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634f6ec4eb
Instead of the ever-messier text parsing of the output of the readelf tool (which is clearly meant for human consumption not to be machine parseable), parse the ELF binaries directly. Add a small dependency-less ELF parser specific to the checks. This is slightly more secure, too, because it removes potential ambiguity due to misparsing and changes in the output format of `elfread`. It also allows for stricter and more specific ELF format checks in the future. This removes the build-time dependency for `readelf`. It passes the test-security-check for me locally, though I haven't checked on all platforms.
324 lines
13 KiB
Python
324 lines
13 KiB
Python
#!/usr/bin/env python3
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# Copyright (c) 2020 Wladimir J. van der Laan
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# Distributed under the MIT software license, see the accompanying
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# file COPYING or http://www.opensource.org/licenses/mit-license.php.
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'''
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Compact, self-contained ELF implementation for bitcoin-core security checks.
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'''
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import struct
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import types
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from typing import Dict, List, Optional, Union, Tuple
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# you can find all these values in elf.h
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EI_NIDENT = 16
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# Byte indices in e_ident
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EI_CLASS = 4 # ELFCLASSxx
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EI_DATA = 5 # ELFDATAxxxx
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ELFCLASS32 = 1 # 32-bit
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ELFCLASS64 = 2 # 64-bit
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ELFDATA2LSB = 1 # little endian
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ELFDATA2MSB = 2 # big endian
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# relevant values for e_machine
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EM_386 = 3
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EM_PPC64 = 21
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EM_ARM = 40
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EM_AARCH64 = 183
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EM_X86_64 = 62
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EM_RISCV = 243
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# relevant values for e_type
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ET_DYN = 3
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# relevant values for sh_type
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SHT_PROGBITS = 1
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SHT_STRTAB = 3
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SHT_DYNAMIC = 6
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SHT_DYNSYM = 11
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SHT_GNU_verneed = 0x6ffffffe
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SHT_GNU_versym = 0x6fffffff
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# relevant values for p_type
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PT_LOAD = 1
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PT_GNU_STACK = 0x6474e551
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PT_GNU_RELRO = 0x6474e552
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# relevant values for p_flags
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PF_X = (1 << 0)
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PF_W = (1 << 1)
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PF_R = (1 << 2)
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# relevant values for d_tag
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DT_NEEDED = 1
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DT_FLAGS = 30
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# relevant values of `d_un.d_val' in the DT_FLAGS entry
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DF_BIND_NOW = 0x00000008
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# relevant d_tags with string payload
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STRING_TAGS = {DT_NEEDED}
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# rrlevant values for ST_BIND subfield of st_info (symbol binding)
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STB_LOCAL = 0
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class ELFRecord(types.SimpleNamespace):
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'''Unified parsing for ELF records.'''
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def __init__(self, data: bytes, offset: int, eh: 'ELFHeader', total_size: Optional[int]) -> None:
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hdr_struct = self.STRUCT[eh.ei_class][0][eh.ei_data]
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if total_size is not None and hdr_struct.size > total_size:
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raise ValueError(f'{self.__class__.__name__} header size too small ({total_size} < {hdr_struct.size})')
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for field, value in zip(self.STRUCT[eh.ei_class][1], hdr_struct.unpack(data[offset:offset + hdr_struct.size])):
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setattr(self, field, value)
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def BiStruct(chars: str) -> Dict[int, struct.Struct]:
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'''Compile a struct parser for both endians.'''
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return {
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ELFDATA2LSB: struct.Struct('<' + chars),
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ELFDATA2MSB: struct.Struct('>' + chars),
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}
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class ELFHeader(ELFRecord):
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FIELDS = ['e_type', 'e_machine', 'e_version', 'e_entry', 'e_phoff', 'e_shoff', 'e_flags', 'e_ehsize', 'e_phentsize', 'e_phnum', 'e_shentsize', 'e_shnum', 'e_shstrndx']
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STRUCT = {
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ELFCLASS32: (BiStruct('HHIIIIIHHHHHH'), FIELDS),
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ELFCLASS64: (BiStruct('HHIQQQIHHHHHH'), FIELDS),
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}
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def __init__(self, data: bytes, offset: int) -> None:
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self.e_ident = data[offset:offset + EI_NIDENT]
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if self.e_ident[0:4] != b'\x7fELF':
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raise ValueError('invalid ELF magic')
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self.ei_class = self.e_ident[EI_CLASS]
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self.ei_data = self.e_ident[EI_DATA]
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super().__init__(data, offset + EI_NIDENT, self, None)
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def __repr__(self) -> str:
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return f'Header(e_ident={self.e_ident!r}, e_type={self.e_type}, e_machine={self.e_machine}, e_version={self.e_version}, e_entry={self.e_entry}, e_phoff={self.e_phoff}, e_shoff={self.e_shoff}, e_flags={self.e_flags}, e_ehsize={self.e_ehsize}, e_phentsize={self.e_phentsize}, e_phnum={self.e_phnum}, e_shentsize={self.e_shentsize}, e_shnum={self.e_shnum}, e_shstrndx={self.e_shstrndx})'
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class Section(ELFRecord):
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name: Optional[bytes] = None
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FIELDS = ['sh_name', 'sh_type', 'sh_flags', 'sh_addr', 'sh_offset', 'sh_size', 'sh_link', 'sh_info', 'sh_addralign', 'sh_entsize']
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STRUCT = {
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ELFCLASS32: (BiStruct('IIIIIIIIII'), FIELDS),
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ELFCLASS64: (BiStruct('IIQQQQIIQQ'), FIELDS),
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}
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def __init__(self, data: bytes, offset: int, eh: ELFHeader) -> None:
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super().__init__(data, offset, eh, eh.e_shentsize)
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self._data = data
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def __repr__(self) -> str:
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return f'Section(sh_name={self.sh_name}({self.name!r}), sh_type=0x{self.sh_type:x}, sh_flags={self.sh_flags}, sh_addr=0x{self.sh_addr:x}, sh_offset=0x{self.sh_offset:x}, sh_size={self.sh_size}, sh_link={self.sh_link}, sh_info={self.sh_info}, sh_addralign={self.sh_addralign}, sh_entsize={self.sh_entsize})'
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def contents(self) -> bytes:
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'''Return section contents.'''
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return self._data[self.sh_offset:self.sh_offset + self.sh_size]
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class ProgramHeader(ELFRecord):
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STRUCT = {
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# different ELF classes have the same fields, but in a different order to optimize space versus alignment
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ELFCLASS32: (BiStruct('IIIIIIII'), ['p_type', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_flags', 'p_align']),
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ELFCLASS64: (BiStruct('IIQQQQQQ'), ['p_type', 'p_flags', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_align']),
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}
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def __init__(self, data: bytes, offset: int, eh: ELFHeader) -> None:
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super().__init__(data, offset, eh, eh.e_phentsize)
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def __repr__(self) -> str:
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return f'ProgramHeader(p_type={self.p_type}, p_offset={self.p_offset}, p_vaddr={self.p_vaddr}, p_paddr={self.p_paddr}, p_filesz={self.p_filesz}, p_memsz={self.p_memsz}, p_flags={self.p_flags}, p_align={self.p_align})'
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class Symbol(ELFRecord):
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STRUCT = {
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# different ELF classes have the same fields, but in a different order to optimize space versus alignment
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ELFCLASS32: (BiStruct('IIIBBH'), ['st_name', 'st_value', 'st_size', 'st_info', 'st_other', 'st_shndx']),
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ELFCLASS64: (BiStruct('IBBHQQ'), ['st_name', 'st_info', 'st_other', 'st_shndx', 'st_value', 'st_size']),
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}
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def __init__(self, data: bytes, offset: int, eh: ELFHeader, symtab: Section, strings: bytes, version: Optional[bytes]) -> None:
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super().__init__(data, offset, eh, symtab.sh_entsize)
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self.name = _lookup_string(strings, self.st_name)
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self.version = version
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def __repr__(self) -> str:
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return f'Symbol(st_name={self.st_name}({self.name!r}), st_value={self.st_value}, st_size={self.st_size}, st_info={self.st_info}, st_other={self.st_other}, st_shndx={self.st_shndx}, version={self.version!r})'
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@property
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def is_import(self) -> bool:
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'''Returns whether the symbol is an imported symbol.'''
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return self.st_bind != STB_LOCAL and self.st_shndx == 0
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@property
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def is_export(self) -> bool:
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'''Returns whether the symbol is an exported symbol.'''
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return self.st_bind != STB_LOCAL and self.st_shndx != 0
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@property
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def st_bind(self) -> int:
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'''Returns STB_*.'''
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return self.st_info >> 4
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class Verneed(ELFRecord):
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DEF = (BiStruct('HHIII'), ['vn_version', 'vn_cnt', 'vn_file', 'vn_aux', 'vn_next'])
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STRUCT = { ELFCLASS32: DEF, ELFCLASS64: DEF }
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def __init__(self, data: bytes, offset: int, eh: ELFHeader) -> None:
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super().__init__(data, offset, eh, None)
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def __repr__(self) -> str:
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return f'Verneed(vn_version={self.vn_version}, vn_cnt={self.vn_cnt}, vn_file={self.vn_file}, vn_aux={self.vn_aux}, vn_next={self.vn_next})'
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class Vernaux(ELFRecord):
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DEF = (BiStruct('IHHII'), ['vna_hash', 'vna_flags', 'vna_other', 'vna_name', 'vna_next'])
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STRUCT = { ELFCLASS32: DEF, ELFCLASS64: DEF }
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def __init__(self, data: bytes, offset: int, eh: ELFHeader, strings: bytes) -> None:
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super().__init__(data, offset, eh, None)
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self.name = _lookup_string(strings, self.vna_name)
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def __repr__(self) -> str:
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return f'Veraux(vna_hash={self.vna_hash}, vna_flags={self.vna_flags}, vna_other={self.vna_other}, vna_name={self.vna_name}({self.name!r}), vna_next={self.vna_next})'
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class DynTag(ELFRecord):
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STRUCT = {
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ELFCLASS32: (BiStruct('II'), ['d_tag', 'd_val']),
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ELFCLASS64: (BiStruct('QQ'), ['d_tag', 'd_val']),
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}
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def __init__(self, data: bytes, offset: int, eh: ELFHeader, section: Section) -> None:
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super().__init__(data, offset, eh, section.sh_entsize)
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def __repr__(self) -> str:
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return f'DynTag(d_tag={self.d_tag}, d_val={self.d_val})'
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def _lookup_string(data: bytes, index: int) -> bytes:
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'''Look up string by offset in ELF string table.'''
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endx = data.find(b'\x00', index)
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assert endx != -1
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return data[index:endx]
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VERSYM_S = BiStruct('H') # .gnu_version section has a single 16-bit integer per symbol in the linked section
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def _parse_symbol_table(section: Section, strings: bytes, eh: ELFHeader, versym: bytes, verneed: Dict[int, bytes]) -> List[Symbol]:
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'''Parse symbol table, return a list of symbols.'''
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data = section.contents()
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symbols = []
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versym_iter = (verneed.get(v[0]) for v in VERSYM_S[eh.ei_data].iter_unpack(versym))
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for ofs, version in zip(range(0, len(data), section.sh_entsize), versym_iter):
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symbols.append(Symbol(data, ofs, eh, section, strings, version))
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return symbols
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def _parse_verneed(section: Section, strings: bytes, eh: ELFHeader) -> Dict[int, bytes]:
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'''Parse .gnu.version_r section, return a dictionary of {versym: 'GLIBC_...'}.'''
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data = section.contents()
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ofs = 0
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result = {}
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while True:
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verneed = Verneed(data, ofs, eh)
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aofs = verneed.vn_aux
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while True:
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vernaux = Vernaux(data, aofs, eh, strings)
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result[vernaux.vna_other] = vernaux.name
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if not vernaux.vna_next:
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break
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aofs += vernaux.vna_next
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if not verneed.vn_next:
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break
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ofs += verneed.vn_next
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return result
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def _parse_dyn_tags(section: Section, strings: bytes, eh: ELFHeader) -> List[Tuple[int, Union[bytes, int]]]:
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'''Parse dynamic tags. Return array of tuples.'''
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data = section.contents()
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ofs = 0
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result = []
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for ofs in range(0, len(data), section.sh_entsize):
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tag = DynTag(data, ofs, eh, section)
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val = _lookup_string(strings, tag.d_val) if tag.d_tag in STRING_TAGS else tag.d_val
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result.append((tag.d_tag, val))
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return result
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class ELFFile:
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sections: List[Section]
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program_headers: List[ProgramHeader]
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dyn_symbols: List[Symbol]
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dyn_tags: List[Tuple[int, Union[bytes, int]]]
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def __init__(self, data: bytes) -> None:
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self.data = data
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self.hdr = ELFHeader(self.data, 0)
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self._load_sections()
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self._load_program_headers()
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self._load_dyn_symbols()
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self._load_dyn_tags()
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self._section_to_segment_mapping()
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def _load_sections(self) -> None:
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self.sections = []
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for idx in range(self.hdr.e_shnum):
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offset = self.hdr.e_shoff + idx * self.hdr.e_shentsize
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self.sections.append(Section(self.data, offset, self.hdr))
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shstr = self.sections[self.hdr.e_shstrndx].contents()
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for section in self.sections:
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section.name = _lookup_string(shstr, section.sh_name)
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def _load_program_headers(self) -> None:
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self.program_headers = []
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for idx in range(self.hdr.e_phnum):
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offset = self.hdr.e_phoff + idx * self.hdr.e_phentsize
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self.program_headers.append(ProgramHeader(self.data, offset, self.hdr))
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def _load_dyn_symbols(self) -> None:
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# first, load 'verneed' section
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verneed = None
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for section in self.sections:
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if section.sh_type == SHT_GNU_verneed:
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strtab = self.sections[section.sh_link].contents() # associated string table
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assert verneed is None # only one section of this kind please
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verneed = _parse_verneed(section, strtab, self.hdr)
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assert verneed is not None
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# then, correlate GNU versym sections with dynamic symbol sections
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versym = {}
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for section in self.sections:
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if section.sh_type == SHT_GNU_versym:
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versym[section.sh_link] = section
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# finally, load dynsym sections
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self.dyn_symbols = []
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for idx, section in enumerate(self.sections):
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if section.sh_type == SHT_DYNSYM: # find dynamic symbol tables
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strtab_data = self.sections[section.sh_link].contents() # associated string table
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versym_data = versym[idx].contents() # associated symbol version table
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self.dyn_symbols += _parse_symbol_table(section, strtab_data, self.hdr, versym_data, verneed)
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def _load_dyn_tags(self) -> None:
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self.dyn_tags = []
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for idx, section in enumerate(self.sections):
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if section.sh_type == SHT_DYNAMIC: # find dynamic tag tables
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strtab = self.sections[section.sh_link].contents() # associated string table
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self.dyn_tags += _parse_dyn_tags(section, strtab, self.hdr)
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def _section_to_segment_mapping(self) -> None:
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for ph in self.program_headers:
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ph.sections = []
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for section in self.sections:
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if ph.p_vaddr <= section.sh_addr < (ph.p_vaddr + ph.p_memsz):
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ph.sections.append(section)
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def query_dyn_tags(self, tag_in: int) -> List[Union[int, bytes]]:
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'''Return the values of all dyn tags with the specified tag.'''
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return [val for (tag, val) in self.dyn_tags if tag == tag_in]
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def load(filename: str) -> ELFFile:
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with open(filename, 'rb') as f:
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data = f.read()
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return ELFFile(data)
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