sysroot: Support for standard-conformance marker file

Add support for standard-conformance marker file loader/entries.srel.

There might be implementations of boot loading infrastructure that are
also using the /loader/entries/ directory, but install files that do
not follow the [1] specification. In order to minimize confusion, a boot
loader implementation may place the file /loader/entries.srel next to the
/loader/entries/ directory containing the ASCII string type1 (followed
by a UNIX newline). Tools that need to determine whether an existing
directory implements the semantics described here may check for this
file and contents: if it exists and contains the mentioned string,
it shall assume a standards-compliant implementation is in place.
If it exists but contains a different string it shall assume other
semantics are implemented. If the file does not exist, no assumptions
should be made.

[1] https://uapi-group.org/specifications/specs/boot_loader_specification/#type-1-boot-loader-entry-keys
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>

Author: igoropaniuk

src/libostree/ostree-sysroot-deploy.c

@@ -2185,6 +2185,58 @@ install_deployment_kernel (OstreeSysroot *sysroot, int new_bootversion,
   return TRUE;
 }
 
+/* Determine whether an existing directory implements the semantics described in
+ * https://uapi-group.org/specifications/specs/boot_loader_specification/#type-1-boot-loader-entry-keys
+ */
+static gboolean
+is_bootconfig_type1_semantics (OstreeSysroot *sysroot, GCancellable *cancellable, GError **error)
+{
+  struct stat stbuf;
+
+  if (!_ostree_sysroot_ensure_boot_fd (sysroot, error))
+    return FALSE;
+
+  if (!glnx_fstatat_allow_noent (sysroot->boot_fd, "loader/entries.srel", &stbuf, AT_SYMLINK_NOFOLLOW,
+                                 error))
+    return FALSE;
+
+
+  if (errno == ENOENT)
+    {
+      g_debug ("Didn't find loader/entries.srel file");
+      return FALSE;
+    }
+  else
+    {
+      /* Get semantics type by reading loader/entries.srel */
+      gsize len;
+      g_autofree char* type =
+        glnx_file_get_contents_utf8_at(sysroot->boot_fd, "loader/entries.srel",
+                                       &len, cancellable, error);
+      if (type == NULL)
+        {
+          g_debug ("Invalid loader/entries.srel file");
+          return FALSE;
+        }
+
+      /* Remove trailing newline symbol if there is any */
+      type[strcspn(type, "\n")] = 0;
+
+      if (g_strcmp0 (type, "type1") == 0)
+        {
+          g_debug ("type1 semantics is found in loader/entries.srel file");
+          return TRUE;
+        }
+      else
+        {
+          g_debug ("Unsupported semantics type ('%s') in loader/entries.srel file", type);
+          return FALSE;
+        }
+    }
+
+  return FALSE;
+}
+
 /* We generate the symlink on disk, then potentially do a syncfs() to ensure
  * that it (and everything else we wrote) has hit disk. Only after that do we
  * rename it into place.
@@ -2512,6 +2564,7 @@ write_deployments_bootswap (OstreeSysroot *self, GPtrArray *new_deployments,
   /* Handle when boot/loader is a link (normal deployment) and as a normal directory (e.g. EFI/vfat) */
   struct stat stbuf;
   gboolean loader_link = FALSE;
+  gboolean force_type1_semantics = is_bootconfig_type1_semantics(self, cancellable, error);
   if (!glnx_fstatat_allow_noent (self->sysroot_fd, "boot/loader", &stbuf, AT_SYMLINK_NOFOLLOW, error))
     return FALSE;
   if (errno == ENOENT)
@@ -2534,6 +2587,9 @@ write_deployments_bootswap (OstreeSysroot *self, GPtrArray *new_deployments,
   else
     return FALSE;
 
+  if (force_type1_semantics && loader_link)
+    return glnx_throw_errno_prefix (error, "type1 semantics, but boot/loader is symlink");
+
   if (loader_link)
     {
       /* Default and when loader is a link is to swap links */