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"""
ELF file parser.
This provides a class ``ELFFile`` that parses an ELF executable in a similar
interface to ``ZipFile``. Only the read interface is implemented.
Based on: https://gist.github.com/lyssdod/f51579ae8d93c8657a5564aefc2ffbca
ELF header: https://refspecs.linuxfoundation.org/elf/gabi4+/ch4.eheader.html
"""
import enum
import os
import struct
from typing import IO, Optional, Tuple
class ELFInvalid(ValueError):
pass
class EIClass(enum.IntEnum):
C32 = 1
C64 = 2
class EIData(enum.IntEnum):
Lsb = 1
Msb = 2
class EMachine(enum.IntEnum):
I386 = 3
S390 = 22
Arm = 40
X8664 = 62
AArc64 = 183
class ELFFile:
"""
Representation of an ELF executable.
"""
def __init__(self, f: IO[bytes]) -> None:
self._f = f
try:
ident = self._read("16B")
except struct.error:
raise ELFInvalid("unable to parse identification")
magic = bytes(ident[:4])
if magic != b"\x7fELF":
raise ELFInvalid(f"invalid magic: {magic!r}")
self.capacity = ident[4] # Format for program header (bitness).
self.encoding = ident[5] # Data structure encoding (endianness).
try:
# e_fmt: Format for program header.
# p_fmt: Format for section header.
# p_idx: Indexes to find p_type, p_offset, and p_filesz.
e_fmt, self._p_fmt, self._p_idx = {
(1, 1): ("<HHIIIIIHHH", "<IIIIIIII", (0, 1, 4)), # 32-bit LSB.
(1, 2): (">HHIIIIIHHH", ">IIIIIIII", (0, 1, 4)), # 32-bit MSB.
(2, 1): ("<HHIQQQIHHH", "<IIQQQQQQ", (0, 2, 5)), # 64-bit LSB.
(2, 2): (">HHIQQQIHHH", ">IIQQQQQQ", (0, 2, 5)), # 64-bit MSB.
}[(self.capacity, self.encoding)]
except KeyError:
raise ELFInvalid(
f"unrecognized capacity ({self.capacity}) or "
f"encoding ({self.encoding})"
)
try:
(
_,
self.machine, # Architecture type.
_,
_,
self._e_phoff, # Offset of program header.
_,
self.flags, # Processor-specific flags.
_,
self._e_phentsize, # Size of section.
self._e_phnum, # Number of sections.
) = self._read(e_fmt)
except struct.error as e:
raise ELFInvalid("unable to parse machine and section information") from e
def _read(self, fmt: str) -> Tuple[int, ...]:
return struct.unpack(fmt, self._f.read(struct.calcsize(fmt)))
@property
def interpreter(self) -> Optional[str]:
"""
The path recorded in the ``PT_INTERP`` section header.
"""
for index in range(self._e_phnum):
self._f.seek(self._e_phoff + self._e_phentsize * index)
try:
data = self._read(self._p_fmt)
except struct.error:
continue
if data[self._p_idx[0]] != 3: # Not PT_INTERP.
continue
self._f.seek(data[self._p_idx[1]])
return os.fsdecode(self._f.read(data[self._p_idx[2]])).strip("\0")
return None
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