Allow fallback to default loader when encountering errors on network …

…boot

Only certain errors trigger fallback to the default loader name.  This change allows fallback when encountering `EFI_TFTP_ERROR` as well.  And ideally would also handle `EFI_HTTP_ERROR` the same way, but that requires updating gnu-efi to a version newer than the shim-15.8 branch.

This fixes the issue reported in rhboot#649 that prevents boot on some models of PC.

Signed-off-by: Nathan O'Meara <Nathan.OMeara@tanium.com>

shim.c

@@ -1258,9 +1258,13 @@ EFI_STATUS init_grub(EFI_HANDLE image_handle)
 	}
 
 	// If the filename is invalid, or the file does not exist,
-	// just fallback to the default loader.
+	// just fallback to the default loader.  Also fallback to the
+	// default loader if we get a TFTP error or HTTP error.
 	if (!use_fb && (efi_status == EFI_INVALID_PARAMETER ||
-	                efi_status == EFI_NOT_FOUND)) {
+	                efi_status == EFI_NOT_FOUND ||
+					// TODO: Add EFI_HTTP_ERROR when gnu-efi is updated to a version that includes it
+	                // efi_status == EFI_HTTP_ERROR ||
+	                efi_status == EFI_TFTP_ERROR)) {
 		console_print(
 			L"start_image() returned %r, falling back to default loader\n",
 			efi_status);