Switch to using a systemd generator for /var

If one wants to set up a mount for `/var` in `/etc/fstab`, it
won't be mounted since `ostree-prepare-root` set up a bind mount for
`/var` to `/sysroot/ostree/$stateroot/var`, and systemd will take
the already extant mount over what's in `/etc/fstab`.

There are a few options to fix this, but what I settled on is parsing
`/etc/fstab` in a generator (exactly like `systemd-fstab-generator` does),
except here we look for an explicit mount for `/var`, and if one *isn't* found,
synthesize the default ostree mount to the stateroot. Another nice property is
that if an admin creates a `var.mount` unit in `/etc` for example, that will
also override our mount.

Note that today ostree doesn't hard depend on systemd, so this behavior only
kicks in if we're built with systemd *and* libmount support (for parsing
`/etc/fstab`).  I didn't really test that case though.

Initially I started writing this as a "pure libc" program, but at one point
decided to use `libostree.so` to find the booted deployment. That didn't work
out because `/boot` wasn't necessarily mounted and hence we couldn't find the
bootloader config. A leftover artifact from this is that the generator code
calls into libostree via the "cmd private" infrastructure. But it's an easy way
to share code, and doesn't hurt.

Closes: #859
Approved by: jlebon

src/switchroot/ostree-mount-util.h

@@ -25,6 +25,10 @@
 #include <err.h>
 #include <stdlib.h>
 #include <sys/statvfs.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <string.h>
 
 static inline int
 path_is_on_readonly_fs (char *path)
@@ -37,4 +41,66 @@ path_is_on_readonly_fs (char *path)
   return (stvfsbuf.f_flag & ST_RDONLY) != 0;
 }
 
+static inline char *
+read_proc_cmdline (void)
+{
+  FILE *f = fopen("/proc/cmdline", "r");
+  char *cmdline = NULL;
+  size_t len;
+
+  if (!f)
+    goto out;
+
+  /* Note that /proc/cmdline will not end in a newline, so getline
+   * will fail unelss we provide a length.
+   */
+  if (getline (&cmdline, &len, f) < 0)
+    goto out;
+  /* ... but the length will be the size of the malloc buffer, not
+   * strlen().  Fix that.
+   */
+  len = strlen (cmdline);
+
+  if (cmdline[len-1] == '\n')
+    cmdline[len-1] = '\0';
+out:
+  if (f)
+    fclose (f);
+  return cmdline;
+}
+
+static inline char *
+read_proc_cmdline_ostree (void)
+{
+  char *cmdline = NULL;
+  const char *iter;
+  char *ret = NULL;
+
+  cmdline = read_proc_cmdline ();
+  if (!cmdline)
+    err (EXIT_FAILURE, "failed to read /proc/cmdline");
+
+  iter = cmdline;
+  while (iter != NULL)
+    {
+      const char *next = strchr (iter, ' ');
+      const char *next_nonspc = next;
+      while (next_nonspc && *next_nonspc == ' ')
+        next_nonspc += 1;
+      if (strncmp (iter, "ostree=", strlen ("ostree=")) == 0)
+        {
+          const char *start = iter + strlen ("ostree=");
+          if (next)
+            ret = strndup (start, next - start);
+          else
+            ret = strdup (start);
+          break;
+        }
+      iter = next_nonspc;
+    }
+
+  free (cmdline);
+  return ret;
+}
+
 #endif /* __OSTREE_MOUNT_UTIL_H_ */

src/switchroot/ostree-prepare-root.c

@@ -46,68 +46,6 @@
 
 #include "ostree-mount-util.h"
 
-static char *
-read_proc_cmdline (void)
-{
-  FILE *f = fopen("/proc/cmdline", "r");
-  char *cmdline = NULL;
-  size_t len;
-
-  if (!f)
-    goto out;
-
-  /* Note that /proc/cmdline will not end in a newline, so getline
-   * will fail unelss we provide a length.
-   */
-  if (getline (&cmdline, &len, f) < 0)
-    goto out;
-  /* ... but the length will be the size of the malloc buffer, not
-   * strlen().  Fix that.
-   */
-  len = strlen (cmdline);
-
-  if (cmdline[len-1] == '\n')
-    cmdline[len-1] = '\0';
-out:
-  if (f)
-    fclose (f);
-  return cmdline;
-}
-
-static char *
-parse_ostree_cmdline (void)
-{
-  char *cmdline = NULL;
-  const char *iter;
-  char *ret = NULL;
-
-  cmdline = read_proc_cmdline ();
-  if (!cmdline)
-    err (EXIT_FAILURE, "failed to read /proc/cmdline");
-
-  iter = cmdline;
-  while (iter != NULL)
-    {
-      const char *next = strchr (iter, ' ');
-      const char *next_nonspc = next;
-      while (next_nonspc && *next_nonspc == ' ')
-        next_nonspc += 1;
-      if (strncmp (iter, "ostree=", strlen ("ostree=")) == 0)
-        {
-          const char *start = iter + strlen ("ostree=");
-          if (next)
-            ret = strndup (start, next - start);
-          else
-            ret = strdup (start);
-          break;
-        }
-      iter = next_nonspc;
-    }
-
-  free (cmdline);
-  return ret;
-}
-
 /* This is an API for other projects to determine whether or not the
  * currently running system is ostree-controlled.
  */
@@ -132,7 +70,7 @@ resolve_deploy_path (const char * root_mountpoint)
   struct stat stbuf;
   char *ostree_target, *deploy_path;
 
-  ostree_target = parse_ostree_cmdline ();
+  ostree_target = read_proc_cmdline_ostree ();
   if (!ostree_target)
     errx (EXIT_FAILURE, "No OSTree target; expected ostree=/ostree/boot.N/...");
 
@@ -211,9 +149,12 @@ main(int argc, char *argv[])
   if (chdir (deploy_path) < 0)
     err (EXIT_FAILURE, "failed to chdir to deploy_path");
 
+  /* In the systemd case, this is handled by ostree-system-generator */
+#ifndef HAVE_SYSTEMD_AND_LIBMOUNT
   /* Link to the deployment's /var */
   if (mount ("../../var", "var", NULL, MS_MGC_VAL|MS_BIND, NULL) < 0)
     err (EXIT_FAILURE, "failed to bind mount ../../var to var");
+#endif
 
   /* If /boot is on the same partition, use a bind mount to make it visible
    * at /boot inside the deployment. */

src/switchroot/ostree-system-generator.c

@@ -0,0 +1,67 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2017 Colin Walters <walters@verbum.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#include "config.h"
+
+#include <err.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdlib.h>
+
+#include "ostree-cmdprivate.h"
+#include "ostree-mount-util.h"
+
+static const char *arg_dest = "/tmp";
+static const char *arg_dest_late = "/tmp";
+
+/* This program is a simple stub that calls the implementation that
+ * lives inside libostree.
+ */
+int
+main(int argc, char *argv[])
+{
+  /* Important: if this isn't an ostree-booted system, do nothing; people could
+   * have the package installed as a dependency for flatpak or whatever.
+   */
+  { struct stat stbuf;
+    if (fstatat (AT_FDCWD, "/run/ostree-booted", &stbuf, 0) < 0)
+      exit (EXIT_SUCCESS);
+  }
+
+  if (argc > 1 && argc != 4)
+    errx (EXIT_FAILURE, "This program takes three or no arguments");
+
+  if (argc > 1)
+    arg_dest = argv[1];
+  if (argc > 3)
+    arg_dest_late = argv[3];
+
+  char *ostree_cmdline = read_proc_cmdline_ostree ();
+  if (!ostree_cmdline)
+    errx (EXIT_FAILURE, "Failed to find ostree= kernel argument");
+
+  { g_autoptr(GError) local_error = NULL;
+    if (!ostree_cmd__private__()->ostree_system_generator (ostree_cmdline, arg_dest, NULL, arg_dest_late, &local_error))
+      errx (EXIT_FAILURE, "%s", local_error->message);
+  }
+
+  exit (EXIT_SUCCESS);
+}