ostree-prepare-root: Use pivot_root if real sysroot is already mounted at /

This allows ostree-prepare-root outside of the initramfs context where the
real rootfs is already mounted at /.  We can't use `mount --move` in this
case because we would be trying to move / into a subdirectory of itself.

Closes: #403
Approved by: cgwalters

src/switchroot/ostree-prepare-root.c

@@ -33,6 +33,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/param.h>
+#include <sys/syscall.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdarg.h>
@@ -184,6 +185,12 @@ resolve_deploy_path (const char * root_mountpoint)
   return deploy_path;
 }
 
+static int
+pivot_root(const char * new_root, const char * put_old)
+{
+  return syscall(__NR_pivot_root, new_root, put_old);
+}
+
 int
 main(int argc, char *argv[])
 {
@@ -197,6 +204,7 @@ main(int argc, char *argv[])
   else
     root_mountpoint = argv[1];
 
+  root_mountpoint = realpath (root_mountpoint, NULL);
   deploy_path = resolve_deploy_path (root_mountpoint);
 
   /* Work-around for a kernel bug: for some reason the kernel
@@ -292,6 +300,22 @@ main(int argc, char *argv[])
 
   touch_run_ostree ();
 
+  if (strcmp(root_mountpoint, "/") == 0)
+    {
+      /* pivot_root rotates two mount points around.  In this instance . (the
+       * deploy location) becomes / and the existing / becomes /sysroot.  We
+       * have to use pivot_root rather than mount --move in this instance
+       * because our deploy location is mounted as a subdirectory of the real
+       * sysroot, so moving sysroot would also move the deploy location.   In
+       * reality attempting mount --move would fail with EBUSY. */
+      if (pivot_root (".", "sysroot") < 0)
+        {
+          perrorv ("failed to pivot_root to deployment");
+          exit (EXIT_FAILURE);
+        }
+    }
+  else
+    {
       /* In this instance typically we have our ready made-up up root at
        * /sysroot/ostree/deploy/.../ (deploy_path) and the real rootfs at
        * /sysroot (root_mountpoint).  We want to end up with our made-up root at
@@ -328,6 +352,7 @@ main(int argc, char *argv[])
           perrorv ("failed to MS_MOVE %s to %s", deploy_path, root_mountpoint);
           exit (EXIT_FAILURE);
         }
+    }
 
   if (getpid() == 1)
     {

tests/test-switchroot.sh

@@ -76,12 +76,32 @@ test_that_prepare_root_sets_sysroot_up_correctly_with_initrd() {
 	echo "ok ostree-prepare-root sets sysroot up correctly with initrd"
 }
 
+setup_no_initrd_env() {
+	mount --bind "$1" "$1"
+	setup_rootfs "$1"
+	setup_bootfs "$1"
+}
+
+test_that_prepare_root_sets_root_up_correctly_with_no_initrd() {
+	find_in_env setup_no_initrd_env >files
+
+	grep -qx "/this_is_ostree_root" files
+	grep -qx "/sysroot/this_is_bootfs" files
+	grep -qx "/sysroot/this_is_real_root" files
+	grep -qx "/var/this_is_ostree_var" files
+	grep -qx "/usr/this_is_ostree_usr" files
+
+	grep -qx "/usr is not writable" files
+	echo "ok ostree-prepare-root sets root up correctly with no initrd"
+}
+
 # This script sources itself so we only want to run tests if we're the parent:
 if [ "${BASH_SOURCE[0]}" = "${0}" ]; then
 	. $(dirname $0)/libtest.sh
 	unshare -m true || \
 	    skip "this test needs to set up mount namespaces, rerun as root"
 
-	echo "1..1"
+	echo "1..2"
 	test_that_prepare_root_sets_sysroot_up_correctly_with_initrd
+	test_that_prepare_root_sets_root_up_correctly_with_no_initrd
 fi