elf: Fix for mismatched app ELF file not detected.

The check that the app ELF file SHA256 matches the one stored in the core dump
would never fail, leading to gdb loading the wrong ELF file and either crashing
or producing misleading debug information.

Specifics:

The note_sec.name field was incorrectly read back as b'ESP_CORE_DUMP_INFO\x00E',
because the namesz length includes the terminating NUL byte and possible junk
padding bytes:
https://github.com/espressif/esp-idf/blob/master/components/espcoredump/src/core_dump_elf.c#L212

In addition, as 'note_sec.name' is a bytes object Python 3 would have never
successfully compared it with a string.

components/espcoredump/corefile/elf.py

@@ -284,6 +284,13 @@ def __init__(self, addr, data, flags):  # type: (int, bytes, int) -> None
         super(ElfNoteSegment, self).__init__(addr, data, flags)
         self.type = ElfFile.PT_NOTE
         self.note_secs = NoteSections.parse(self.data)
+        for note in self.note_secs:
+            # note.name should include a terminating NUL byte, plus possible
+            # padding
+            #
+            # (note: construct.PaddingString can't parse this if there
+            # are non-zero padding bytes after the NUL, it also parses those.)
+            note.name = note.name.split(b'\x00')[0]
 
     @staticmethod
     def _type_str():  # type: () -> str

components/espcoredump/corefile/loader.py

@@ -261,7 +261,7 @@ def _extract_elf_corefile(self, exe_name=None, e_machine=ESPCoreDumpElfFile.EM_X
         for seg in core_elf.note_segments:
             for note_sec in seg.note_secs:
                 # Check for version info note
-                if note_sec.name == 'ESP_CORE_DUMP_INFO' \
+                if note_sec.name == b'ESP_CORE_DUMP_INFO' \
                         and note_sec.type == ESPCoreDumpElfFile.PT_INFO \
                         and exe_name:
                     exe_elf = ElfFile(exe_name)