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Copy pathverify_shellpp_elf.py
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executable file
·457 lines (417 loc) · 17.1 KB
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#!/usr/bin/env python3
"""Verify a Shell++ ARM ET_REL module against its generated target ABI."""
from __future__ import annotations
import argparse
import re
import struct
import sys
from dataclasses import dataclass
from pathlib import Path
ELF_HEADER = struct.Struct("<16sHHIIIIIHHHHHH")
SECTION_HEADER = struct.Struct("<IIIIIIIIII")
SYMBOL = struct.Struct("<IIIBBH")
RELOCATION = struct.Struct("<II")
ET_REL = 1
EM_ARM = 40
EV_CURRENT = 1
ELFCLASS32 = 1
ELFDATA2LSB = 1
EV_SYSV = 0
SHT_SYMTAB = 2
SHT_STRTAB = 3
SHT_RELA = 4
SHT_NOBITS = 8
SHT_REL = 9
SHF_ALLOC = 0x2
SHF_WRITE = 0x1
SHF_EXECINSTR = 0x4
SHN_UNDEF = 0
STT_FUNC = 2
R_ARM_ABS32 = 2
R_ARM_TARGET1 = 38
ABI_DEFINE = re.compile(
r"^#define\s+(SHELLPP_ABI_[A-Z0-9_]+_ADDR)\s+"
r"(0x[0-9a-fA-F]+|[0-9]+)u?$"
)
DATA_ADDRESS_MACROS = {
"SHELLPP_ABI_STYLE_MISANS_DEMIBOLD_32_ADDR",
"SHELLPP_ABI_SETTINGS_TEMPLATE_ADDR",
"SHELLPP_ABI_SETTINGS_LIST_SLOT_ADDR",
"SHELLPP_ABI_SETTINGS_SECOND_LIST_SLOT_ADDR",
}
class VerificationError(Exception):
pass
@dataclass(frozen=True)
class Section:
name_offset: int
type: int
flags: int
address: int
offset: int
size: int
link: int
info: int
alignment: int
entry_size: int
name: str = ""
@dataclass(frozen=True)
class ElfSymbol:
name: str
value: int
size: int
info: int
other: int
section_index: int
def fail(message: str) -> None:
raise VerificationError(message)
def read_abi_addresses(path: Path) -> tuple[set[int], set[int]]:
functions: set[int] = set()
data_addresses: set[int] = set()
for raw in path.read_text(encoding="ascii").splitlines():
match = ABI_DEFINE.fullmatch(raw.strip())
if not match:
continue
name, value_text = match.groups()
value = int(value_text, 0)
if name in DATA_ADDRESS_MACROS:
data_addresses.add(value)
else:
functions.add(value)
if not functions:
fail("generated ABI header contains no firmware function addresses")
if not data_addresses:
fail("generated ABI header contains no firmware data addresses")
if any((address & 1) == 0 for address in functions):
fail("generated ABI header contains a non-Thumb function address")
return functions, data_addresses
def require_range(data: bytes, offset: int, size: int, label: str) -> None:
if offset < 0 or size < 0 or offset + size > len(data):
fail(label + " is outside the ELF file")
def c_string(data: bytes, offset: int) -> str:
if offset < 0 or offset >= len(data):
fail("string offset is outside its string table")
end = data.find(b"\0", offset)
if end < 0:
fail("unterminated string in ELF string table")
return data[offset:end].decode("ascii", "replace")
def has_flag(value: int, flag: int) -> bool:
return (value & flag) == flag
def section_footprint(sections: list[Section]) -> int:
footprint = 0
for section in sections:
if not has_flag(section.flags, SHF_ALLOC):
continue
alignment = max(section.alignment, 1)
footprint = (footprint + alignment - 1) // alignment * alignment
footprint += section.size
return footprint
def literal_ranges(
sections: list[Section], symbols: list[ElfSymbol]
) -> dict[int, list[tuple[int, int]]]:
result: dict[int, list[tuple[int, int]]] = {}
for section_index, section in enumerate(sections):
if section.name != ".text":
continue
mappings = sorted(
(symbol.value, symbol.name)
for symbol in symbols
if symbol.section_index == section_index
and (
symbol.name in ("$d", "$t")
or symbol.name.startswith("$d.")
or symbol.name.startswith("$t.")
)
)
ranges: list[tuple[int, int]] = []
for index, (start, kind) in enumerate(mappings):
end = mappings[index + 1][0] if index + 1 < len(mappings) else section.size
if (kind == "$d" or kind.startswith("$d.")) and start < end:
ranges.append((start, end))
result[section_index] = ranges
return result
def scan_absolute_addresses(
data: bytes,
sections: list[Section],
symbols: list[ElfSymbol],
relocation_offsets: set[tuple[int, int]],
firmware_functions: set[int],
firmware_data: set[int],
) -> set[int]:
ranges_by_section = literal_ranges(sections, symbols)
found: set[int] = set()
for section_index, section in enumerate(sections):
if not has_flag(section.flags, SHF_ALLOC) or section.type == SHT_NOBITS:
continue
if section.name == ".text":
ranges = ranges_by_section.get(section_index, [])
elif section.name == ".data":
ranges = [(0, section.size)]
else:
ranges = []
for range_start, range_end in ranges:
aligned_start = (range_start + 3) & ~3
for relative in range(aligned_start, range_end - 3, 4):
if (section_index, relative) in relocation_offsets:
continue
value = struct.unpack_from("<I", data, section.offset + relative)[0]
location = section.name + "+0x" + format(relative, "x")
if value in firmware_functions or value in firmware_data:
found.add(value)
if 0x2C000000 <= value < 0x2D000000:
fail(
"analysis-only firmware address at "
+ location
+ ": 0x"
+ format(value, "08x")
)
if 0x0C000000 <= value < 0x0D000000:
if (value & 1) == 0:
fail(
"firmware function address is not Thumb at "
+ location
+ ": 0x"
+ format(value, "08x")
)
if value not in firmware_functions:
fail(
"firmware function is not in the target whitelist at "
+ location
+ ": 0x"
+ format(value, "08x")
)
if (
0x20000000 <= value < 0x21000000
and not 0x20200000 <= value < 0x20300000
and value not in firmware_data
):
fail(
"firmware data address is not in the target whitelist at "
+ location
+ ": 0x"
+ format(value, "08x")
)
# Clang may fold nearby absolute call targets into one literal base plus
# an immediate offset. Therefore an address can be used without appearing
# byte-for-byte in the literal pool. The scan remains fail-closed for every
# absolute firmware address that is emitted directly.
return found
def parse_elf(
path: Path, firmware_functions: set[int], firmware_data: set[int]
) -> tuple[dict[str, int], list[Section], list[ElfSymbol], set[int], set[int]]:
data = path.read_bytes()
if len(data) < ELF_HEADER.size:
fail("file is smaller than an ELF32 header")
header = ELF_HEADER.unpack_from(data)
ident = header[0]
e_type, e_machine, e_version, e_entry = header[1:5]
e_phoff, e_shoff, e_flags = header[5:8]
e_ehsize, _e_phentsize, e_phnum, e_shentsize, e_shnum, e_shstrndx = header[8:]
if ident[:4] != b"\x7fELF":
fail("missing ELF magic")
if ident[4] != ELFCLASS32 or ident[5] != ELFDATA2LSB or ident[6] != EV_CURRENT:
fail("expected ELF32 little-endian version 1")
if ident[7] != EV_SYSV:
fail("expected System V ELF ABI")
if e_type != ET_REL or e_machine != EM_ARM or e_version != EV_CURRENT:
fail("expected ARM ELF32 ET_REL")
if ((e_flags >> 24) & 0xFF) != 5:
fail("expected ARM EABI5 e_flags")
if (e_flags & 0x00FFFFFF) not in (0, 0x200):
fail("unsupported ARM float ABI flags")
if e_ehsize != ELF_HEADER.size or e_phnum != 0 or e_phoff != 0:
fail("ET_REL must not contain program headers")
if e_shentsize != SECTION_HEADER.size or e_shnum == 0 or e_shstrndx >= e_shnum:
fail("invalid section-header table")
require_range(data, e_shoff, e_shnum * e_shentsize, "section-header table")
sections: list[Section] = []
for index in range(e_shnum):
values = SECTION_HEADER.unpack_from(data, e_shoff + index * e_shentsize)
section = Section(*values)
if section.type != SHT_NOBITS:
require_range(data, section.offset, section.size, "section " + str(index))
sections.append(section)
names_section = sections[e_shstrndx]
if names_section.type != SHT_STRTAB:
fail("section-name table is not a string table")
names = data[names_section.offset : names_section.offset + names_section.size]
sections = [
Section(
section.name_offset,
section.type,
section.flags,
section.address,
section.offset,
section.size,
section.link,
section.info,
section.alignment,
section.entry_size,
c_string(names, section.name_offset) if section.name_offset else "",
)
for section in sections
]
symbol_sections = [section for section in sections if section.type == SHT_SYMTAB]
if len(symbol_sections) != 1:
fail("expected exactly one .symtab section")
symbol_section = symbol_sections[0]
if symbol_section.name != ".symtab" or symbol_section.entry_size != SYMBOL.size:
fail("symbol table format is invalid")
if symbol_section.link >= len(sections):
fail("symbol table string-table link is invalid")
strings_section = sections[symbol_section.link]
if strings_section.type != SHT_STRTAB or symbol_section.size % symbol_section.entry_size:
fail("symbol table format is invalid")
strings = data[strings_section.offset : strings_section.offset + strings_section.size]
symbols: list[ElfSymbol] = []
for offset in range(
symbol_section.offset,
symbol_section.offset + symbol_section.size,
symbol_section.entry_size,
):
values = SYMBOL.unpack_from(data, offset)
name = c_string(strings, values[0]) if values[0] else ""
symbols.append(ElfSymbol(name, *values[1:]))
undefined = [
symbol.name or "<anonymous>"
for symbol in symbols[1:]
if symbol.section_index == SHN_UNDEF
]
if undefined:
fail("undefined imports are not allowed: " + ", ".join(undefined))
entry_symbols = [symbol for symbol in symbols if symbol.name == "module_initialize"]
if len(entry_symbols) != 1:
fail("module_initialize must be exported exactly once")
entry_symbol = entry_symbols[0]
if entry_symbol.section_index == SHN_UNDEF or (entry_symbol.info & 0x0F) != STT_FUNC:
fail("module_initialize is not a defined function")
if e_entry != 0:
fail("XiaomiVela ET_REL modules must have a zero ELF entry")
required_sections = {
".text": (SHF_ALLOC | SHF_EXECINSTR, False),
".rodata": (SHF_ALLOC, False),
".data": (SHF_ALLOC | SHF_WRITE, False),
".bss": (SHF_ALLOC | SHF_WRITE, True),
}
by_name = {section.name: section for section in sections}
for name, (required_flags, must_be_nobits) in required_sections.items():
section = by_name.get(name)
if section is None:
fail("missing required NuttX module section " + name)
if not has_flag(section.flags, required_flags):
fail("section " + name + " has incompatible flags")
if must_be_nobits != (section.type == SHT_NOBITS):
fail("section " + name + " has an incompatible type")
allowed_alloc_sections = set(required_sections) | {".ARM.exidx", ".init_array"}
unexpected_alloc = sorted(
section.name or "<anonymous>"
for section in sections
if has_flag(section.flags, SHF_ALLOC) and section.name not in allowed_alloc_sections
)
if unexpected_alloc:
fail("unexpected allocated sections: " + ", ".join(unexpected_alloc))
if any(section.name == ".preinit_array" for section in sections):
fail(".preinit_array is prohibited")
relocation_offsets: set[tuple[int, int]] = set()
for section in sections:
if section.type == SHT_RELA:
fail("SHT_RELA relocations are not accepted by this target profile")
if section.type != SHT_REL:
continue
if section.entry_size != RELOCATION.size or section.size % section.entry_size:
fail("invalid REL relocation section " + section.name)
if section.info == 0 or section.info >= len(sections):
fail("relocation section has no valid target: " + section.name)
for offset in range(section.offset, section.offset + section.size, section.entry_size):
relocation_offset, _ = RELOCATION.unpack_from(data, offset)
relocation_offsets.add((section.info, relocation_offset))
found_addresses = scan_absolute_addresses(
data,
sections,
symbols,
relocation_offsets,
firmware_functions,
firmware_data,
)
relocations: set[int] = set()
for section in sections:
if section.type != SHT_REL:
continue
if section.link >= len(sections) or sections[section.link].type != SHT_SYMTAB:
fail("relocation section has no valid symbol table: " + section.name)
target = sections[section.info]
if not has_flag(target.flags, SHF_ALLOC):
fail("relocation target is not allocated: " + section.name)
for offset in range(section.offset, section.offset + section.size, section.entry_size):
relocation_offset, info = RELOCATION.unpack_from(data, offset)
if relocation_offset + 4 > target.size:
fail("relocation is outside target section: " + section.name)
symbol_index = info >> 8
if symbol_index >= len(symbols):
fail("relocation references an out-of-range symbol")
if symbol_index and symbols[symbol_index].section_index == SHN_UNDEF:
fail("relocation references an undefined symbol")
relocation_type = info & 0xFF
allowed_types = {R_ARM_ABS32}
if section.name == ".rel.ARM.exidx":
allowed_types.update({0, 42})
if section.name == ".rel.init_array":
allowed_types.add(R_ARM_TARGET1)
if relocation_type not in allowed_types:
fail(
"unsupported relocation type "
+ str(relocation_type)
+ " in "
+ section.name
)
relocations.add(relocation_type)
metadata = {
"entry": e_entry,
"entry_symbol": entry_symbol.value,
"loaded_size": section_footprint(sections),
"file_size": len(data),
"bss_size": by_name[".bss"].size,
}
return metadata, sections, symbols, relocations, found_addresses
def main() -> int:
parser = argparse.ArgumentParser(description=__doc__)
parser.add_argument("module", type=Path)
parser.add_argument("--abi-header", type=Path, required=True)
parser.add_argument("--max-loaded-size", type=int, required=True)
parser.add_argument("--max-bss-size", type=int, required=True)
args = parser.parse_args()
try:
firmware_functions, firmware_data = read_abi_addresses(args.abi_header)
metadata, sections, symbols, relocations, addresses = parse_elf(
args.module, firmware_functions, firmware_data
)
if metadata["loaded_size"] >= args.max_loaded_size:
fail(
"SHF_ALLOC size "
+ str(metadata["loaded_size"])
+ " must be strictly less than "
+ str(args.max_loaded_size)
)
if metadata["bss_size"] > args.max_bss_size:
fail(
".bss size "
+ str(metadata["bss_size"])
+ " exceeds limit "
+ str(args.max_bss_size)
)
except (OSError, VerificationError) as error:
print("FAIL: " + str(error), file=sys.stderr)
return 1
relocation_list = ", ".join(str(value) for value in sorted(relocations)) or "none"
print("OK: " + str(args.module))
print(" file size: " + str(metadata["file_size"]) + " bytes")
print(" SHF_ALLOC total: " + str(metadata["loaded_size"]) + " bytes")
print(" .bss: " + str(metadata["bss_size"]) + " bytes")
print(" entry: 0x" + format(metadata["entry"], "08x"))
print(" module_initialize: 0x" + format(metadata["entry_symbol"], "08x"))
print(" defined symbols: " + str(len(symbols) - 1))
print(" direct target ABI literals: " + str(len(addresses)))
print(" ARM relocation types: " + relocation_list)
print(" sections: " + ", ".join(section.name or "<null>" for section in sections))
return 0
if __name__ == "__main__":
raise SystemExit(main())
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