmi_panic);
 
 static void panic_print_sys_info(void)
 {
+	if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
+		console_flush_on_panic(CONSOLE_REPLAY_ALL);
+
 	if (panic_print & PANIC_PRINT_TASK_INFO)
 		show_state();
 
@@ -277,7 +281,7 @@ void panic(const char *fmt, ...)
 	 * panic() is not being callled from OOPS.
 	 */
 	debug_locks_off();
-	console_flush_on_panic();
+	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
 
 	panic_print_sys_info();
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 02ca827..54f93de 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2525,10 +2525,11 @@ void console_unblank(void)
 
 /**
  * console_flush_on_panic - flush console content on panic
+ * @mode: flush all messages in buffer or just the pending ones
  *
  * Immediately output all pending messages no matter what.
  */
-void console_flush_on_panic(void)
+void console_flush_on_panic(enum con_flush_mode mode)
 {
 	/*
 	 * If someone else is holding the console lock, trylock will fail
@@ -2539,6 +2540,11 @@ void console_flush_on_panic(void)
 	 */
 	console_trylock();
 	console_may_schedule = 0;
+
+	if (mode == CONSOLE_REPLAY_ALL) {
+		console_seq = log_first_seq;
+		console_idx = log_first_idx;
+	}
 	console_unlock();
 }
 
-- 
2.7.4