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# SPDX-FileCopyrightText: 2026 Tomasz Kramkowski <tomasz@kramkow.ski>
# SPDX-License-Identifier: GPL-3.0-or-later
"""Compile C/assembly hooks into one Linux x86-64 ELF patch payload."""
import struct
import subprocess
from pathlib import Path
from typing import BinaryIO
from capstone import CS_ARCH_X86, CS_MODE_64, Cs
from elftools.construct import Container
from elftools.elf.constants import P_FLAGS
from elftools.elf.elffile import ELFFile
from elftools.elf.relocation import RelocationSection
from elftools.elf.sections import SymbolTableSection
from pydantic import BaseModel, ConfigDict, Field
class Hook(BaseModel):
model_config = ConfigDict(strict=True, frozen=True, extra="forbid")
address: int = Field(ge=0)
length: int = Field(ge=5)
target: str = Field(min_length=1)
class CompiledPayload:
"""Position-independent code with resolved entry addresses inside it."""
code: bytes
entries: dict[str, int]
def __init__(self, source: BinaryIO, targets: list[str]):
elf = ELFFile(source)
for section in elf.iter_sections():
if (
section["sh_flags"] & 2
and section["sh_size"]
and section.name != ".payload"
):
raise ValueError(f"unsupported allocated section: {section.name}")
# PC32 (2) PLT32 (4) (PC-relative)
if isinstance(section, RelocationSection):
if any(
r["r_info_type"] not in (2, 4) for r in section.iter_relocations()
):
raise ValueError("helper contains a non-relative relocation")
payload = elf.get_section_by_name(".payload")
if payload is None:
raise ValueError("missing compiled payload")
self.code = payload.data()
symbols = elf.get_section_by_name(".symtab")
if not isinstance(symbols, SymbolTableSection):
raise ValueError("missing compiled symbol table")
payload_index = elf.get_section_index(".payload")
address = payload["sh_addr"]
self.entries = {}
for target in targets:
matches = symbols.get_symbol_by_name(target) or []
if len(matches) != 1 or matches[0]["st_shndx"] != payload_index:
raise ValueError(
f"hook target is not a unique payload symbol: {target}"
)
entry = matches[0]["st_value"]
if not address <= entry < address + len(self.code):
raise ValueError(
f"hook target is outside the compiled payload: {target}"
)
self.entries[target] = entry
def align(value: int, boundary: int) -> int:
return (value + boundary - 1) & -boundary
def run(*args: str) -> str:
return subprocess.check_output(args, text=True)
def resolve_hook_sites(elf: ELFFile, hooks: list[Hook]) -> list[tuple[Hook, int]]:
"""Find file offsets and verify that each hook covers whole instructions."""
source = elf.stream
sites = []
disassembler = Cs(CS_ARCH_X86, CS_MODE_64)
for hook in hooks:
offset = next(elf.address_offsets(hook.address, hook.length))
source.seek(offset)
instructions = disassembler.disasm(source.read(hook.length), hook.address)
if sum(i.size for i in instructions) != hook.length:
raise ValueError("hook length cuts an instruction or contains invalid code")
sites.append((hook, offset))
return sites
def compile_code(
sources: list[Path],
imports: dict[str, int],
targets: list[str],
code_va: int,
output: Path,
) -> CompiledPayload:
"""Compile and link the patch sources at their destination address."""
output.mkdir(parents=True, exist_ok=False)
objects = []
flags = [
"-Os",
"-ffreestanding",
"-fPIE",
"-fvisibility=hidden",
"-fno-stack-protector",
"-fno-strict-aliasing",
"-mno-red-zone",
"-fno-asynchronous-unwind-tables",
"-fno-unwind-tables",
"-fcf-protection=none",
"-Wall",
"-Wextra",
]
for number, source_path in enumerate(sources):
obj = output / f"{number}.o"
source_flags = ["-std=c17"] if source_path.suffix == ".c" else []
run("cc", *flags, *source_flags, "-c", str(source_path), "-o", str(obj))
objects.append(str(obj))
linker_script = output / "layout.ld"
linker_script.write_text(
"PHDRS { code PT_LOAD FLAGS(5); }\n" # PF_R | PF_X: readable/executable.
f"SECTIONS {{ . = {code_va:#x};\n"
" .payload : { *(.text.entry) *(.text*) *(.rodata*) } :code\n"
" /DISCARD/ : { *(.comment) *(.note.GNU-stack) }\n}\n"
)
import_args = [f"--defsym={name}={value:#x}" for name, value in imports.items()]
linked = output / "helper.elf"
run(
"ld",
"--build-id=none",
"--emit-relocs",
f"--script={linker_script}",
*import_args,
f"--output={linked}",
*objects,
)
with linked.open("rb") as compiled:
return CompiledPayload(compiled, targets)
def make_hook_edits(
sites: list[tuple[Hook, int]], entries: dict[str, int]
) -> list[tuple[int, bytes]]:
"""Redirect each hook site to its compiled entry with a relative jump."""
edits = []
for hook, offset in sites:
displacement = entries[hook.target] - hook.address - 5
if not -(1 << 31) <= displacement < 1 << 31:
raise ValueError(
f"hook target is outside relative jump range: {hook.target}"
)
jump = b"\xe9" + struct.pack("<i", displacement) + b"\x90" * (hook.length - 5)
edits.append((offset, jump))
return edits
def compile_segment(
elf: ELFFile,
sources: list[Path],
imports: dict[str, int],
targets: list[str],
output: Path,
) -> tuple[CompiledPayload, list[tuple[int, bytes]]]:
"""Place the compiled code and enlarged program-header table in a new segment."""
phdr = elf.structs.Elf_Phdr
headers = [segment.header for segment in elf.iter_segments()]
# Place the new segment after all existing loadable mappings.
load_end = max(
header["p_vaddr"] + header["p_memsz"]
for header in headers
if header["p_type"] == "PT_LOAD"
)
segment_va = align(load_end, 4096)
segment_offset = align(elf.stream_len, 4096)
table_size = (len(headers) + 1) * phdr.sizeof()
code_delta = align(table_size, 16)
code_va = segment_va + code_delta
compiled = compile_code(sources, imports, targets, code_va, output)
# Relocate the enlarged program-header table into the new segment.
table_header = next(header for header in headers if header["p_type"] == "PT_PHDR")
table_header.update(
p_flags=P_FLAGS.PF_R,
p_offset=segment_offset,
p_vaddr=segment_va,
p_paddr=segment_va,
p_filesz=table_size,
p_memsz=table_size,
p_align=8,
)
segment_size = code_delta + len(compiled.code)
headers.append(
Container(
p_type="PT_LOAD",
p_flags=P_FLAGS.PF_R | P_FLAGS.PF_X,
p_offset=segment_offset,
p_vaddr=segment_va,
p_paddr=segment_va,
p_filesz=segment_size,
p_memsz=segment_size,
p_align=4096,
)
)
edits = [
(0x20, struct.pack("<Q", segment_offset)), # Elf64_Ehdr.e_phoff
(0x38, struct.pack("<H", len(headers))), # Elf64_Ehdr.e_phnum
(segment_offset, b"".join(phdr.build(header) for header in headers)),
(segment_offset + code_delta, compiled.code),
]
return compiled, edits
def compile_hooks(
source: BinaryIO,
sources: list[Path],
imports: dict[str, int],
hooks: list[Hook],
output: Path,
) -> list[tuple[int, bytes]]:
elf = ELFFile(source)
sites = resolve_hook_sites(elf, hooks)
compiled, segment_edits = compile_segment(
elf, sources, imports, [hook.target for hook in hooks], output
)
return make_hook_edits(sites, compiled.entries) + segment_edits
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