[Endless] sd-boot: Read fake symlinks

ostree uses symlinks on the boot filesystem, this isn't great when that
filesystem is vfat, which can't do symlinks. Since the EFI ESP must be
vfat on most implementation, this makes ostree incompaible with sd-boot.

To allow ostree to keep making symlinks, we make a fake symlink that's
just a text file with the name of the file that would be linked to.

https://phabricator.endlessm.com/T27040

Author: Derek Foreman

src/boot/efi/boot.c

@@ -1573,8 +1573,39 @@ static void config_entry_add_from_file(
         TAKE_PTR(entry);
 }
 
+static CHAR16 *resolve_link(EFI_FILE *root_dir, CHAR16 *link, CHAR16 *file) {
+  EFI_STATUS err;
+  _cleanup_freepool_ CHAR8 *contents = NULL;
+  _cleanup_freepool_ CHAR16 *linkname = NULL;
+  _cleanup_freepool_ CHAR16 *target = NULL;
+  CHAR16 *out = NULL;
+
+  linkname = AllocatePool(StrSize(link) + StrSize(L".sln"));
+  if (!linkname)
+    return NULL;
+  StrCpy(linkname, link);
+  StrCat(linkname, L".sln");
+
+  err = file_read(root_dir, linkname, 0, 0, &contents, NULL);
+  if (EFI_ERROR(err))
+    return NULL;
+
+  target = xstra_to_str(contents);
+  if (!target)
+    return NULL;
+
+  out = AllocatePool(StrSize(target) + StrSize(file));
+  if (!out)
+    return NULL;
+  StrCpy(out, target);
+  StrCat(out, file);
+
+  return out;
+}
+
 static void config_load_defaults(Config *config, EFI_FILE *root_dir) {
         _cleanup_freepool_ CHAR8 *content = NULL;
+        _cleanup_freepool_ CHAR16 *link = NULL;
         UINTN value;
         EFI_STATUS err;
 
@@ -1593,7 +1624,10 @@ static void config_load_defaults(Config *config, EFI_FILE *root_dir) {
                 .timeout_sec_efivar = TIMEOUT_UNSET,
         };
 
-        err = file_read(root_dir, L"\\loader\\loader.conf", 0, 0, &content, NULL);
+        link = resolve_link(root_dir, L"\\loader", L"\\entries");
+        if (!link)
+          link = L"\\loader\\entries";
+        err = file_read(root_dir, link, 0, 0, &content, NULL);
         if (!EFI_ERROR(err))
                 config_defaults_load_from_file(config, content);
 
@@ -1639,14 +1673,18 @@ static void config_load_entries(
         _cleanup_freepool_ EFI_FILE_INFO *f = NULL;
         UINTN f_size = 0;
         EFI_STATUS err;
+        _cleanup_freepool_ CHAR16 *link = NULL;
 
         assert(config);
         assert(device);
         assert(root_dir);
 
         /* Adds Boot Loader Type #1 entries (i.e. /loader/entries/….conf) */
 
-        err = open_directory(root_dir, L"\\loader\\entries", &entries_dir);
+        link = resolve_link(root_dir, L"\\loader", L"\\entries");
+        if (!link)
+          link = L"\\loader\\entries";
+        err = open_directory(root_dir, link, &entries_dir);
         if (EFI_ERROR(err))
                 return;