Add minimum_length to extended header for BC

Summary:
#14320

Error happening when we have older PTE files with extended header size 24. 

When we call 'from_bytes', we expect header size 32 after adding segment_data_size field in   D81938296  

This is BC on C++ side because we have a minimum length.
Add minimum length to python to make the change BC.

Reviewed By: JacobSzwejbka

Differential Revision: D82492169

Author: lucylq

exir/_serialize/_program.py

@@ -136,8 +136,7 @@ class _ExtendedHeader:
 
     # The magic bytes that should be at the beginning of the header.
     EXPECTED_MAGIC: ClassVar[bytes] = b"eh00"
-    # The length of the header in bytes.
-    EXPECTED_LENGTH: ClassVar[int] = (
+    MINIMUM_LENGTH: ClassVar[int] = (
         # Header magic
         4
         # Header length
@@ -146,10 +145,19 @@ class _ExtendedHeader:
         + 8
         # Segment base offset
         + 8
+    )
+    # The length of the header in bytes.
+    EXPECTED_LENGTH: ClassVar[int] = (
+        MINIMUM_LENGTH
         # Segment data size
         + 8
     )
 
+    # To find the header, callers should provide at least this many bytes of
+    # the head of the serialized Program data. Keep this in sync with
+    # kNumHeadBytes in //executorch/schema/extended_header.cpp
+    NUM_HEAD_BYTES: ClassVar[int] = 64
+
     # Instance attributes. @dataclass will turn these into ctor args.
 
     # The size of the serialized program data in bytes.
@@ -187,21 +195,29 @@ def from_bytes(data: bytes) -> "_ExtendedHeader":
                 + f"< {_ExtendedHeader.EXPECTED_LENGTH}"
             )
 
+        magic = data[0:4]
+        length = int.from_bytes(data[4:8], byteorder=_HEADER_BYTEORDER)
+        program_size = int.from_bytes(data[8:16], byteorder=_HEADER_BYTEORDER)
+        segment_base_offset = int.from_bytes(data[16:24], byteorder=_HEADER_BYTEORDER)
+        segment_data_size = (
+            int.from_bytes(data[24:32], byteorder=_HEADER_BYTEORDER)
+            if length > _ExtendedHeader.MINIMUM_LENGTH
+            else 0
+        )
+
         return _ExtendedHeader(
-            magic=data[0:4],
-            length=int.from_bytes(data[4:8], byteorder=_HEADER_BYTEORDER),
-            program_size=int.from_bytes(data[8:16], byteorder=_HEADER_BYTEORDER),
-            segment_base_offset=int.from_bytes(
-                data[16:24], byteorder=_HEADER_BYTEORDER
-            ),
-            segment_data_size=int.from_bytes(data[24:32], byteorder=_HEADER_BYTEORDER),
+            magic=magic,
+            length=length,
+            program_size=program_size,
+            segment_base_offset=segment_base_offset,
+            segment_data_size=segment_data_size,
         )
 
     def is_valid(self) -> bool:
         """Returns true if the extended header appears to be well-formed."""
         return (
             self.magic == _ExtendedHeader.EXPECTED_MAGIC
-            and self.length >= _ExtendedHeader.EXPECTED_LENGTH
+            and self.length >= _ExtendedHeader.MINIMUM_LENGTH
         )
 
     def to_bytes(self) -> bytes:

exir/_serialize/test/test_program.py

@@ -1009,14 +1009,30 @@ def test_named_data_segments(self) -> None:
 EXAMPLE_HEADER_DATA: bytes = (
     # Magic bytes
     b"eh00"
-    # uint32_t header size (little endian)
+    # uint32_t header size (little endian). 0x20 --> 32 bytes.
     + b"\x20\x00\x00\x00"
     # uint64_t program size
     + b"\x44\x33\x44\x33\x22\x11\x22\x11"
     # uint64_t segment base offset
     + b"\x88\x77\x88\x77\x66\x55\x66\x55"
     # uint64_t segment data size
     + b"\x22\x33\x22\x33\x44\x55\x44\x55"
+    # Padding; provide at least NUM_HEAD_BYTES for the header.
+    + b"\x99" * (_ExtendedHeader.NUM_HEAD_BYTES - 32)
+)
+
+# Minimum fields in an extended header (no segment data size).
+EXAMPLE_HEADER_DATA_MIN: bytes = (
+    # Magic bytes
+    b"eh00"
+    # uint32_t header size (little endian). 0x18 --> 24 bytes.
+    + b"\x18\x00\x00\x00"
+    # uint64_t program size
+    + b"\x44\x33\x44\x33\x22\x11\x22\x11"
+    # uint64_t segment base offset
+    + b"\x88\x77\x88\x77\x66\x55\x66\x55"
+    # Padding; provide at least NUM_HEAD_BYTES for the header.
+    + b"\x99" * (_ExtendedHeader.NUM_HEAD_BYTES - 24)
 )
 
 
@@ -1060,6 +1076,20 @@ def test_from_bytes_valid(self) -> None:
         self.assertEqual(eh.segment_base_offset, EXAMPLE_SEGMENT_BASE_OFFSET)
         self.assertEqual(eh.segment_data_size, EXAMPLE_SEGMENT_DATA_SIZE)
 
+    def test_from_bytes_minimum(self) -> None:
+        # Parse the serialized extended header.
+        eh = _ExtendedHeader.from_bytes(EXAMPLE_HEADER_DATA_MIN)
+
+        # This is a valid header: good magic and length.
+        self.assertTrue(eh.is_valid())
+
+        self.assertEqual(eh.magic, _ExtendedHeader.EXPECTED_MAGIC)
+        self.assertEqual(eh.length, _ExtendedHeader.MINIMUM_LENGTH)
+        self.assertEqual(eh.program_size, EXAMPLE_PROGRAM_SIZE)
+        self.assertEqual(eh.segment_base_offset, EXAMPLE_SEGMENT_BASE_OFFSET)
+        # Does not contain segment_data_size; should be 0
+        self.assertEqual(eh.segment_data_size, 0)
+
     def test_from_bytes_with_more_data_than_necessary(self) -> None:
         # Pass in more data than necessary to parse the header.
         header_data_with_suffix = EXAMPLE_HEADER_DATA + b"\x55" * 16
@@ -1167,4 +1197,5 @@ def test_from_bytes_invalid_length(self) -> None:
         self.assertEqual(eh.length, 16)
         self.assertEqual(eh.program_size, EXAMPLE_PROGRAM_SIZE)
         self.assertEqual(eh.segment_base_offset, EXAMPLE_SEGMENT_BASE_OFFSET)
-        self.assertEqual(eh.segment_data_size, EXAMPLE_SEGMENT_DATA_SIZE)
+        # Length cut short; segment_data_size parsed as 0.
+        self.assertEqual(eh.segment_data_size, 0)

schema/extended_header.h

@@ -22,7 +22,8 @@ namespace runtime {
 struct ExtendedHeader {
   /**
    * To find the header, callers should provide at least this many bytes of the
-   * head of the serialized Program data.
+   * head of the serialized Program data. Keep this in sync with NUM_HEAD_BYTES
+   * in //executorch/exir/_serialize/program.py
    */
   static constexpr size_t kNumHeadBytes = 64;