deploy: Create fake symlinks when on FAT filesystems

When trying to deploy on a vfat partitions, such as the EFI ESP, we
fail to create the loader symlink, and deployment fails.

We'd like to use sd-boot, which only works with the ESP, so we need
a way to deploy without using symlinks.

For a quick and dirty minimally invasive fix, I've made deployment
create a bogus symlink - a text file named loader.sln that contains
the name of the real loader dir.

I've modified systemd-boot to read these bogus symlinks as well.

https://phabricator.endlessm.com/T27040

src/libostree/ostree-sysroot-deploy.c

@@ -31,6 +31,8 @@
 #include <sys/poll.h>
 #include <linux/fs.h>
 #include <err.h>
+#include <sys/statfs.h>
+#include <linux/magic.h>
 
 #ifdef HAVE_LIBMOUNT
 #include <libmount.h>
@@ -59,6 +61,35 @@
 static gboolean
 is_ro_mount (const char *path);
 
+static gboolean
+fs_is_fat (int parent_dfd)
+{
+  struct statfs buf;
+
+  if (TEMP_FAILURE_RETRY (fstatfs (parent_dfd, &buf)) < 0)
+    return FALSE;
+
+  if (buf.f_type == MSDOS_SUPER_MAGIC)
+    return TRUE;
+
+  return FALSE;
+}
+
+static gboolean
+symlink_fat (const char    *target,
+             int            parent_dfd,
+             const char    *linkpath,
+             GCancellable  *cancellable,
+             GError       **error)
+{
+  if (!glnx_file_replace_contents_at(parent_dfd, linkpath, (guint8*)target,
+                                     -1, GLNX_FILE_REPLACE_DATASYNC_NEW,
+                                     cancellable, error))
+    return FALSE;
+
+  return TRUE;
+}
+
 /*
  * Like symlinkat() but overwrites (atomically) an existing
  * symlink.
@@ -70,6 +101,9 @@ symlink_at_replace (const char    *oldpath,
                     GCancellable  *cancellable,
                     GError       **error)
 {
+  g_autofree char *dest_dir = g_path_get_dirname(newpath);
+  glnx_autofd int dest_dir_dfd = -1;
+  gboolean fat;
   /* Possibly in the future generate a temporary random name here,
    * would need to move "generate a temporary name" code into
    * libglnx or glib?
@@ -79,12 +113,35 @@ symlink_at_replace (const char    *oldpath,
   /* Clean up any stale temporary links */
   (void) unlinkat (parent_dfd, temppath, 0);
 
-  /* Create the temp link */
-  if (TEMP_FAILURE_RETRY (symlinkat (oldpath, parent_dfd, temppath)) < 0)
-    return glnx_throw_errno_prefix (error, "symlinkat");
+  /* Clean up any stale FAT "symlinks" */
+  g_autofree char *temp_fat_path = g_strconcat (temppath, ".sln", NULL);
+  (void) unlinkat (parent_dfd, temp_fat_path, 0);
+
+  if (!glnx_opendirat (parent_dfd, dest_dir, TRUE, &dest_dir_dfd, error))
+    return FALSE;
 
+  fat = fs_is_fat (dest_dir_dfd);
+
+  /* Create the temp link */
+  if (!fat)
+    {
+      if (TEMP_FAILURE_RETRY (symlinkat (oldpath, parent_dfd, temppath)) < 0)
+        return glnx_throw_errno_prefix (error, "symlinkat");
+    }
+  else
+    {
+      if (!symlink_fat (oldpath, parent_dfd, temp_fat_path, cancellable, error))
+        return glnx_throw_errno_prefix (error, "symlinkat");
+    }
   /* Rename it into place */
-  if (!glnx_renameat (parent_dfd, temppath, parent_dfd, newpath, error))
+  if (fat)
+    {
+      g_autofree char *new_fat_path = g_strconcat (newpath, ".sln", NULL);
+      if (!glnx_renameat (parent_dfd, temp_fat_path, parent_dfd, new_fat_path, error))
+        return FALSE;
+      return TRUE;
+    }
+  else if (!glnx_renameat (parent_dfd, temppath, parent_dfd, newpath, error))
     return FALSE;
 
   return TRUE;
@@ -2076,17 +2133,27 @@ swap_bootloader (OstreeSysroot  *sysroot,
   g_assert ((current_bootversion == 0 && new_bootversion == 1) ||
             (current_bootversion == 1 && new_bootversion == 0));
 
+  gboolean fat;
   glnx_autofd int boot_dfd = -1;
   if (!glnx_opendirat (sysroot->sysroot_fd, "boot", TRUE, &boot_dfd, error))
     return FALSE;
 
+  fat = fs_is_fat (boot_dfd);
+
   /* The symlink was already written, and we used syncfs() to ensure
    * its data is in place.  Renaming now should give us atomic semantics;
    * see https://bugzilla.gnome.org/show_bug.cgi?id=755595
    */
-  if (!glnx_renameat (boot_dfd, "loader.tmp", boot_dfd, "loader", error))
-    return FALSE;
-
+  if (!fat)
+    {
+      if (!glnx_renameat (boot_dfd, "loader.tmp", boot_dfd, "loader", error))
+         return FALSE;
+    }
+  else
+    {
+      if (!glnx_renameat (boot_dfd, "loader.tmp.sln", boot_dfd, "loader.sln", error))
+        return FALSE;
+    }
   /* Now we explicitly fsync this directory, even though it
    * isn't required for atomicity, for two reasons:
    *  - It should be very cheap as we're just syncing whatever

src/libostree/ostree-sysroot.c

@@ -580,32 +580,37 @@ read_current_bootversion (OstreeSysroot *self,
                           GCancellable  *cancellable,
                           GError       **error)
 {
-  int ret_bootversion;
+  int ret_bootversion = 0;
   struct stat stbuf;
+  g_autofree char *target = NULL;
 
   if (!glnx_fstatat_allow_noent (self->sysroot_fd, "boot/loader", &stbuf, AT_SYMLINK_NOFOLLOW, error))
     return FALSE;
   if (errno == ENOENT)
     {
-      ret_bootversion = 0;
+      /* Don't share the error because we want error handling to be the same as before we
+       * added fake symlinks */
+      target = glnx_file_get_contents_utf8_at(self->sysroot_fd, "boot/loader.sln", NULL, cancellable, NULL);
+      if (!target) goto not_found;
     }
   else
     {
       if (!S_ISLNK (stbuf.st_mode))
         return glnx_throw (error, "Not a symbolic link: boot/loader");
-
-      g_autofree char *target =
+      target =
         glnx_readlinkat_malloc (self->sysroot_fd, "boot/loader", cancellable, error);
       if (!target)
         return FALSE;
-      if (g_strcmp0 (target, "loader.0") == 0)
-        ret_bootversion = 0;
-      else if (g_strcmp0 (target, "loader.1") == 0)
-        ret_bootversion = 1;
-      else
-        return glnx_throw (error, "Invalid target '%s' in boot/loader", target);
     }
 
+  if (g_strcmp0 (target, "loader.0") == 0)
+    ret_bootversion = 0;
+  else if (g_strcmp0 (target, "loader.1") == 0)
+    ret_bootversion = 1;
+  else
+    return glnx_throw (error, "Invalid target '%s' in boot/loader", target);
+
+not_found:
   *out_bootversion = ret_bootversion;
   return TRUE;
 }