Allow to change flash parameters only when no SHA256 digest is appended

Partial fix for espressif/esp-idf#8798

esptool/cmds.py

@@ -260,16 +260,44 @@ def _update_image_flash_params(esp, address, args, image):
         )
         return image
 
+    # After the 8-byte header comes the extended header for chips others than ESP8266.
+    # The 15th byte of the extended header indicates if the image is protected by
+    # a SHA256 checksum. In that case we should not modify the header because
+    # the checksum check would fail.
+    sha_implies_keep = args.chip != "esp8266" and image[8 + 15] == 1
+
+    def print_keep_warning(arg_to_keep, arg_used):
+        print(
+            "Warning: Image file at {addr} is protected with a hash checksum, "
+            "so not changing the flash {arg} setting. "
+            "Use the --flash_{arg}=keep option instead of --flash_{arg}={arg_orig} "
+            "in order to remove this warning".format(
+                addr=hex(address), arg=arg_to_keep, arg_orig=arg_used
+            )
+        )
+
     if args.flash_mode != "keep":
-        flash_mode = FLASH_MODES[args.flash_mode]
+        new_flash_mode = FLASH_MODES[args.flash_mode]
+        if flash_mode != new_flash_mode and sha_implies_keep:
+            print_keep_warning("mode", args.flash_mode)
+        else:
+            flash_mode = new_flash_mode
 
     flash_freq = flash_size_freq & 0x0F
     if args.flash_freq != "keep":
-        flash_freq = esp.parse_flash_freq_arg(args.flash_freq)
+        new_flash_freq = esp.parse_flash_freq_arg(args.flash_freq)
+        if flash_freq != new_flash_freq and sha_implies_keep:
+            print_keep_warning("frequency", args.flash_freq)
+        else:
+            flash_freq = new_flash_freq
 
     flash_size = flash_size_freq & 0xF0
     if args.flash_size != "keep":
-        flash_size = esp.parse_flash_size_arg(args.flash_size)
+        new_flash_size = esp.parse_flash_size_arg(args.flash_size)
+        if flash_size != new_flash_size and sha_implies_keep:
+            print_keep_warning("size", args.flash_size)
+        else:
+            flash_size = new_flash_size
 
     flash_params = struct.pack(b"BB", flash_mode, flash_size + flash_freq)
     if flash_params != image[2:4]:

test/test_esptool.py

@@ -759,9 +759,17 @@ def test_detect_size_changes_size(self):
         )
         readback = self.readback(self.flash_offset, 8)
         self.assertEqual(self.header[:3], readback[:3])  # first 3 bytes unchanged
-        self.assertNotEqual(self.header[3], readback[3])  # size_freq byte changed
+        if chip in ["esp8266", "esp32"]:
+            self.assertNotEqual(self.header[3], readback[3])  # size_freq byte changed
+        else:
+            # Not changed because protected by SHA256 digest
+            self.assertEqual(self.header[3], readback[3])  # size_freq byte unchanged
         self.assertEqual(self.header[4:], readback[4:])  # rest unchanged
 
+    @unittest.skipUnless(
+        chip in ["esp8266", "esp32"],
+        "Bootloader header needs to be modifiable - without sha256",
+    )
     def test_explicit_set_size_freq_mode(self):
         self.run_esptool(
             "write_flash -fs 2MB -fm dout -ff 80m 0x%x %s"
@@ -855,7 +863,14 @@ def test_flash_header_rewrite(self):
         output = self.run_esptool(
             "write_flash -fm dout -ff 20m 0x%x %s" % (self.BL_OFFSET, bl_image)
         )
-        self.assertIn("Flash params set to", output)
+        if chip in ["esp8266", "esp32"]:
+            # There is no SHA256 digest so the header can be changed - ESP8266 doesn't
+            # support this; The test image for ESP32 just doesn't have it.
+            self.assertIn("Flash params set to", output)
+        else:
+            self.assertNotIn("Flash params set to", output)
+            self.assertIn("not changing the flash mode setting", output)
+            self.assertIn("not changing the flash frequency setting", output)
 
     def test_flash_header_no_magic_no_rewrite(self):
         # first image doesn't start with magic byte, second image does