opkrun: Don't rely on external program for mounting

When calling the external mount program, the child process we create by
the fork() system call may not have the right to mount devices or files,
for instance in the case where capabilities are used.

Signed-off-by: Paul Cercueil <[email protected]>

Author: pcercuei

opkrun.c

@@ -3,6 +3,8 @@
 
 #include "opk.h"
 
+#include <errno.h>
+#include <fcntl.h>
 #include <getopt.h>
 #include <limits.h>
 #include <stdbool.h>
@@ -11,9 +13,12 @@
 #include <string.h>
 #include <sys/mount.h>
 #include <sys/stat.h>
+#include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
+#include <linux/loop.h>
+
 #ifndef MY_NAME
 #define MY_NAME "opkrun"
 #endif
@@ -232,6 +237,61 @@ static char *get_url(const char *file)
 	return tmp;
 }
 
+static int logetfree(void)
+{
+	int fd = open("/dev/loop-control", O_RDWR);
+	if (fd < 0) {
+		fprintf(stderr, "Failed to open '/dev/loop-control': %d\n", fd);
+		return -1;
+	}
+
+	int devnr = ioctl(fd, LOOP_CTL_GET_FREE, NULL);
+	if (devnr < 0) {
+		fprintf(stderr, "Failed to acquire free loop device: %d\n", devnr);
+	}
+
+	close(fd);
+	return devnr;
+}
+
+static int losetup(const char *loop, const char *file)
+{
+	unsigned int i;
+	int filefd, loopfd, ret;
+
+	filefd = open(file, O_RDONLY);
+	if (filefd < 0) {
+		fprintf(stderr, "losetup: cannot open '%s': %d\n", file, filefd);
+		return -1;
+	}
+
+	/* We try to open the loop device just a bit after it was created.
+	 * Give some time to udev so that it can set the proper rights. */
+	for (i = 0; i < 100; i++) {
+		loopfd = open(loop, O_RDONLY);
+		if (loopfd < 0 && errno == EACCES)
+			usleep(10000);
+		else
+			break;
+	}
+	if (loopfd < 0) {
+		fprintf(stderr, "losetup: cannot open '%s': %d\n", loop, loopfd);
+		close(filefd);
+		return loopfd;
+	}
+
+	ret = ioctl(loopfd, LOOP_SET_FD, (void *)(intptr_t)filefd);
+	if (ret < 0) {
+		fprintf(stderr, "Cannot setup loop device '%s': %d\n", loop, ret);
+		close(loopfd);
+		close(filefd);
+		return ret;
+	}
+
+	close(filefd);
+	return loopfd;
+}
+
 int main(int argc, char **argv)
 {
 	if (argc < 2) {
@@ -328,10 +388,20 @@ int main(int argc, char **argv)
 	umount(params.mountpoint);
 	mkdir(params.mountpoint, 0755);
 
-	char buf[256];
-	sprintf(buf, "mount -o loop,nodev,nosuid,ro \'%s\' \'%s\' >/dev/null 2>&1",
-				opk_name, params.mountpoint);
-	ret = system(buf);
+	int devnr = logetfree();
+	if (devnr < 0)
+		return devnr;
+
+	char loop_dev[9 + 10 + 1];
+	sprintf(loop_dev, "/dev/loop%i", devnr);
+
+	int loopfd = losetup(loop_dev, opk_name);
+	if (loopfd < 0) {
+		perror("Failed to losetup");
+		return loopfd;
+	}
+
+	ret = mount(loop_dev, params.mountpoint, "squashfs", MS_NODEV | MS_NOSUID | MS_RDONLY, 0);
 	if (ret < 0) {
 		perror("Unable to mount OPK");
 		free(params.mountpoint);
@@ -370,6 +440,9 @@ int main(int argc, char **argv)
 	rmdir(params.mountpoint);
 	free(params.mountpoint);
 
+	ioctl(loopfd, LOOP_CLR_FD, (void *)0);
+	close(loopfd);
+
 	for (char **ptr = args; *ptr; ptr++)
 		free(*ptr);