Save a copy of kernel log to /cache during Android init. NOTE: this code is NOT complete!

kmsg-from-init.diff

diff --git a/init/reboot_utils.cpp b/init/reboot_utils.cpp
index de085cc9b..80edeb237 100644
--- a/init/reboot_utils.cpp
+++ b/init/reboot_utils.cpp
@@ -33,6 +33,75 @@
 namespace android {
 namespace init {
 
+extern "C" {
+    #include <errno.h>
+    #include <sys/mount.h>
+    #include <sys/stat.h>
+    #include <sys/types.h>
+    #include <sys/klog.h>
+    #include <unistd.h>
+    #include <string.h>
+    #include <stdlib.h>
+    #include <fcntl.h>
+
+    /* Must be an existing directory */
+    #define CACHE_TMP_PATH "/cache"
+    #define KLOG_FILE "/cache/dmesg"
+    #define SYSLOG_BUF_SIZE (2*1024*1024)
+
+    void copy_dmesg_to_cache(const char *cache_dev, const char *fs) {
+        int ret = 0;
+        struct stat statbuf;
+        char *syslog_buf = NULL;
+        int syslog_len = -1;
+        int fd_to = -1;
+
+        if (fs == NULL) {
+            fs = "ext4";
+        }
+
+        memset(&statbuf, 0, sizeof(statbuf));
+        ret = stat(CACHE_TMP_PATH, &statbuf);
+        if (ret != 0 && errno == -ENOENT) {
+            /* Error and it does exist */
+            goto error;
+        } else if (!(statbuf.st_mode & S_IFDIR)) {
+            /* No error or it does not exist or it's a file */
+            unlink(CACHE_TMP_PATH);
+        }
+
+        ret = mount(cache_dev, CACHE_TMP_PATH, fs, 0, NULL);
+        if (ret != 0) {
+            goto error;
+        }
+
+        /* Must be read at once */
+        syslog_buf = (char *)malloc(SYSLOG_BUF_SIZE);
+        if (syslog_buf == NULL) {
+            goto error;
+        }
+
+        if ((syslog_len = klogctl(3 /* SYSLOG_ACTION_READ_ALL */, syslog_buf, SYSLOG_BUF_SIZE)) < 0) {
+            goto error;
+        }
+
+        fd_to = open(KLOG_FILE, O_WRONLY | O_CREAT | O_EXCL, 0666);
+        if (fd_to < 0) {
+            goto error;
+        }
+
+        write(fd_to, syslog_buf, syslog_len);
+        close(fd_to);
+        umount(CACHE_TMP_PATH);
+
+    error:
+        if (syslog_buf) {
+            free(syslog_buf);
+        }
+        return;
+    }
+}
+
 static std::string init_fatal_reboot_target = "bootloader";
 
 void SetFatalRebootTarget() {
@@ -150,6 +219,7 @@ void InstallRebootSignalHandlers() {
             _exit(signal);
         }
 
+        copy_dmesg_to_cache("/dev/block/mmcblk0p6", "ext4");
         // Calling DoReboot() or LOG(FATAL) is not a good option as this is a signal handler.
         // RebootSystem uses syscall() which isn't actually async-signal-safe, but our only option
         // and probably good enough given this is already an error case and only enabled for