panic/reboot: allow specifying reboot_mode for panic only

Allow specifying reboot_mode for panic only.  This is needed on systems
where ramoops is used to store panic logs, and user wants to use warm
reset to preserve those, while still having cold reset on normal
reboots.

Link: http://lkml.kernel.org/r/20190322004735.27702-1-aaro.koskinen@iki.fiSigned-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4064,7 +4064,9 @@
 				[[,]s[mp]#### \
 				[[,]b[ios] | a[cpi] | k[bd] | t[riple] | e[fi] | p[ci]] \
 				[[,]f[orce]
-			Where reboot_mode is one of warm (soft) or cold (hard) or gpio,
+			Where reboot_mode is one of warm (soft) or cold (hard) or gpio
+					(prefix with 'panic_' to set mode for panic
+					reboot only),
 			      reboot_type is one of bios, acpi, kbd, triple, efi, or pci,
 			      reboot_force is either force or not specified,
 			      reboot_cpu is s[mp]#### with #### being the processor

include/linux/reboot.h

@@ -14,6 +14,7 @@ struct device;
 #define SYS_POWER_OFF	0x0003	/* Notify of system power off */
 
 enum reboot_mode {
+	REBOOT_UNDEFINED = -1,
 	REBOOT_COLD = 0,
 	REBOOT_WARM,
 	REBOOT_HARD,
@@ -21,6 +22,7 @@ enum reboot_mode {
 	REBOOT_GPIO,
 };
 extern enum reboot_mode reboot_mode;
+extern enum reboot_mode panic_reboot_mode;
 
 enum reboot_type {
 	BOOT_TRIPLE	= 't',

kernel/panic.c

@@ -306,6 +306,8 @@ void panic(const char *fmt, ...)
 		 * shutting down.  But if there is a chance of
 		 * rebooting the system it will be rebooted.
 		 */
+		if (panic_reboot_mode != REBOOT_UNDEFINED)
+			reboot_mode = panic_reboot_mode;
 		emergency_restart();
 	}
 #ifdef __sparc__

kernel/reboot.c

@@ -31,6 +31,7 @@ EXPORT_SYMBOL(cad_pid);
 #define DEFAULT_REBOOT_MODE
 #endif
 enum reboot_mode reboot_mode DEFAULT_REBOOT_MODE;
+enum reboot_mode panic_reboot_mode = REBOOT_UNDEFINED;
 
 /*
  * This variable is used privately to keep track of whether or not
@@ -519,6 +520,8 @@ EXPORT_SYMBOL_GPL(orderly_reboot);
 static int __init reboot_setup(char *str)
 {
 	for (;;) {
+		enum reboot_mode *mode;
+
 		/*
 		 * Having anything passed on the command line via
 		 * reboot= will cause us to disable DMI checking
@@ -526,17 +529,24 @@ static int __init reboot_setup(char *str)
 		 */
 		reboot_default = 0;
 
+		if (!strncmp(str, "panic_", 6)) {
+			mode = &panic_reboot_mode;
+			str += 6;
+		} else {
+			mode = &reboot_mode;
+		}
+
 		switch (*str) {
 		case 'w':
-			reboot_mode = REBOOT_WARM;
+			*mode = REBOOT_WARM;
 			break;
 
 		case 'c':
-			reboot_mode = REBOOT_COLD;
+			*mode = REBOOT_COLD;
 			break;
 
 		case 'h':
-			reboot_mode = REBOOT_HARD;
+			*mode = REBOOT_HARD;
 			break;
 
 		case 's':
@@ -553,11 +563,11 @@ static int __init reboot_setup(char *str)
 				if (rc)
 					return rc;
 			} else
-				reboot_mode = REBOOT_SOFT;
+				*mode = REBOOT_SOFT;
 			break;
 		}
 		case 'g':
-			reboot_mode = REBOOT_GPIO;
+			*mode = REBOOT_GPIO;
 			break;
 
 		case 'b':