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#!/usr/bin/env python3
"""Mechanically re-derive every claim in CANDIDATE-SYSDRV-E04B68-COUNT-UNDERFLOW.md.
The chain was assembled by hand over many passes, and in that stretch four separate readings
turned out to be wrong (a truncated listing, immediates mistaken for a stored value, a
misattributed base register, a register reassignment missed). Prose is therefore not adequate
evidence. This script asserts each structural claim directly against the bytes of
`psp_out/sys_drv_code.bin`, so the candidate stands or falls on reproducible checks.
Every check prints PASS/FAIL and the instruction it matched. A FAIL means the write-up is
wrong, not that the firmware changed.
Usage: ./verify_e04b68_chain.py
"""
from __future__ import annotations
import re
import struct
import sys
from pathlib import Path
import capstone as cs
REPO = Path(__file__).resolve().parent.parent
BASE = 0xE00100
THUMB = cs.Cs(cs.CS_ARCH_ARM, cs.CS_MODE_THUMB)
B = (REPO / "psp_out" / "sys_drv_code.bin").read_bytes()
def dis(lo: int, n: int):
return list(THUMB.disasm(B[lo - BASE:lo - BASE + n], lo))
def at(lo: int, n: int, addr: int):
for i in dis(lo, n):
if i.address == addr:
return i
return None
def literal(i) -> int | None:
"""Resolve `ldr rX,[pc,#imm]` to the loaded word."""
if not (i.mnemonic.startswith("ldr") and "pc" in (i.op_str or "")):
return None
m = re.search(r"#(0x[0-9a-f]+|\d+)", i.op_str)
if not m:
return None
lit = ((i.address + 4) & ~3) + int(m.group(1), 0)
off = lit - BASE
if not (0 <= off < len(B) - 4):
return None
return struct.unpack_from("<I", B, off)[0]
RESULTS: list[tuple[bool, str, str]] = []
def check(name: str, ok: bool, detail: str) -> None:
RESULTS.append((ok, name, detail))
def main() -> int:
# --- Step 3: sub_E04B68 reads buf[0x88], clamps to 0x400, subtracts UNSIGNED ----------
i = at(0xE04B68, 0x80, 0xE04B70)
check("E04B68 reads the count at buf+0x88 before any validation",
i is not None and i.mnemonic.startswith("ldr") and "#0x88" in (i.op_str or ""),
f"{i.address:#x} {i.mnemonic} {i.op_str}" if i else "not found")
i = at(0xE04B68, 0x80, 0xE04B8E)
clamp = i is not None and i.mnemonic.startswith("mov") and "0x400" in (i.op_str or "")
check("E04B68 clamps the total to 0x400", clamp,
f"{i.address:#x} {i.mnemonic} {i.op_str}" if i else "not found")
i = at(0xE04B68, 0x80, 0xE04BBA)
check("E04B68 computes copy2 length as an UNSIGNED subtraction (subs r2, r7, r0)",
i is not None and i.mnemonic == "subs" and re.match(r"r2,\s*r7,\s*r0", i.op_str or ""),
f"{i.address:#x} {i.mnemonic} {i.op_str}" if i else "not found")
i = at(0xE04B68, 0x80, 0xE04BAE)
check("E04B68 copy1 uses the count unclamped (call follows ldr r2,[r4,#0x88])",
i is not None and i.mnemonic in ("blx", "bl"),
f"{i.address:#x} {i.mnemonic} {i.op_str}" if i else "not found")
# --- Step 2: sub_E1D6BC arm copies payload into slot->buf, bounded only by capacity ----
i = at(0xE1D6BC, 0x260, 0xE1D8A4)
check("E1D6BC arm loads the slot buffer (ldr r5,[r4,#0x4c])",
i is not None and "#0x4c" in (i.op_str or ""),
f"{i.address:#x} {i.mnemonic} {i.op_str}" if i else "not found")
i = at(0xE1D6BC, 0x260, 0xE1D8AA)
check("E1D6BC arm performs the payload copy into that buffer",
i is not None and i.mnemonic in ("blx", "bl"),
f"{i.address:#x} {i.mnemonic} {i.op_str}" if i else "not found")
i = at(0xE1D6BC, 0x260, 0xE1D6F8)
j = at(0xE1D6BC, 0x260, 0xE1D6FC)
check("E1D6BC bounds the message length only by slot capacity [slot+0x48]",
i is not None and "#0x48" in (i.op_str or "") and j is not None and j.mnemonic == "bhi",
f"{i.address:#x} {i.mnemonic} {i.op_str} ; {j.address:#x} {j.mnemonic}" if i and j else "not found")
# --- arm selection: slot[0] == 0x80000032 -------------------------------------------
sel = None
for i in dis(0xE1D6BC, 0x260):
if literal(i) == 0x7FFFFFCE:
sel = i
break
check("E1D6BC selects that arm via the 0x7fffffce normalisation (slot[0] == 0x80000032)",
sel is not None, f"{sel.address:#x} {sel.mnemonic} {sel.op_str} ;=0x7fffffce" if sel else "not found")
# --- Step 1: the allocator accepts format 0x80000032 the same way ---------------------
acc = None
for i in dis(0xE055D0, 0x3AA):
if literal(i) == 0x7FFFFFCE:
acc = i
break
ok_acc = False
detail = "not found"
if acc is not None:
nxt = [x for x in dis(0xE055D0, 0x3AA) if acc.address < x.address <= acc.address + 6]
ok_acc = any(x.mnemonic.startswith("adds") for x in nxt) and any(x.mnemonic == "beq" for x in nxt)
detail = f"{acc.address:#x} ldr ;=0x7fffffce then " + ", ".join(
f"{x.mnemonic} {x.op_str}" for x in nxt)
check("allocator accepts format 0x80000032 via the same adds/beq normalisation",
ok_acc, detail)
# --- capacity is caller-scaled and stored at slot+0x48 --------------------------------
i = at(0xE055D0, 0x3AA, 0xE05900)
j = at(0xE055D0, 0x3AA, 0xE05902)
k = at(0xE055D0, 0x3AA, 0xE05908)
ok_cap = (i is not None and i.mnemonic == "lsrs" and "#4" in (i.op_str or "")
and j is not None and "lsl #3" in (j.op_str or "")
and k is not None and "#0x48" in (k.op_str or ""))
check("capacity := 9*(n>>4)+0x10 stored to slot+0x48",
ok_cap,
"; ".join(f"{x.address:#x} {x.mnemonic} {x.op_str}" for x in (i, j, k) if x))
# the operand of that shift must be reloaded from the request, not the format code
reload_site = at(0xE055D0, 0x3AA, 0xE058DA)
check("that capacity operand is a request field (ldr r7,[r2,#0xc]), NOT the format code",
reload_site is not None and re.match(r"r7,\s*\[r2,\s*#0xc\]", reload_site.op_str or ""),
f"{reload_site.address:#x} {reload_site.mnemonic} {reload_site.op_str}" if reload_site else "not found")
# --- slot[0] is the caller-supplied format code --------------------------------------
i = at(0xE055D0, 0x3AA, 0xE0590C)
j = at(0xE055D0, 0x3AA, 0xE0590E)
check("slot[0] := the caller's format code (ldr r3,[r2,#4] ; strd r3,r6,[r1])",
i is not None and re.match(r"r3,\s*\[r2,\s*#4\]", i.op_str or "")
and j is not None and j.mnemonic.startswith("strd"),
"; ".join(f"{x.address:#x} {x.mnemonic} {x.op_str}" for x in (i, j) if x))
# --- the count field is inside the message-writable region ---------------------------
caps = [9 * (n >> 4) + 0x10 for n in (0xE0, 0x100, 0x400)]
check("caller-selectable capacities exceed the count offset 0x8c",
all(c >= 0x8C for c in caps), f"n=0xe0/0x100/0x400 -> {[hex(c) for c in caps]}")
# --- the underflow itself --------------------------------------------------------
count, clampv = 0x500, 0x400
check("count > clamp underflows copy2's length",
((clampv - count) & 0xFFFFFFFF) > 0xFFFF0000,
f"0x400 - 0x500 = {((clampv - count) & 0xFFFFFFFF):#x}")
width = max(len(n) for _, n, _ in RESULTS)
bad = 0
for ok, name, detail in RESULTS:
print(f"[{'PASS' if ok else 'FAIL'}] {name.ljust(width)} {detail}")
bad += 0 if ok else 1
print(f"\n{len(RESULTS) - bad}/{len(RESULTS)} structural claims verified against the binary")
if bad:
print("FAIL means the write-up is wrong and must be corrected.")
return 1 if bad else 0
if __name__ == "__main__":
sys.exit(main())
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