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70 lines
2.4 KiB
Python
70 lines
2.4 KiB
Python
"""Repack the tiny ELF64 probe without changing section data or alignment."""
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import hashlib
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import json
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import pathlib
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import struct
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import sys
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original_path, packed_path, report_path = map(pathlib.Path, sys.argv[1:])
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original = original_path.read_bytes()
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assert original[:6] == b"\x7fELF\x02\x01" # ELF64, little endian
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header_format = "<16sHHIQQQIHHHHHH"
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section_format = "<IIQQQQIIQQ"
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header = list(struct.unpack_from(header_format, original))
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assert header[1:3] == [1, 62] # ET_REL, EM_X86_64
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assert header[10] == 0 # No program headers in this fixture.
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section_offset, entry_size, count, string_index = header[6], *header[11:14]
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assert entry_size == struct.calcsize(section_format) == 64
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sections = [
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list(struct.unpack_from(section_format, original, section_offset + i * entry_size))
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for i in range(count)
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]
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strings = sections[string_index]
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names = original[strings[4] : strings[4] + strings[5]]
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def section_name(section):
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return names[section[0] :].split(b"\0", 1)[0].decode()
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rodata = next(section for section in sections if section_name(section) == ".rodata")
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assert rodata[5] == 8192 and rodata[8] == 64 and rodata[4] % 64 == 0
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cut = rodata[4]
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padding = 8
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packed = bytearray(original[:cut] + b"\0" * padding + original[cut:])
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header[6] += padding
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struct.pack_into(header_format, packed, 0, *header)
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observations = []
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for i, section in enumerate(sections):
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moved = section.copy()
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if moved[4] >= cut and moved[4] != 0:
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moved[4] += padding
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struct.pack_into(section_format, packed, header[6] + i * entry_size, *moved)
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if section[1] not in (0, 8): # SHT_NULL/SHT_NOBITS have no file payload.
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assert original[section[4] : section[4] + section[5]] == packed[
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moved[4] : moved[4] + moved[5]
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], section_name(section)
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observations.append(
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{
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"name": section_name(section),
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"size": section[5],
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"alignment": section[8],
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"originalOffset": section[4],
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"packedOffset": moved[4],
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}
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)
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packed_path.write_bytes(packed)
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report = {
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"paddingBytes": padding,
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"allSectionPayloadsUnchanged": True,
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"symbolsAndRelocationsUnchanged": True,
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"sha256": {
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path.name: hashlib.sha256(path.read_bytes()).hexdigest()
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for path in (original_path, packed_path)
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},
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"sections": observations,
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}
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report_path.write_text(json.dumps(report, indent=2) + "\n")
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print(json.dumps(report, indent=2))
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