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"""Declarative structure decoding for firmware memory.
A schema is plain data (JSON-friendly), so the same description can come from
the repo, a Ghidra export, or a plugin:
{
"name": "gsp_ctrl",
"fields": [
{"name": "magic", "offset": 0, "fmt": "u32"},
{"name": "state", "offset": 8, "fmt": "u32", "enum": {"0": "IDLE", "1": "RUN"}},
{"name": "buf", "offset": 64, "fmt": "bytes", "size": 256},
{"name": "name", "offset": 64, "fmt": "cstr", "size": 32}
]
}
Supported formats: u8/u16/u32/u64, s8/s16/s32/s64, f32/f64, bool, bytes,
cstr, ptr (u64 by default; override with ``size``).
"""
from __future__ import annotations
import struct as _struct
from dataclasses import dataclass, field as _field
from .errors import StructError
_INT_FORMATS = {
"u8": ("B", 1),
"u16": ("H", 2),
"u32": ("I", 4),
"u64": ("Q", 8),
"s8": ("b", 1),
"s16": ("h", 2),
"s32": ("i", 4),
"s64": ("q", 8),
"bool": ("B", 1),
}
_FLOAT_FORMATS = {"f32": ("f", 4), "f64": ("d", 8)}
@dataclass
class Field:
name: str
offset: int
fmt: str
size: int = 0
enum: dict | None = None
note: str | None = None
count: int = 1
@property
def nbytes(self) -> int:
if self.fmt in _INT_FORMATS:
return _INT_FORMATS[self.fmt][1]
if self.fmt in _FLOAT_FORMATS:
return _FLOAT_FORMATS[self.fmt][1]
if self.fmt in ("bytes", "cstr"):
return self.size
if self.fmt == "ptr":
return self.size or 8
if self.fmt == "u64x":
return 8
raise StructError(f"field {self.name!r}: unknown format {self.fmt!r}")
@dataclass
class Schema:
name: str
fields: list[Field] = _field(default_factory=list)
size: int = 0
def __post_init__(self) -> None:
computed = max((f.offset + f.nbytes for f in self.fields), default=0)
if not self.size:
self.size = computed
elif self.size < computed:
raise StructError(
f"schema {self.name!r}: declared size {self.size:#x} < fields end {computed:#x}"
)
self.validate()
def validate(self) -> None:
seen: set[str] = set()
for field in self.fields:
if field.name in seen:
raise StructError(f"schema {self.name!r}: duplicate field {field.name!r}")
seen.add(field.name)
if field.offset < 0:
raise StructError(f"schema {self.name!r}: negative offset for {field.name!r}")
if field.nbytes <= 0:
raise StructError(f"schema {self.name!r}: zero-size field {field.name!r}")
def decode(self, data: bytes, base: int = 0) -> dict:
if len(data) < self.size:
raise StructError(
f"schema {self.name!r}: need {self.size} bytes, got {len(data)}"
)
out: dict = {}
for field in self.fields:
out[field.name] = _decode_field(field, data, base)
return out
def _decode_field(field: Field, data: bytes, base: int):
start = field.offset
if field.fmt in _INT_FORMATS:
code, width = _INT_FORMATS[field.fmt]
value = _struct.unpack_from("<" + code, data, start)[0]
if field.fmt == "bool":
value = bool(value)
return value
if field.fmt in _FLOAT_FORMATS:
code, _width = _FLOAT_FORMATS[field.fmt]
return _struct.unpack_from("<" + code, data, start)[0]
if field.fmt == "ptr":
return int.from_bytes(data[start : start + field.nbytes], "little")
if field.fmt == "bytes":
return bytes(data[start : start + field.size])
if field.fmt == "cstr":
raw = data[start : start + field.size]
return raw.split(b"\x00", 1)[0].decode("utf-8", "replace")
raise StructError(f"unknown format {field.fmt!r}")
def decode(schema: Schema, data: bytes, base: int = 0) -> dict:
return schema.decode(data, base=base)
def schema_from_json(payload: dict) -> Schema:
if not isinstance(payload, dict) or "fields" not in payload:
raise StructError("schema JSON must be an object with a 'fields' list")
fields = []
for item in payload["fields"]:
try:
fields.append(
Field(
name=item["name"],
offset=int(item["offset"]),
fmt=item.get("fmt", "bytes"),
size=int(item.get("size", 0)),
enum=item.get("enum"),
note=item.get("note"),
count=int(item.get("count", 1)),
)
)
except KeyError as exc:
raise StructError(f"schema field missing key: {exc}") from exc
return Schema(name=payload.get("name", "schema"), fields=fields, size=int(payload.get("size", 0)))
def schema_to_json(schema: Schema) -> dict:
return {
"name": schema.name,
"size": schema.size,
"fields": [
{
k: v
for k, v in {
"name": f.name,
"offset": f.offset,
"fmt": f.fmt,
"size": f.size or None,
"enum": f.enum,
"note": f.note,
}.items()
if v is not None
}
for f in schema.fields
],
}
def render(schema: Schema, values: dict, indent: str = " ") -> str:
lines = [f"{schema.name} ({{base}}+{schema.size:#x} bytes)"]
width = max((len(f.name) for f in schema.fields), default=0)
for field in schema.fields:
value = values.get(field.name)
lines.append(f"{indent}{field.name:<{width}} @+0x{field.offset:03x} = {_format_value(field, value)}")
if field.note:
lines.append(f"{indent}{'':<{width}} ; {field.note}")
return "\n".join(lines)
def _format_value(field: Field, value) -> str:
if isinstance(value, bool):
return str(value)
if isinstance(value, float):
return f"{value:g}"
if isinstance(value, int):
text = f"{value:#x}" if not field.enum else f"{value} ({field.enum.get(str(value), '?')})"
if field.enum and field.fmt not in ("bool",):
text = f"{value:#x} ({field.enum.get(str(value), field.enum.get(value, '?'))})"
return text
if isinstance(value, (bytes, bytearray)):
head = bytes(value[:16]).hex(" ")
suffix = " ..." if len(value) > 16 else ""
return f"[{len(value)}B] {head}{suffix}"
return repr(value)
|
