From a5bb76b2db7b6093cc1f350d75c810d9fab6d993 Mon Sep 17 00:00:00 2001
From: Thadeu Lima de Souza Cascardo <cascardo@cascardo.eti.br>
Date: Thu, 20 Oct 2016 10:02:53 +0000
Subject: [PATCH] Mount root and exec GNU bash.

Mounts a specific partition as root, chroots, then execs GNU bash.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@cascardo.eti.br>
---
 main.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 73 insertions(+), 9 deletions(-)

diff --git a/main.c b/main.c
index f370220..b910217 100644
--- a/main.c
+++ b/main.c
@@ -21,11 +21,12 @@
 #include <stdlib.h>
 #include <fcntl.h>
 #include <sys/ioctl.h>
-#include <sys/mman.h>
+#include <sys/mount.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <stdlib.h>
 #include <string.h>
+#include <errno.h>
 
 #define PREFIX "/sys/class/android_usb/android0/"
 
@@ -43,17 +44,61 @@ int do_read(char *fname, char *buffer, size_t len)
 	int fd;
 	int r;
 	fd = open(fname, O_RDONLY);
+	if (fd < 0) {
+		fprintf(stderr, "failed to open %s: %s\n", fname,
+			strerror(errno));
+		return -1;
+	}
 	r = read(fd, buffer, len);
 	close(fd);
 	return r;
 }
 
-int main(int argc, char **argv)
+int
+do_mknod(char *path, char *name, int block, int wait)
 {
+	char *fullpath;
+	size_t len;
 	int r;
-	int fd = -1;
 	char dev[33];
 	int maj, min;
+	len = strlen(path) + strlen(name) + 6;
+	fullpath = malloc(len);
+	if (!fullpath) {
+		fprintf(stderr, "failed to allocate buffer\n");
+		return -1;
+	}
+	snprintf(fullpath, len, "%s/%s/dev", path, name);
+	do {
+		r = do_read(fullpath, dev, sizeof(dev));
+		if (r < 0 && !wait) {
+			fprintf(stderr, "failed to read %s: %s\n", fullpath,
+					strerror(errno));
+			goto out;
+		}
+	} while (r < 0 && wait);
+	r = sscanf(dev, "%d:%d", &maj, &min);
+	if (r < 0) {
+		fprintf(stderr, "failed to read devno from %s\n", dev);
+		goto out;
+	}
+	snprintf(fullpath, len, "/dev/%s", name);
+	r = mknod(fullpath, 0666 | (block ? S_IFBLK : S_IFCHR), makedev(maj, min));
+	if (r < 0) {
+		fprintf(stderr, "failed to mknod %s: %s\n", fullpath,
+				strerror(errno));
+	}
+out:
+	free(fullpath);
+	return r;
+}
+
+int main(int argc, char **argv)
+{
+	int r;
+	int fd = -1;
+	int i;
+	char *args[] = { "bash", "-l", NULL };
 	r = devmount_setup();
 	if (r < 0) {
 		fprintf(stderr, "failed to mount devtmpfs, proceeding anyway\n");
@@ -68,15 +113,34 @@ int main(int argc, char **argv)
 	do_write(PREFIX "/f_acm/acm_transports", "tty");
 	do_write(PREFIX "/functions", "acm");
 	do_write(PREFIX "/enable", "1");
-	while (do_read("/sys/class/tty/ttyGS0/dev", dev, sizeof(dev)) < 0) {
-		sleep(1);
-	}
-	sscanf(dev, "%d:%d", &maj, &min);
-	mknod("/dev/ttyGS0", 0666 | S_IFCHR, makedev(maj, min));
+	do_mknod("/sys/class/tty", "ttyGS0", 0, 1);
 	fd = open("/dev/ttyGS0", O_RDWR);
-	while (1) {
+	for (i = 0; i < 10; i++) {
 		write(fd, "Hello World!\n", 13);
 		sleep(1);
 	}
+	write(fd, "G\n", 2);
+	dup2(fd, 0);
+	dup2(fd, 1);
+	dup2(fd, 2);
+	printf("N\n");
+	r = do_mknod("/sys/block/mmcblk1", "mmcblk1p3", 1, 1);
+	if (r < 0) {
+		fprintf(stderr, "Failed to create mmcblk1p3\n");
+	}
+	printf("U\n");
+	printf("Mounting root\n");
+	r = mkdir("/root", 0755);
+	if (r) printf("failed mkdir\n");
+	r = mount("/dev/mmcblk1p3", "/root", "ext4", MS_NOATIME, NULL);
+	if (r) printf("failed mount: %d\n", errno);
+	printf("Chroot and chdir\n");
+	r = chdir("/root");
+	if (r) printf("failed first chdir\n");
+	r = chroot("/root");
+	if (r) printf("failed chroot\n");
+	r = chdir("/");
+	if (r) printf("failed second chdir\n");
+	execve("/bin/bash", args, NULL);
 	return 0;
 }
-- 
2.20.1