boot: Fix readdir_harder() on VirtualBox

Fixes: #22073

Author: medhefgo

Diff for: src/boot/efi/boot.c

@@ -1577,7 +1577,7 @@ static void config_load_entries(
                 _cleanup_freepool_ CHAR8 *content = NULL;
 
                 err = readdir_harder(entries_dir, &f, &f_size);
-                if (f_size == 0 || EFI_ERROR(err))
+                if (EFI_ERROR(err) || !f)
                         break;
 
                 if (f->FileName[0] == '.')
@@ -2019,7 +2019,7 @@ static void config_entry_add_linux(
                 CHAR8 *key, *value;
 
                 err = readdir_harder(linux_dir, &f, &f_size);
-                if (f_size == 0 || EFI_ERROR(err))
+                if (EFI_ERROR(err) || !f)
                         break;
 
                 if (f->FileName[0] == '.')

Diff for: src/boot/efi/util.c

@@ -592,7 +592,12 @@ EFI_STATUS readdir_harder(
          * the specified buffer needs to be freed by caller, after final use. */
 
         if (!*buffer) {
-                sz = offsetof(EFI_FILE_INFO, FileName) /* + 256 */;
+                /* Some broken firmware violates the EFI spec by still advancing the readdir
+                 * position when returning EFI_BUFFER_TOO_SMALL, effectively skipping over any files when
+                 * the buffer was too small. Therefore, start with a buffer that should handle FAT32 max
+                 * file name length.
+                 * As a side effect, most readdir_harder() calls will now be slightly faster. */
+                sz = sizeof(EFI_FILE_INFO) + 256 * sizeof(CHAR16);
                 *buffer = xallocate_pool(sz);
                 *buffer_size = sz;
         } else