InitFatalReboot: Trigger panic explicitly for init_fatal_panic

The exit of init panics the system *after* process context (mm, stack,
...etc.) are recycled, according to Linux kernel's 'do_exit'
implementation. To preserve most init process context for debugging,
triggers the panic via proc-sysrq explicitly.

Note: after this change, there will be no "Attempt to kill init" panic
when androidboot.init_fatal_panic is set.

Test: Insert data abort fault in init, the full process context is
      preserved in memory dump captured after panic.
Bug: 155940351
Change-Id: I3393bd00f99b8cb432cfa19a105b7d636b411764
(cherry picked from commit be1cf9006a)

init/reboot.cpp

@@ -66,8 +66,6 @@
 #include "sigchld_handler.h"
 #include "util.h"
 
-#define PROC_SYSRQ "/proc/sysrq-trigger"
-
 using namespace std::literals;
 
 using android::base::boot_clock;

init/reboot_utils.cpp

@@ -29,6 +29,7 @@
 #include <cutils/android_reboot.h>
 
 #include "capabilities.h"
+#include "reboot_utils.h"
 
 namespace android {
 namespace init {
@@ -138,6 +139,9 @@ void __attribute__((noreturn)) InitFatalReboot(int signal_number) {
         LOG(ERROR) << backtrace->FormatFrameData(i);
     }
     if (init_fatal_panic) {
+        LOG(ERROR) << __FUNCTION__ << ": Trigger crash";
+        android::base::WriteStringToFile("c", PROC_SYSRQ);
+        LOG(ERROR) << __FUNCTION__ << ": Sys-Rq failed to crash the system; fallback to exit().";
         _exit(signal_number);
     }
     RebootSystem(ANDROID_RB_RESTART2, init_fatal_reboot_target);

init/reboot_utils.h

@@ -18,6 +18,8 @@
 
 #include <string>
 
+#define PROC_SYSRQ "/proc/sysrq-trigger"
+
 namespace android {
 namespace init {