netboot: Convert file not found error to EFI_NOT_FOUND

This will result in falling back to the default second stage loader like
disk based lookups. The error code is available in the TftpError field
of the Mode interface. Unfortunately, some firmware doesn't fill in the
error fields, so a generic EFI_TFTP_ERROR to EFI_NOT_FOUND conversion is
included.

Author: dbnicholson

netboot.c

@@ -16,6 +16,16 @@
 #define ntohs(x) __builtin_bswap16(x)	/* supported both by GCC and clang */
 #define htons(x) ntohs(x)
 
+/* TFTP error codes from RFC 1350 */
+#define TFTP_ERROR_NOT_DEFINED  0  /* Not defined, see error message (if any). */
+#define TFTP_ERROR_NOT_FOUND    1  /* File not found. */
+#define TFTP_ERROR_ACCESS       2  /* Access violation. */
+#define TFTP_ERROR_NO_SPACE     3  /* Disk full or allocation exceeded. */
+#define TFTP_ERROR_ILLEGAL_OP   4  /* Illegal TFTP operation. */
+#define TFTP_ERROR_UNKNOWN_ID   5  /* Unknown transfer ID. */
+#define TFTP_ERROR_EXISTS       6  /* File already exists. */
+#define TFTP_ERROR_NO_USER      7  /* No such user. */
+
 static EFI_PXE_BASE_CODE *pxe;
 static EFI_IP_ADDRESS tftp_addr;
 static CHAR8 *full_path;
@@ -362,8 +372,34 @@ EFI_STATUS FetchNetbootimage(EFI_HANDLE image_handle UNUSED, VOID **buffer, UINT
 		goto try_again;
 	}
 
-	if (EFI_ERROR(efi_status) && *buffer) {
-		FreePool(*buffer);
+	if (EFI_ERROR(efi_status)) {
+		if (pxe->Mode->TftpErrorReceived) {
+			console_print(L"TFTP error %u: %a\n",
+				      pxe->Mode->TftpError.ErrorCode,
+				      pxe->Mode->TftpError.ErrorString);
+
+			switch (pxe->Mode->TftpError.ErrorCode) {
+			case TFTP_ERROR_NOT_FOUND:
+				efi_status = EFI_NOT_FOUND;
+				break;
+			default:
+				break;
+			}
+		} else if (efi_status == EFI_TFTP_ERROR) {
+			/*
+			 * Unfortunately, some firmware doesn't fill in the
+			 * error details. Treat all TFTP errors like file not
+			 * found so shim falls back to the default loader.
+			 *
+			 * https://github.com/tianocore/edk2/pull/6287
+			 */
+			console_print(L"Unknown TFTP error, treating as file not found\n");
+			efi_status = EFI_NOT_FOUND;
+		}
+
+		if (*buffer) {
+			FreePool(*buffer);
+		}
 	}
 	return efi_status;
 }