switchroot: Stop making /sysroot mount private

Back in 2b8d586, /sysroot was changed to be a private mount so that
submounts of /var do not propagate back to the stateroot /var. That's
laudible, but it makes /sysroot different than every other shared mount
in the root namespace. In particular, it means that submounts of
/sysroot do not propagate into separate mount namespaces.

Rather than make /sysroot private, make /var a slave+shared mount so
that it receives mount events from /sysroot but not vice versa. That
achieves the same effect of preventing /var submount events from
propagating back to /sysroot while allowing /sysroot mount events to
propagate forward like every other system mount. See
mount_namespaces(7)[1] and the linux shared subtrees[2] documentation
for details on slave+shared mount propagation.

When /var is mounted in the initramfs, this is accomplished with
mount(2) syscalls. When /var is mounted after switching to the real
root, the mount propagation flags are applied as options in the
generated var.mount unit. This depends on a mount(8) feature that has
been present since util-linux 2.23. That's available in RHEL 7 and every
non-EOL Debian and Ubuntu release. Applying the propagation from
var.mount fixes a small race, too. Previously, if a /var submount was
added before /sysroot was made private, it would have propagated back
into /sysroot. That was possible since ostree-remount.service orders
itself after var.mount but not before any /var submounts.

1. https://man7.org/linux/man-pages/man7/mount_namespaces.7.html
2. https://docs.kernel.org/filesystems/sharedsubtree.html

Fixes: #2086

(cherry picked from commit 2973ec591008be94d74d08807079d242b914dcd2
without the test since the kola tests aren't run for Endless)

https://phabricator.endlessm.com/T35640

src/libostree/ostree-impl-system-generator.c

@@ -216,6 +216,15 @@ fstab_generator (const char *ostree_cmdline, const char *normal_dir, const char
    *
    * Note that our unit doesn't run if systemd.volatile is enabled;
    * see https://github.com/ostreedev/ostree/pull/856
+   *
+   * To avoid having submounts of /var propagate into $stateroot/var, the mount
+   * is made with slave+shared propagation. This means that /var will receive
+   * mount events from the parent /sysroot mount, but not vice versa. Adding a
+   * shared peer group below the slave group means that submounts of /var will
+   * inherit normal shared propagation. See mount_namespaces(7), Linux
+   * Documentation/filesystems/sharedsubtree.txt and
+   * https://github.com/ostreedev/ostree/issues/2086. This also happens in
+   * ostree-prepare-root.c for the INITRAMFS_MOUNT_VAR case.
    */
   if (!g_output_stream_printf (outstream, &bytes_written, cancellable, error,
                                "##\n# Automatically generated by ostree-system-generator\n##\n\n"
@@ -227,7 +236,7 @@ fstab_generator (const char *ostree_cmdline, const char *normal_dir, const char
                                "[Mount]\n"
                                "Where=%s\n"
                                "What=%s\n"
-                               "Options=bind\n",
+                               "Options=bind,slave,shared\n",
                                var_path, stateroot_var_path))
     return FALSE;
   if (!g_output_stream_flush (outstream, cancellable, error))

src/switchroot/ostree-prepare-root.c

@@ -492,6 +492,16 @@ main (int argc, char *argv[])
     {
       if (mount ("../../var", TMP_SYSROOT "/var", NULL, MS_BIND | MS_SILENT, NULL) < 0)
         err (EXIT_FAILURE, "failed to bind mount ../../var to var");
+
+      /* To avoid having submounts of /var propagate into $stateroot/var, the
+       * mount is made with slave+shared propagation. See the comment in
+       * ostree-impl-system-generator.c when /var isn't mounted in the
+       * initramfs for further explanation.
+       */
+      if (mount (NULL, TMP_SYSROOT "/var", NULL, MS_SLAVE | MS_SILENT, NULL) < 0)
+        err (EXIT_FAILURE, "failed to change /var to slave mount");
+      if (mount (NULL, TMP_SYSROOT "/var", NULL, MS_SHARED | MS_SILENT, NULL) < 0)
+        err (EXIT_FAILURE, "failed to change /var to slave+shared mount");
     }
 
   /* We only stamp /run now if we're running in an initramfs, i.e. we're
@@ -551,17 +561,6 @@ main (int argc, char *argv[])
         }
     }
 
-  /* The /sysroot mount needs to be private to avoid having a mount for e.g. /var/cache
-   * also propagate to /sysroot/ostree/deploy/$stateroot/var/cache
-   *
-   * Now in reality, today this is overridden by systemd: the *actual* way we fix this up
-   * is in ostree-remount.c.  But let's do it here to express the semantics we want
-   * at the very start (perhaps down the line systemd will have compile/runtime option
-   * to say that the initramfs environment did everything right from the start).
-   */
-  if (mount ("none", "sysroot", NULL, MS_PRIVATE | MS_SILENT, NULL) < 0)
-    err (EXIT_FAILURE, "remounting 'sysroot' private");
-
   if (running_as_pid1)
     {
       execl ("/sbin/init", "/sbin/init", NULL);

src/switchroot/ostree-remount.c

@@ -86,15 +86,6 @@ main (int argc, char *argv[])
    */
   touch_run_ostree ();
 
-  /* The /sysroot mount needs to be private to avoid having a mount for e.g. /var/cache
-   * also propagate to /sysroot/ostree/deploy/$stateroot/var/cache
-   *
-   * Today systemd remounts / (recursively) as shared, so we're undoing that as early
-   * as possible.  See also a copy of this in ostree-prepare-root.c.
-   */
-  if (mount ("none", "/sysroot", NULL, MS_REC | MS_PRIVATE, NULL) < 0)
-    perror ("warning: While remounting /sysroot MS_PRIVATE");
-
   bool root_is_composefs = false;
   struct stat stbuf;
   if (fstatat (AT_FDCWD, _OSTREE_COMPOSEFS_ROOT_STAMP, &stbuf, 0) == 0)