Rename programs: 9player -> 9ns, introspect -> 9proc, app -> web (9web)

Directories, binaries, build options (-D9ns, -D9proc), step names, module
name (9proc), thread and fs names, env var NINEPLAYER_MOUNT -> NINE_MOUNT,
docs and test scripts. Browser assets move to web/static.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

1 files changed, 1018 insertions, 0 deletions

diff --git a/9proc/test/adv_core_hostile.py b/9proc/test/adv_core_hostile.py
new file mode 100755
index 0000000..464876f
--- /dev/null
+++ b/9proc/test/adv_core_hostile.py
@@ -0,0 +1,1018 @@
+#!/usr/bin/env python3
+"""Hostile raw-9P2000 client aimed at the 9proc *core* (stdlib only).
+
+Complements adv_9proc_hostile.py in this directory (framing, tags, scratch, floods)
+with attacks on the freestanding engine's own paths: the /vars tree and its
+comptime renderers, snapshot slots, the static tree, the fid table at its
+configured maximum, directory-read offsets, msize 24, the ctl staging rule,
+and the demo's debug providers driven as black boxes.
+
+Usage:
+    adv_core_hostile.py --server zig-out/bin/9proc-demo   # spawns it on a temp unix socket
+    adv_core_hostile.py --socket PATH                      # attacks a running server
+
+Exit status is non-zero if any check fails or the server dies.
+"""
+import argparse
+import os
+import signal
+import struct
+import subprocess
+import sys
+import tempfile
+import threading
+import time
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+import adv_9proc_hostile as base  # noqa: E402
+from adv_9proc_hostile import (  # noqa: E402
+    NOTAG, Tversion, Tflush, Rflush, Twalk, Rwalk, Topen, Ropen, Rcreate,
+    Tread, Rread, Twrite, Rwrite, Tclunk, Rclunk, Tremove, Rremove, Tstat, Rstat, Twstat, Rwstat,
+    Rerror, OREAD, OWRITE, ORDWR, OEXEC, OTRUNC, ORCLOSE, DMDIR,
+    Nine, frame, s16, mkstat, parse_stat, ok, healthy, expect_dead,
+)
+
+MAX_FIDS = 32768          # demo/main.zig cfg.max_fids
+SNAPSHOT_SLOTS = 8        # demo/main.zig cfg.snapshot_slots (per connection)
+SCRATCH_BUDGET = 512 << 20
+SCRATCH_MAX_FILE = 64 << 20
+
+
+def records(d):
+    """Splits a directory read into (name, raw-record) pairs."""
+    out = []
+    while d:
+        n, = struct.unpack_from("<H", d)
+        out.append((parse_stat(d[:n + 2])["name"], d[:n + 2]))
+        d = d[n + 2:]
+    return out
+
+
+def qid_of_walk(rb):
+    n, = struct.unpack_from("<H", rb)
+    return [struct.unpack_from("<BIQ", rb, 2 + 13 * i) for i in range(n)]
+
+
+def worker_tid(c):
+    """The tid of the demo's worker thread, via /threads/<tid>/name."""
+    c.walk_ok(0, 40, [b"threads"])
+    c.open(40, OREAD)
+    d = c.read_all(40)
+    c.clunk(40)
+    for name, _ in records(d):
+        if c.path_read([b"threads", name, b"name"], fid=41) == b"worker":
+            return name
+    return None
+
+
+# --------------------------------------------------------------------------- /vars
+
+
+def attack_vars(path):
+    print("# /vars: deep walks, hostile names, renderer edge cases, hostile writes")
+    c = Nine(path)
+    c.session(1 << 20)
+    deep = [b"vars", b"state", b"f", b"last_job", b"f", b"id", b".", b"..", b"id", b".", b"..", b"id", b".", b"..", b"id", b"value"]
+    assert len(deep) == 16
+    ok("16-element walk deep into /vars/state/f/... succeeds", c.walk_ok(0, 1, deep) == 16)
+    rt, _, _ = c.open(1, OREAD)
+    ok("deep walk lands on a readable value file", rt == Ropen, rt)
+    c.clunk(1)
+    up = [b"vars", b"state", b"f", b"inner"] if False else [b"vars", b"state", b"f", b"last_job"] + [b".."] * 12
+    n = c.walk_ok(0, 1, up)
+    ok("12 x '..' from inside /vars climbs to the root and stays there", n == 16, n)
+    rt, st = c.stat(1)
+    ok("fid after the climb is the root directory", rt == Rstat and st["qid"][2] == 0xFF << 56, st)
+    c.clunk(1)
+    # names that are hex/decimal edge cases or otherwise hostile: never anything but Rerror/partial walk
+    for nm in (b"0", b"-1", b"0x", b"0x0", b"state\x00", b"State", b" state", b"state ", b"a" * 255, b"a" * 65535, b"\xff\xfe", b"..\x00", b"f", b"value"):
+        n = c.walk_ok(0, 1, [b"vars", nm])
+        ok(f"walk /vars/{nm[:12]!r}{'...' if len(nm) > 12 else ''} is a partial walk (1)", n == 1, n)
+        ok("  and newfid stays unbound", c.err(Tclunk, struct.pack("<I", 1)) == "unknown fid")
+    for nm in (b"0", b"F", b"f\x00", b"ticks", b"value ", b"raw\x00"):
+        n = c.walk_ok(0, 1, [b"vars", b"state", nm])
+        ok(f"walk /vars/state/{nm!r} is a partial walk (2)", n == 2, n)
+    # . and .. on var files and directories
+    ok("walk '.' from a var file is 'not a directory'", c.walk_ok(0, 1, [b"vars", b"state", b"value"]) == 3 and c.err(Twalk, struct.pack("<IIH", 1, 2, 1) + s16(b".")) == "not a directory")
+    ok("walk '..' from a var file is 'not a directory'", c.err(Twalk, struct.pack("<IIH", 1, 2, 1) + s16(b"..")) == "not a directory")
+    c.clunk(1)
+    c.walk_ok(0, 1, [b"vars", b"state", b"f"])
+    rt, _, rb = c.walk(1, 2, [b".", b"..", b"..", b".."])
+    q = qid_of_walk(rb) if rt == Rwalk else []
+    ok("'.' and '..' through the var tree: f -> state -> /vars -> /", len(q) == 4 and q[3][2] == 0xFF << 56 and (q[1][0] & 0x80), q)
+    c.clunk(1)
+    c.clunk(2)
+    # every file under /vars/state reads; raw reads beyond @sizeOf are empty
+    size = int(c.path_read([b"vars", b"state", b"size"]))
+    ok("/vars/state/size is a number", size > 0, size)
+    raw = c.path_read([b"vars", b"state", b"raw"])
+    ok("/vars/state/raw has exactly @sizeOf bytes", raw is not None and len(raw) == size, (len(raw) if raw else raw, size))
+    c.walk_ok(0, 1, [b"vars", b"state", b"raw"])
+    c.open(1, OREAD)
+    rt, d = c.read(1, size, 100)
+    ok("raw read at offset @sizeOf is empty", rt == Rread and d == b"", (rt, d))
+    rt, d = c.read(1, size - 1, 100)
+    ok("raw read at @sizeOf-1 returns one byte", rt == Rread and len(d) == 1, (rt, d))
+    rt, d = c.read(1, (1 << 64) - 1, 100)
+    ok("raw read at 2^64-1 is empty", rt == Rread and d == b"")
+    rt, d = c.read(1, 0, 0xFFFFFFFF)
+    ok("raw read with count 2^32-1 is clamped", rt == Rread and len(d) == size, (rt, len(d) if d else d))
+    rt, st = c.stat(1)
+    ok("raw stat length is @sizeOf and mode 0444", rt == Rstat and st["length"] == size and st["mode"] == 0o444, st)
+    ok("raw is read-only", c.err(Twrite, struct.pack("<IQI", 1, 0, 1) + b"x") is not None)
+    c.clunk(1)
+    for leaf in (b"type", b"size", b"addr", b"value"):
+        c.walk_ok(0, 1, [b"vars", b"state", leaf])
+        e = c.err(Topen, struct.pack("<IB", 1, OWRITE))
+        ok(f"/vars/state/{leaf.decode()} refuses OWRITE", e == "permission denied", e)
+        e = c.err(Topen, struct.pack("<IB", 1, OREAD | OTRUNC))
+        ok(f"/vars/state/{leaf.decode()} refuses OTRUNC", e == "permission denied", e)
+        c.clunk(1)
+    v = c.path_read([b"vars", b"state", b"value"])
+    ok("/vars/state/value renders every field", v is not None and all(k in v for k in (b"ticks:", b"phase:", b"last_job:", b"id:", b"cost:")), v)
+    ok("nested struct is indented", b"\n  id: " in (v or b""), v)
+    # dynamic file: read at offset 0 regenerates, offset 1 is the tail of the same snapshot
+    c.walk_ok(0, 1, [b"vars", b"state", b"value"])
+    c.open(1, OREAD)
+    rt, d = c.read(1, 0, 8192)
+    rt2, d2 = c.read(1, 1, 8192)
+    ok("value read at offset 1 is the tail of the snapshot", rt == Rread and rt2 == Rread and d2 == d[1:], (d, d2))
+    rt3, d3 = c.read(1, len(d), 8192)
+    ok("value read at the end is empty", rt3 == Rread and d3 == b"")
+    rt4, d4 = c.read(1, (1 << 63) + 5, 10)
+    ok("value read at 2^63+5 is empty", rt4 == Rread and d4 == b"")
+    rt, st = c.stat(1)
+    ok("value stat reports length 0 (dynamic)", rt == Rstat and st["length"] == 0, st)
+    c.clunk(1)
+    # hostile writes to scalar values: garbage, huge, negative, floats with exponents, NULs, empty
+    c.walk_ok(0, 1, [b"vars", b"state", b"f", b"ticks", b"value"])
+    rt, _, _ = c.open(1, OWRITE | OTRUNC)
+    ok("open ticks/value OWRITE|OTRUNC", rt == Ropen, rt)
+    for bad in (b"abc", b"99999999999999999999999", b"-1", b"1e3", b"", b"   ", b"4\x002", b"1.5", b"0x", b"+", b"\xd9\xa1\xd9\xa2", b"12 34", b"0b102"):
+        e = c.err(Twrite, struct.pack("<IQI", 1, 0, len(bad)) + bad)
+        ok(f"write {bad!r} to u64 value is 'bad value'", e == "bad value", e)
+    ok("read on the write-only value fid is 'file not open'", c.err(Tread, struct.pack("<IQI", 1, 0, 10)) == "file not open")
+    for good, want in ((b"  4200  \n", 4200), (b"0x10", 16), (b"+7", 7), (b"0b1010", 10), (b"0o17", 15), (b"1_000", 1000), (b"18446744073709551615", (1 << 64) - 1)):
+        rt, _, rb = c.write(1, (1 << 64) - 1, good)  # offset is ignored for values
+        got = c.path_read([b"vars", b"state", b"f", b"ticks", b"value"])
+        try:
+            gv = int(got)
+        except (TypeError, ValueError):
+            gv = None
+        # the worker keeps incrementing (wrapping), so allow a small drift
+        ok(f"write {good!r} stores {want}", rt == Rwrite and gv is not None and (gv - want) % (1 << 64) < 100_000, (rt, got))
+    c.write(1, 0, b"1")
+    c.clunk(1)
+    # enum and float and u32 leaves
+    c.walk_ok(0, 1, [b"vars", b"state", b"f", b"phase", b"value"])
+    c.open(1, ORDWR)
+    for bad in (b"trap\x00ped", b"IDLE", b"2", b"", b"idle extra", b"\x00idle\x00x", b"idl", b"idle\x00\x00x"):
+        e = c.err(Twrite, struct.pack("<IQI", 1, 0, len(bad)) + bad)
+        ok(f"enum write {bad!r} is 'bad value'", e == "bad value", e)
+    rt, _, _ = c.write(1, 0, b"\n idle \x00")
+    rt2, d = c.read(1, 0, 100)
+    ok("enum write with surrounding whitespace/NUL is accepted", rt == Rwrite and d in (b"idle", b"working", b"trapped"), (rt, d))
+    rt, st = c.stat(1)
+    ok("enum value is 0644", rt == Rstat and st["mode"] == 0o644, st)
+    c.clunk(1)
+    c.walk_ok(0, 1, [b"vars", b"state", b"f", b"last_job", b"f", b"cost", b"value"])
+    c.open(1, ORDWR)
+    for bad in (b"abc", b"1.5.5", b"e5", b"", b"0x", b"1e", b"--1"):
+        ok(f"float write {bad!r} is 'bad value'", c.err(Twrite, struct.pack("<IQI", 1, 0, len(bad)) + bad) == "bad value")
+    for good in (b"1.5e3", b"-0x1p-2", b"1e999", b"nan", b"-0", b"2.5"):
+        rt, _, _ = c.write(1, 0, good)
+        rt2, d = c.read(1, 0, 100)
+        ok(f"float write {good!r} accepted and renders ({d!r})", rt == Rwrite and rt2 == Rread and d != b"", (rt, d))
+    c.write(1, 0, b"0")
+    c.clunk(1)
+    c.walk_ok(0, 1, [b"vars", b"state", b"f", b"last_job", b"f", b"id", b"value"])
+    c.open(1, OWRITE)
+    ok("u32 write 4294967296 is 'bad value'", c.err(Twrite, struct.pack("<IQI", 1, 0, 10) + b"4294967296") == "bad value")
+    ok("u32 write -0 is accepted as 0 (std.fmt.parseInt semantics)", c.write(1, 0, b"-0")[0] == Rwrite)
+    ok("u32 write -1 is 'bad value'", c.err(Twrite, struct.pack("<IQI", 1, 0, 2) + b"-1") == "bad value")
+    rt, _, _ = c.write(1, 0, b"4294967295")
+    ok("u32 write 4294967295 accepted", rt == Rwrite)
+    c.write(1, 0, b"0")
+    c.clunk(1)
+    # struct values and the f directory are not writable; the tree admits no create/remove/wstat
+    for names in ([b"vars"], [b"vars", b"state"], [b"vars", b"state", b"f"], [b"vars", b"state", b"f", b"last_job"], [b"vars", b"state", b"f", b"last_job", b"f"]):
+        c.walk_ok(0, 1, names)
+        p = b"/".join(names).decode()
+        ok(f"create in {p} is denied", c.err(base.Tcreate, struct.pack("<I", 1) + s16(b"x") + struct.pack("<IB", 0o644, OWRITE)) == "permission denied")
+        ok(f"wstat of {p} is denied", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(name=b"y"))) == "permission denied")
+        ok(f"open {p} for write is 'is a directory'", c.err(Topen, struct.pack("<IB", 1, OWRITE)) == "is a directory")
+        ok(f"remove {p} is denied", c.err(Tremove, struct.pack("<I", 1)) == "permission denied")
+        ok(f"  and the fid was clunked", c.err(Tclunk, struct.pack("<I", 1)) == "unknown fid")
+    for names in ([b"vars", b"state", b"value"], [b"vars", b"state", b"f", b"last_job", b"value"], [b"vars", b"state", b"type"]):
+        c.walk_ok(0, 1, names)
+        p = b"/".join(names).decode()
+        ok(f"wstat of {p} is denied", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(length=0))) == "permission denied")
+        ok(f"{p} is not writable", c.err(Topen, struct.pack("<IB", 1, ORDWR)) == "permission denied")
+        ok(f"remove {p} is denied", c.err(Tremove, struct.pack("<I", 1)) == "permission denied")
+    # directory listing of /vars/state and of f/, exact record boundaries
+    c.walk_ok(0, 1, [b"vars", b"state"])
+    c.open(1, OREAD)
+    d = c.read_all(1)
+    names = [n for n, _ in records(d)]
+    ok("/vars/state lists value,type,size,addr,raw,f", sorted(names) == sorted([b"value", b"type", b"size", b"addr", b"raw", b"f"]), names)
+    c.clunk(1)
+    c.walk_ok(0, 1, [b"vars", b"state", b"f"])
+    c.open(1, OREAD)
+    d = c.read_all(1)
+    names = [n for n, _ in records(d)]
+    ok("/vars/state/f lists the three fields", sorted(names) == [b"last_job", b"phase", b"ticks"], names)
+    c.clunk(1)
+    c.close()
+    ok("server healthy after /vars attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- snapshot slots
+
+
+def attack_snapshots(path):
+    print("# snapshot slots: exhaustion, hold, release by clunk and by Tversion; per-connection")
+    dyn = [[b"runtime", b"pid"], [b"runtime", b"ppid"], [b"runtime", b"uptime"], [b"runtime", b"fn", b"now"], [b"runtime", b"fn", b"fib30"],
+           [b"vars", b"state", b"value"], [b"vars", b"state", b"addr"], [b"vars", b"state", b"f", b"ticks", b"value"], [b"vars", b"state", b"f", b"phase", b"value"], [b"runtime", b"fn", b"uname"]]
+    c = Nine(path)
+    c.session()
+    opened = 0
+    err = None
+    for i, names in enumerate(dyn):
+        c.walk_ok(0, 100 + i, names)
+        rt, _, rb = c.open(100 + i, OREAD)
+        if rt == Ropen:
+            opened += 1
+        else:
+            err = c.err.__self__ and rb
+            n, = struct.unpack_from("<H", rb)
+            err = rb[2:2 + n].decode()
+            break
+    ok(f"exactly {SNAPSHOT_SLOTS} dynamic files open per connection", opened == SNAPSHOT_SLOTS, opened)
+    ok("the next open is 'too many open dynamic files'", err == "too many open dynamic files", err)
+    ok("the refused fid is still unopened (read is 'file not open')", c.err(Tread, struct.pack("<IQI", 100 + opened, 0, 10)) == "file not open")
+    # every held snapshot is still readable and consistent at offset 1
+    for i in range(opened):
+        rt, d = c.read(100 + i, 0, 8192)
+        rt2, d2 = c.read(100 + i, 1, 8192)
+        ok(f"held snapshot {i} reads and its offset-1 read is the tail", rt == Rread and rt2 == Rread and d2 == d[1:], (rt, rt2))
+    # static and provider files need no slot
+    ok("static file opens while slots are exhausted", c.path_read([b"build", b"zig_version"]) not in (None, b""))
+    ok("/vars/state/type opens while slots are exhausted", c.path_read([b"vars", b"state", b"type"]) not in (None, b""))
+    ok("/vars/state/raw opens while slots are exhausted", c.path_read([b"vars", b"state", b"raw"]) not in (None, b""))
+    ok("scratch root lists while slots are exhausted", c.walk_ok(0, 50, [b"scratch"]) == 1 and c.open(50, OREAD)[0] == Ropen)
+    c.clunk(50)
+    # a second connection has its own slots
+    c2 = Nine(path)
+    c2.session()
+    n2 = 0
+    for i, names in enumerate(dyn[:SNAPSHOT_SLOTS]):
+        c2.walk_ok(0, 100 + i, names)
+        n2 += c2.open(100 + i, OREAD)[0] == Ropen
+    ok("a second connection opens its own 8 dynamic files", n2 == SNAPSHOT_SLOTS, n2)
+    c2.close()
+    # clunk one -> the refused one now opens; clunk via Tremove (denied) also frees the slot
+    c.clunk(100)
+    rt, _, _ = c.open(100 + opened, OREAD)
+    ok("after one clunk the refused open succeeds", rt == Ropen, rt)
+    ok("remove of an open dynamic file is denied", c.err(Tremove, struct.pack("<I", 101)) == "permission denied")
+    c.walk_ok(0, 60, dyn[0])
+    rt, _, _ = c.open(60, OREAD)
+    ok("the failed-remove fid's slot was released", rt == Ropen, rt)
+    # a clone of an open dynamic fid takes no slot and is unopened
+    rt, _, _ = c.walk(60, 61, [])
+    ok("clone of an open dynamic fid is allowed", rt == Rwalk, rt)
+    ok("the clone is not open", c.err(Tread, struct.pack("<IQI", 61, 0, 10)) == "file not open")
+    ok("the clone cannot open (slots exhausted again)", c.err(Topen, struct.pack("<IB", 61, OREAD)) == "too many open dynamic files")
+    # Tversion releases everything: 8 opens succeed again
+    rt, ms, _ = c.version(65536)
+    ok("mid-session Tversion", rt == base.Rversion)
+    c.attach()
+    n3 = 0
+    for i, names in enumerate(dyn[:SNAPSHOT_SLOTS]):
+        c.walk_ok(0, 100 + i, names)
+        n3 += c.open(100 + i, OREAD)[0] == Ropen
+    ok("after Tversion all 8 slots are free again", n3 == SNAPSHOT_SLOTS, n3)
+    c.close()
+    ok("server healthy after snapshot attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- the static tree
+
+
+def attack_static(path):
+    print("# static tree: create/remove/wstat everywhere, '.'/'..' on files and provider roots")
+    c = Nine(path)
+    c.session()
+    dirs = [[], [b"build"], [b"comptime"], [b"comptime", b"types"], [b"comptime", b"types", b"Qid"], [b"runtime"], [b"runtime", b"fn"]]
+    for names in dirs:
+        p = "/" + b"/".join(names).decode()
+        ok(f"walk {p}", c.walk_ok(0, 1, names) == len(names))
+        ok(f"create in {p} is denied", c.err(base.Tcreate, struct.pack("<I", 1) + s16(b"x") + struct.pack("<IB", 0o644, OWRITE)) == "permission denied")
+        ok(f"mkdir in {p} is denied", c.err(base.Tcreate, struct.pack("<I", 1) + s16(b"d") + struct.pack("<IB", DMDIR | 0o755, OREAD)) == "permission denied")
+        ok(f"wstat of {p} is denied", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(mtime=1))) == "permission denied")
+        ok(f"wstat of {p} with all don't-care is denied too", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat())) == "permission denied")
+        ok(f"open {p} ORDWR is 'is a directory'", c.err(Topen, struct.pack("<IB", 1, ORDWR)) == "is a directory")
+        ok(f"open {p} OEXEC works like OREAD", c.open(1, OEXEC)[0] == Ropen)
+        ok(f"write to open {p} is 'is a directory'", c.err(Twrite, struct.pack("<IQI", 1, 0, 1) + b"x") is not None)
+        ok(f"remove {p} is denied", c.err(Tremove, struct.pack("<I", 1)) == "permission denied")
+        ok(f"  and clunked", c.err(Tclunk, struct.pack("<I", 1)) == "unknown fid")
+    files = [[b"README"], [b"build", b"time"], [b"comptime", b"decls"], [b"comptime", b"types", b"Qid", b"fields"], [b"runtime", b"pid"], [b"runtime", b"fn", b"fib30"], [b"runtime", b"ctl"]]
+    for names in files:
+        p = "/" + b"/".join(names).decode()
+        ok(f"walk {p}", c.walk_ok(0, 1, names) == len(names))
+        ok(f"'.' from {p} is 'not a directory'", c.err(Twalk, struct.pack("<IIH", 1, 2, 1) + s16(b".")) == "not a directory")
+        ok(f"'..' from {p} is 'not a directory'", c.err(Twalk, struct.pack("<IIH", 1, 2, 1) + s16(b"..")) == "not a directory")
+        ok(f"wstat of {p} is denied", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(length=0))) == "permission denied")
+        ok(f"remove {p} is denied", c.err(Tremove, struct.pack("<I", 1)) == "permission denied")
+        ok(f"  and clunked", c.err(Tclunk, struct.pack("<I", 1)) == "unknown fid")
+        if names[-1] != b"ctl":
+            c.walk_ok(0, 1, names)
+            ok(f"open {p} OWRITE is denied", c.err(Topen, struct.pack("<IB", 1, OWRITE)) == "permission denied")
+            ok(f"open {p} OREAD|OTRUNC is denied", c.err(Topen, struct.pack("<IB", 1, OREAD | OTRUNC)) == "permission denied")
+            ok(f"open {p} ORCLOSE alone reads (no removal on clunk)", c.open(1, OREAD | ORCLOSE)[0] == Ropen and c.read(1, 0, 10)[0] == Rread)
+            c.clunk(1)
+            ok(f"{p} still exists after ORCLOSE clunk", c.walk_ok(0, 1, names) == len(names))
+            c.clunk(1)
+    # '.' and '..' on provider roots: '.' is the same qid, '..' is the server root
+    for prov in (b"scratch", b"threads", b"addr", b"mem", b"hex", b"breakpoints", b"panic"):
+        n = c.walk_ok(0, 1, [prov])
+        rt, _, rb = c.walk(1, 2, [b".", b".."])
+        q = qid_of_walk(rb) if rt == Rwalk else []
+        rt2, st = c.stat(1)
+        ok(f"/{prov.decode()}: '.' keeps the qid and '..' reaches the root", n == 1 and len(q) == 2 and q[0][2] == st["qid"][2] and q[1][2] == 0xFF << 56, (n, q))
+        ok(f"/{prov.decode()}: root stat name is the mount name", rt2 == Rstat and st["name"] == prov, st)
+        ok(f"/{prov.decode()}: rename of the provider root is denied", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(name=b"other"))) == "permission denied")
+        ok(f"/{prov.decode()}: remove of the provider root is denied", c.err(Tremove, struct.pack("<I", 1)) == "permission denied")
+        c.clunk(2)
+    # qid path spaces do not collide: static tag 0xFF, vars 0xFE, providers 0..n
+    seen = {}
+    for names in dirs + files + [[b"vars"], [b"vars", b"state"], [b"vars", b"state", b"value"], [b"scratch"], [b"threads"], [b"panic", b"message"], [b"mem", b"maps"]]:
+        c.walk_ok(0, 1, names)
+        rt, st = c.stat(1)
+        c.clunk(1)
+        key = st["qid"][2]
+        ok(f"qid path of /{b'/'.join(names).decode()} is unique", key not in seen, (key, seen.get(key)))
+        seen[key] = names
+    # walks through the whole tree with 16 elements of '..' never leave the root
+    ok("16 x '..' from root stays at root", c.walk_ok(0, 1, [b".."] * 16) == 16)
+    rt, st = c.stat(1)
+    ok("  and it is the root", rt == Rstat and st["qid"][2] == 0xFF << 56)
+    c.clunk(1)
+    c.close()
+    ok("server healthy after static attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- debug providers as black boxes
+
+
+def attack_debug_providers(path):
+    print("# debug providers: hostile names, offsets, writes; the server must answer or Rerror, never die")
+    c = Nine(path, timeout=15)
+    c.session()
+    tid = worker_tid(c)
+    ok("worker thread found under /threads", tid is not None, tid)
+    hostile = [b"0", b"0x", b"0x0", b"-1", b"+1", b"00", b"ffffffffffffffff", b"0xffffffffffffffff", b"1" * 13, b"1" * 12, b"zzz", b"1e5", b" 1", b"1 ", b"0x0000000000000001", b"8", b"0x7fffffffffff", b"ffffffffffff", b"\x00", b"." * 3, b"a" * 255]
+    for tree in (b"addr", b"hex", b"mem"):
+        for nm in hostile:
+            n = c.walk_ok(0, 1, [tree, nm])
+            if n == 2:
+                rt, _, rb = c.open(1, OREAD)
+                if rt == Ropen:
+                    rt2, d = c.read(1, 0, 4096)
+                    rt3, d3 = c.read(1, (1 << 64) - 1, 4096)
+                    rt4, d4 = c.read(1, (1 << 63), 4096)
+                    ok(f"/{tree.decode()}/{nm[:16]!r}: reads at 0, 2^63 and 2^64-1 are answered", rt2 in (Rread, Rerror) and rt3 in (Rread, Rerror) and rt4 in (Rread, Rerror), (rt2, rt3, rt4))
+                else:
+                    ok(f"/{tree.decode()}/{nm[:16]!r}: open answered", rt == Rerror, rt)
+                c.clunk(1)
+            else:
+                ok(f"/{tree.decode()}/{nm[:16]!r}: walk refused or partial", n in (1, None), n)
+    for nm in hostile + [b"1", b"4294967295", b"4294967296", b"99999999999", b"0" + (tid or b"1")]:
+        n = c.walk_ok(0, 1, [b"threads", nm])
+        if n == 2:
+            for leaf in (b"name", b"stat", b"stack", b"regs"):
+                rt, _, _ = c.walk(1, 2, [leaf])
+                if rt == Rwalk:
+                    rt, _, _ = c.open(2, OREAD)
+                    if rt == Ropen:
+                        c.read(2, 0, 8192)
+                    c.clunk(2)
+            c.clunk(1)
+            ok(f"/threads/{nm!r} walked (a live tid) and its files answered", True)
+        else:
+            ok(f"/threads/{nm!r}: walk refused or partial", n in (1, None), n)
+        n = c.walk_ok(0, 1, [b"breakpoints", nm])
+        ok(f"/breakpoints/{nm!r}: walk refused (nothing paused)", n in (1, None), n)
+    # files under a thread: '.'/'..' and walking through them
+    if tid:
+        ok("/threads/<tid>/stack/x is 'not a directory'", c.walk_ok(0, 1, [b"threads", tid, b"stack"]) == 3 and c.err(Twalk, struct.pack("<IIH", 1, 2, 1) + s16(b"x")) == "not a directory")
+        ok("'..' from /threads/<tid>/stack is 'not a directory'", c.err(Twalk, struct.pack("<IIH", 1, 2, 1) + s16(b"..")) == "not a directory")
+        c.clunk(1)
+        ok("/threads/<tid>/../../<tid>/name walks", c.walk_ok(0, 1, [b"threads", tid, b"..", b"..", b"threads", tid, b"name"]) == 7)
+        c.clunk(1)
+        c.walk_ok(0, 1, [b"threads", tid])
+        ok("create under /threads/<tid> is denied", c.err(base.Tcreate, struct.pack("<I", 1) + s16(b"x") + struct.pack("<IB", 0o644, OWRITE)) == "permission denied")
+        ok("wstat of /threads/<tid> is denied", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(name=b"y"))) == "permission denied")
+        ok("remove of /threads/<tid> is denied", c.err(Tremove, struct.pack("<I", 1)) == "permission denied")
+        stack = c.path_read([b"threads", tid, b"stack"])
+        ok("worker stack reads", stack is not None and b"workerLoop" in stack, stack)
+        # thread stack at offset 1 is the tail (dynamic snapshot in the provider)
+        c.walk_ok(0, 1, [b"threads", tid, b"stack"])
+        c.open(1, OREAD)
+        rt, d = c.read(1, 0, 65000)
+        rt2, d2 = c.read(1, 1, 65000)
+        ok("thread stack offset-1 read is the tail of the same snapshot", rt == Rread and rt2 == Rread and d2 == d[1:], (len(d), len(d2)))
+        c.clunk(1)
+    # /mem: read of unmapped memory is an error, write of unmapped memory is an error; writes at wild offsets too
+    c.walk_ok(0, 1, [b"mem", b"8"])
+    rt, _, _ = c.open(1, ORDWR)
+    ok("/mem/8 opens", rt == Ropen, rt)
+    rt, d = c.read(1, 0, 16)
+    ok("read of unmapped memory is an Rerror", rt == Rerror, rt)
+    rt, _, _ = c.write(1, 0, b"x")
+    ok("write to unmapped memory is an Rerror", rt == Rerror, rt)
+    rt, _, _ = c.write(1, (1 << 64) - 9, b"x")
+    ok("write at a wrapping offset is answered", rt in (Rerror, Rwrite), rt)
+    rt, _, _ = c.write(1, 0, b"")
+    ok("empty write to /mem is answered", rt in (Rerror, Rwrite), rt)
+    c.clunk(1)
+    addr = c.path_read([b"vars", b"state", b"addr"])
+    ok("/vars/state/addr reads", addr is not None and addr.startswith(b"0x"), addr)
+    if addr:
+        hx = addr[2:]
+        size = int(c.path_read([b"vars", b"state", b"size"]))
+        c.walk_ok(0, 1, [b"mem", hx])
+        c.open(1, OREAD)
+        rt, d = c.read(1, 0, size)
+        ok("/mem/<state addr> reads @sizeOf bytes", rt == Rread and len(d) == size, (rt, len(d) if d else d))
+        rt, d = c.read(1, 0, 0xFFFFFFFF)
+        ok("/mem read with count 2^32-1 is clamped and answered", rt in (Rread, Rerror), rt)
+        c.clunk(1)
+        hexd = c.path_read([b"hex", hx])
+        ok("/hex/<state addr> is a hexdump", hexd is not None and len(hexd) > 64, hexd[:40] if hexd else hexd)
+        # a value written through /mem must render, not trap: corrupt the phase enum and read /vars/state/value
+        phase_addr = int(c.path_read([b"vars", b"state", b"f", b"phase", b"addr"]), 16)
+        c.walk_ok(0, 1, [b"mem", b"%x" % phase_addr])
+        c.open(1, OWRITE)
+        rt, _, _ = c.write(1, 0, b"\xee")
+        ok("write a corrupt enum byte through /mem", rt == Rwrite, rt)
+        c.clunk(1)
+        v = c.path_read([b"vars", b"state", b"value"])
+        ok("/vars/state/value renders the corrupt enum as a number instead of trapping", v is not None and b"phase: 238" in v, v)
+        pv = c.path_read([b"vars", b"state", b"f", b"phase", b"value"])
+        ok("/vars/state/f/phase/value renders 238", pv == b"238", pv)
+        c.walk_ok(0, 1, [b"vars", b"state", b"f", b"phase", b"value"])
+        c.open(1, OWRITE)
+        rt, _, _ = c.write(1, 0, b"idle")
+        ok("the enum can be repaired through /vars", rt == Rwrite, rt)
+        c.clunk(1)
+    # /panic: ctl refuses reads and garbage; message/stack read
+    ok("/panic/message reads (empty, no panic)", c.path_read([b"panic", b"message"]) == b"")
+    ok("/panic/stack reads", c.path_read([b"panic", b"stack"]) is not None)
+    c.walk_ok(0, 1, [b"panic", b"ctl"])
+    ok("/panic/ctl refuses OREAD", c.err(Topen, struct.pack("<IB", 1, OREAD)) == "permission denied")
+    c.clunk(1)
+    c.walk_ok(0, 1, [b"panic", b"ctl"])
+    rt, _, _ = c.open(1, OWRITE)
+    ok("/panic/ctl opens OWRITE", rt == Ropen, rt)
+    for bad in (b"garbage", b"", b"continue please", b"\x00continue"):
+        rt, _, _ = c.write(1, 0, bad)
+        ok(f"/panic/ctl write {bad!r} without a panic is an Rerror", rt == Rerror, rt)
+    c.clunk(1)
+    # /breakpoints is empty; the debug providers refuse create/wstat/remove
+    for prov in (b"threads", b"addr", b"mem", b"hex", b"breakpoints", b"panic"):
+        c.walk_ok(0, 1, [prov])
+        ok(f"create in /{prov.decode()} is denied", c.err(base.Tcreate, struct.pack("<I", 1) + s16(b"x") + struct.pack("<IB", 0o644, OWRITE)) == "permission denied")
+        ok(f"wstat of /{prov.decode()} is denied", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(mtime=5))) == "permission denied")
+        c.open(1, OREAD)
+        d = c.read_all(1)
+        ok(f"/{prov.decode()} lists", d is not None)
+        c.clunk(1)
+    # the provider's snapshot pool (shared by every connection) recovers after exhaustion
+    held = []
+    err = None
+    for i in range(16):
+        names = [b"addr", b"%x" % (0x1000 + i)]
+        c.walk_ok(0, 200 + i, names)
+        rt, _, rb = c.open(200 + i, OREAD)
+        if rt == Ropen:
+            held.append(200 + i)
+        else:
+            n, = struct.unpack_from("<H", rb)
+            err = rb[2:2 + n].decode()
+            break
+    ok("debug provider snapshot pool exhausts with an Rerror", err is not None and len(held) >= 1, (len(held), err))
+    for f in held:
+        c.clunk(f)
+    ok("after clunking, /addr opens again", c.path_read([b"addr", b"1000"]) is not None)
+    # Tversion with debug files open (hexdumps of the exposed state: mapped memory)
+    base_addr = int(addr, 16) if addr else 0
+    opened = 0
+    for i in range(4):
+        c.walk_ok(0, 300 + i, [b"hex", b"%x" % (base_addr + i)])
+        opened += c.open(300 + i, OREAD)[0] == Ropen
+    ok("four /hex snapshots open", opened == 4, opened)
+    rt, _, _ = c.version(65536)
+    ok("Tversion with debug snapshots open", rt == base.Rversion)
+    c.attach()
+    ok("/hex still opens after the reset", c.path_read([b"hex", b"%x" % base_addr]) is not None)
+    ok("/hex of unmapped memory is an Rerror at open, not a crash", c.walk_ok(0, 1, [b"hex", b"3000"]) == 2 and c.open(1, OREAD)[0] == Rerror)
+    c.clunk(1)
+    c.close()
+    ok("server healthy after debug provider attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- fids at the maximum
+
+
+def flood(c, ids, names):
+    """Pipelines one Twalk per id and counts the Rwalk replies; returns (ok_count, error_count, seconds)."""
+    got = [0, 0]
+    dead = [False]
+
+    def reader():
+        try:
+            for _ in ids:
+                rt, _, _ = c.recv_frame()
+                if rt == Rwalk:
+                    got[0] += 1
+                else:
+                    got[1] += 1
+        except (EOFError, OSError):
+            dead[0] = True
+
+    t = threading.Thread(target=reader)
+    t.start()
+    t0 = time.time()
+    body = b"".join(frame(Twalk, i & 0xFFFE, struct.pack("<IIH", 0, fid, len(names)) + b"".join(s16(n) for n in names)) for i, fid in enumerate(ids))
+    c.raw(body)
+    t.join(120)
+    return got[0], got[1], time.time() - t0, dead[0] or t.is_alive()
+
+
+def attack_fid_table(path):
+    print(f"# fid table: {MAX_FIDS} fids, adversarial ids, clunk all, reuse")
+    c = Nine(path, timeout=60)
+    c.session()
+    n = MAX_FIDS - 1  # fid 0 is the attach
+    ids = []
+    for i in range(n):
+        k = i % 3
+        ids.append(i * 8192 + 1 if k == 0 else (0x80000000 | i) if k == 1 else 0xFFFFFFFE - i)
+    assert len(set(ids)) == n and 0 not in ids
+    good, bad, dt, dead = flood(c, ids, [b"scratch"])
+    ok(f"{n} walks with adversarial fid numbers all succeed", good == n and bad == 0 and not dead, (good, bad, dead))
+    print(f"      {n} clones (provider handles) in {dt:.2f}s")
+    ok("the next fid is 'too many fids'", c.err(Twalk, struct.pack("<IIH", 0, 7, 0)) == "too many fids")
+    ok("attach at the limit is 'too many fids'", c.err(base.Tattach, struct.pack("<II", 7, base.NOFID) + s16(b"u") + s16(b"")) == "too many fids")
+    ok("an existing id is 'fid in use'", c.err(Twalk, struct.pack("<IIH", 0, ids[12345], 0)) == "fid in use")
+    ok("a self-walk at the limit works", c.walk_ok(ids[5], ids[5], [b".."]) == 1)
+    ok("an unknown fid at the limit is 'unknown fid'", c.err(Tstat, struct.pack("<I", 7)) == "unknown fid")
+    # clunk all, pipelined, in a hostile order (every third first, then the rest reversed)
+    order = ids[::3] + ids[1::3][::-1] + ids[2::3][::-1]
+    got = [0]
+    dead = [False]
+
+    def reader():
+        try:
+            for _ in order:
+                rt, _, _ = c.recv_frame()
+                got[0] += rt == Rclunk
+        except (EOFError, OSError):
+            dead[0] = True
+
+    t = threading.Thread(target=reader)
+    t.start()
+    t0 = time.time()
+    c.raw(b"".join(frame(Tclunk, i & 0xFFFE, struct.pack("<I", fid)) for i, fid in enumerate(order)))
+    t.join(120)
+    ok(f"{n} clunks all answered", got[0] == n and not dead[0] and not t.is_alive(), (got[0], dead[0]))
+    print(f"      {n} clunks in {time.time() - t0:.2f}s")
+    ok("clunk of a clunked fid is 'unknown fid'", c.err(Tclunk, struct.pack("<I", ids[100])) == "unknown fid")
+    # reuse: the whole table is available again with dense ids
+    good, bad, dt, dead = flood(c, list(range(1, n + 1)), [])
+    ok(f"{n} clones with dense ids after the churn all succeed", good == n and bad == 0 and not dead, (good, bad, dead))
+    print(f"      {n} clones (reuse) in {dt:.2f}s")
+    ok("still 'too many fids' at the limit", c.err(Twalk, struct.pack("<IIH", 0, n + 1, 0)) == "too many fids")
+    rt, _, _ = c.version(65536)
+    ok("Tversion after the fid churn", rt == base.Rversion)
+    c.attach()
+    good, bad, dt, dead = flood(c, list(range(1, 1001)), [b"scratch"])
+    ok("1000 clones after Tversion", good == 1000 and bad == 0, (good, bad))
+    c.close()
+    ok("server healthy after the fid table attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- flush storm
+
+
+def attack_flush(path):
+    print("# Tflush storm")
+    c = Nine(path, timeout=30)
+    c.session()
+    n = 2000
+    blob = b"".join(frame(Tflush, i & 0xFFFE, struct.pack("<H", (i * 7919) & 0xFFFF)) for i in range(n))
+    got = [0]
+
+    def reader():
+        try:
+            for _ in range(n):
+                rt, _, _ = c.recv_frame()
+                got[0] += rt == Rflush
+        except (EOFError, OSError):
+            pass
+
+    t = threading.Thread(target=reader)
+    t.start()
+    c.raw(blob)
+    t.join(60)
+    ok(f"{n} pipelined Tflush (random oldtags, including own tag) all Rflush", got[0] == n, got[0])
+    rt, tag, _ = c.call(Tflush, struct.pack("<H", 5), 5)
+    ok("Tflush of its own tag is Rflush", rt == Rflush and tag == 5, (rt, tag))
+    rt, st = c.stat(0)
+    ok("a normal request after the storm works", rt == Rstat, rt)
+    c.close()
+    ok("server healthy after the flush storm", healthy(path))
+
+
+# --------------------------------------------------------------------------- msize 24
+
+
+def attack_msize24(path):
+    print("# msize 24: everything that fits is served, everything else is an Rerror that fits")
+    c = Nine(path)
+    rt, ms, ver = c.version(24)
+    ok("Tversion 24", rt == base.Rversion and ms == 24, (rt, ms))
+    rt, _, _ = c.attach(uname=b"u")  # 20 bytes; Rattach is 20
+    ok("Tattach at msize 24", rt == base.Rattach, rt)
+    e = c.err(Tstat, struct.pack("<I", 0))
+    ok("Tstat: Rerror truncated to 15 bytes ('reply too large')", e == "reply too large", e)
+    rt, _, rb = c.walk(0, 1, [b"build"])  # Twalk 24, Rwalk 22
+    ok("Twalk of one 5-byte name", rt == Rwalk, rt)
+    e = c.err(Twalk, struct.pack("<IIH", 0, 2, 2) + s16(b".") + s16(b"."))  # 23 bytes; Rwalk would be 35
+    ok("Twalk of two names cannot be answered: 'reply too large'", e == "reply too large", e)
+    ok("newfid unbound after the refused walk", c.err(Tclunk, struct.pack("<I", 2)) == "unknown fid")
+    rt, _, _ = c.open(1, OREAD)  # Ropen 24
+    ok("Topen at msize 24", rt == Ropen, rt)
+    rt, d = c.read(1, 0, 4096)  # count clamped to msize - iohdrsz = 0
+    ok("Tread of a directory at msize 24 answers an empty Rread (no split record)", rt == Rread and d == b"", (rt, d))
+    e = c.err(Tread, struct.pack("<IQI", 1, 1, 4096))
+    ok("dir read at offset 1 is 'bad offset'", e == "bad offset", e)
+    rt, _, _ = c.clunk(1)
+    ok("Tclunk at msize 24", rt == Rclunk, rt)
+    rt, _, _ = c.walk(0, 1, [b"vars"])  # Twalk 23
+    rt, _, _ = c.walk(1, 1, [b"state"])  # 23
+    rt, _, _ = c.walk(1, 1, [b"value"])  # 23
+    ok("walk to /vars/state/value in 3 self-walks", rt == Rwalk, rt)
+    rt, _, _ = c.open(1, OREAD)
+    ok("open a dynamic file at msize 24", rt == Ropen, rt)
+    rt, d = c.read(1, 0, 4096)
+    ok("read of a dynamic file at msize 24 is an empty Rread", rt == Rread and d == b"", (rt, d))
+    rt, _, _ = c.clunk(1)
+    for nm in (b"vars", b"state", b"f", b"ticks", b"value"):  # each Twalk <= 24 bytes
+        rt, _, _ = c.walk(0 if nm == b"vars" else 1, 1, [nm])
+    ok("walk to /vars/state/f/ticks/value in 5 self-walks", rt == Rwalk, rt)
+    rt, _, _ = c.open(1, OWRITE)
+    ok("open a writable value at msize 24", rt == Ropen, rt)
+    rt, _, _ = c.write(1, 0, b"5")  # Twrite 24, Rwrite 11
+    ok("Twrite of one byte at msize 24", rt == Rwrite, rt)
+    e = c.err(Twrite, struct.pack("<IQI", 1, 0, 0) + b"")
+    ok("empty write to a value at msize 24 is 'bad value'", e == "bad value", e)
+    rt, _, _ = c.clunk(1)
+    ok("clunk at msize 24", rt == Rclunk, rt)
+    rt, ms, _ = c.version(65536)
+    ok("renegotiate a big msize on the same connection", rt == base.Rversion and ms == 65536, (rt, ms))
+    c.attach()
+    ok("normal service resumes", c.path_read([b"build", b"zig_version"]) not in (None, b""))
+    c.close()
+    # frames larger than 24 after negotiating 24 kill the connection
+    c = Nine(path)
+    c.version(24)
+    c.raw(frame(base.Tattach, 1, struct.pack("<II", 0, base.NOFID) + s16(b"longer-name") + s16(b"")))
+    ok("a 30-byte Tattach at msize 24: connection closed", expect_dead(c))
+    c.close()
+    ok("server healthy after msize-24 attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- directory offsets
+
+
+def attack_dir_offsets(path):
+    print("# directory reads: exact record boundaries vs off by one")
+    c = Nine(path)
+    c.session()
+    tag = os.urandom(3).hex().encode()
+    root = b"do-" + tag
+    c.walk_ok(0, 1, [b"scratch"])
+    c.create(1, root, DMDIR | 0o755, OREAD)
+    c.clunk(1)
+    for nm in (b"alpha", b"beta-with-a-longer-name", b"g"):
+        c.walk_ok(0, 1, [b"scratch", root])
+        c.create(1, nm, 0o644, OWRITE)
+        c.clunk(1)
+    for names in ([], [b"vars", b"state"], [b"comptime", b"types"], [b"scratch", root], [b"threads"], [b"panic"]):
+        p = "/" + b"/".join(names).decode()
+        c.walk_ok(0, 1, names)
+        c.open(1, OREAD)
+        rt, d = c.read(1, 0, 65000)
+        recs = records(d)
+        if len(recs) < 2:
+            ok(f"{p}: at least two entries", False, len(recs))
+            c.clunk(1)
+            continue
+        r0 = len(recs[0][1])
+        r1 = len(recs[1][1])
+        rt, d0 = c.read(1, 0, r0)
+        ok(f"{p}: count = first record length returns exactly that record", rt == Rread and d0 == recs[0][1], (rt, len(d0) if d0 else d0, r0))
+        rt, d1 = c.read(1, r0, r1)
+        ok(f"{p}: read at the record boundary returns the next record", rt == Rread and d1 == recs[1][1], (rt, len(d1) if d1 else d1))
+        e = c.err(Tread, struct.pack("<IQI", 1, r0 + r1 + 1, 65000))
+        ok(f"{p}: offset boundary+1 is 'bad offset'", e == "bad offset", e)
+        e = c.err(Tread, struct.pack("<IQI", 1, r0 + r1 - 1, 65000))
+        ok(f"{p}: offset boundary-1 is 'bad offset'", e == "bad offset", e)
+        e = c.err(Tread, struct.pack("<IQI", 1, r0, 65000))
+        ok(f"{p}: re-reading an earlier boundary is 'bad offset'", e == "bad offset", e)
+        rt, rest = c.read(1, r0 + r1, 65000)
+        ok(f"{p}: after a bad offset the good boundary still continues", rt == Rread and rest == d[r0 + r1:], rt)
+        rt, dd = c.read(1, 0, r0 - 1)
+        ok(f"{p}: count one short of a record returns nothing (no split)", rt == Rread and dd == b"", (rt, dd))
+        rt, dd = c.read(1, 0, 65000)
+        ok(f"{p}: offset 0 restarts and yields the same bytes", rt == Rread and dd == d)
+        rt, dd = c.read(1, len(d), 65000)
+        ok(f"{p}: read at the end is empty", rt == Rread and dd == b"")
+        rt, dd = c.read(1, len(d), 65000)
+        ok(f"{p}: read at the end twice is empty twice", rt == Rread and dd == b"")
+        c.clunk(1)
+    for nm in (b"alpha", b"beta-with-a-longer-name", b"g"):
+        c.walk_ok(0, 1, [b"scratch", root, nm])
+        c.remove(1)
+    c.walk_ok(0, 1, [b"scratch", root])
+    ok("cleanup of the directory-offset test root", c.remove(1)[0] == Rremove)
+    c.close()
+    ok("server healthy after directory offset attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- ctl staging
+
+
+def attack_ctl(path):
+    print("# ctl: a failed command leaves the previous result, length and qid version untouched")
+    c = Nine(path)
+    c.session(1 << 20)
+    c.walk_ok(0, 1, [b"runtime", b"ctl"])
+    c.open(1, ORDWR)
+    rt, _, _ = c.write(1, 0, b"echo persist")
+    rt, st = c.stat(1)
+    v = st["qid"][1]
+    ok("echo persist", st["length"] == 7, st)
+    for bad in (b"nope", b"fib 94", b"add 1", b"", b"\x00", b"echo\x00hidden"):
+        e = c.err(Twrite, struct.pack("<IQI", 1, 0, len(bad)) + bad)
+        rt, d = c.read(1, 0, 100)
+        rt2, st2 = c.stat(1)
+        ok(f"after failed {bad!r}: result still 'persist'", d == b"persist", d)
+        ok(f"after failed {bad!r}: length 7 and qid version unchanged", st2["length"] == 7 and st2["qid"][1] == v, (st2["length"], st2["qid"][1], v))
+    # a second connection sees the same result and version
+    c2 = Nine(path)
+    c2.session()
+    ok("other connection reads the surviving result", c2.path_read([b"runtime", b"ctl"]) == b"persist")
+    c2.walk_ok(0, 1, [b"runtime", b"ctl"])
+    rt, st3 = c2.stat(1)
+    ok("other connection sees the same version", st3["qid"][1] == v, (st3["qid"][1], v))
+    c2.close()
+    # a successful command bumps the version and replaces the result; an empty result is a result
+    rt, _, _ = c.write(1, 0, b"echo")
+    rt, st4 = c.stat(1)
+    rt, d = c.read(1, 0, 100)
+    ok("echo with no argument yields an empty result with a new version", d == b"" and st4["length"] == 0 and st4["qid"][1] != v, (d, st4))
+    e = c.err(Twrite, struct.pack("<IQI", 1, 0, 4) + b"nope")
+    rt, d = c.read(1, 0, 100)
+    ok("a failure after an empty result keeps it empty", d == b"", d)
+    # the largest result: echo of a 60000-byte line
+    big = b"echo " + b"y" * 60000
+    rt, _, _ = c.write(1, 0, big)
+    d = c.read_all(1)
+    ok("60000-byte ctl result round-trips", rt == Rwrite and d == b"y" * 60000, (rt, len(d) if d else d))
+    rt, _, _ = c.write(1, 0, b"echo " + b"z" * 70000)  # exceeds ctl_bytes (64 KiB) -> the handler's writer fails
+    rt2, d = c.read(1, 0, 100)
+    rt3, st5 = c.stat(1)
+    ok("an over-long result is an Rerror and the previous result survives", rt == Rerror and d == b"y" * 100 and st5["length"] == 60000, (rt, d[:10] if d else d, st5["length"]))
+    c.clunk(1)
+    c.close()
+    ok("server healthy after ctl attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- fid state machine on scratch
+
+
+def attack_fid_states(path):
+    print("# fid state machine on /scratch")
+    c = Nine(path)
+    c.session()
+    tag = os.urandom(3).hex().encode()
+    root = b"fs-" + tag
+    c.walk_ok(0, 1, [b"scratch"])
+    c.create(1, root, DMDIR | 0o755, OREAD)
+    c.clunk(1)
+    S = [b"scratch", root]
+    c.walk_ok(0, 1, S)
+    rt, _, _ = c.create(1, b"f", 0o644, ORDWR)
+    ok("create f", rt == Rcreate)
+    ok("open of an open fid is 'file already open'", c.err(Topen, struct.pack("<IB", 1, OREAD)) == "file already open")
+    ok("walk with names from an open fid is 'file already open'", c.err(Twalk, struct.pack("<IIH", 1, 2, 1) + s16(b".")) == "file already open")
+    ok("create on an open fid is 'file already open'", c.err(base.Tcreate, struct.pack("<I", 1) + s16(b"g") + struct.pack("<IB", 0o644, OWRITE)) == "file already open")
+    rt, _, _ = c.walk(1, 2, [])
+    ok("clone of an open fid is allowed", rt == Rwalk)
+    ok("the clone is not open", c.err(Tread, struct.pack("<IQI", 2, 0, 10)) == "file not open")
+    rt, _, _ = c.open(2, OREAD)
+    ok("the clone opens independently", rt == Ropen)
+    c.write(1, 0, b"data")
+    rt, d = c.read(2, 0, 10)
+    ok("the clone sees the write", rt == Rread and d == b"data", d)
+    rt, _, _ = c.wstat(2, mkstat(name=b"renamed"))
+    ok("wstat through an open fid works", rt == Rwstat)
+    rt, _, _ = c.remove(1)
+    ok("remove through the open writer fid", rt == Rremove)
+    rt, d = c.read(2, 0, 10)
+    ok("the other open fid still reads the removed file", rt == Rread and d == b"data", d)
+    ok("stat of the removed file still answers", c.stat(2)[0] == Rstat)
+    ok("open of a removed file through a new walk is impossible (not found)", c.walk_ok(0, 3, S + [b"renamed"]) == 2)
+    c.clunk(2)
+    # newfid == fid walks on unopened fids rebind; on the same fid with a failing later element they do nothing
+    c.walk_ok(0, 3, S)
+    rt, _, rb = c.walk(3, 3, [b"..", root, b"nope"])
+    n = struct.unpack_from("<H", rb)[0] if rt == Rwalk else None
+    ok("partial self-walk returns 2 and leaves the fid where it was", n == 2 and c.stat(3)[1]["name"] == root, (n,))
+    rt, _, _ = c.walk(3, 3, [b"..", b".."])
+    rt, st = c.stat(3)
+    ok("self-walk with names rebinds the fid", rt == Rstat and st["qid"][2] == 0xFF << 56, st)
+    c.clunk(3)
+    # Tversion while a removed-but-held file exists: nothing leaks, the server keeps serving
+    c.walk_ok(0, 4, S)
+    c.create(4, b"held", 0o644, OWRITE)
+    c.write(4, 0, b"x" * 1000)
+    c.walk_ok(0, 5, S + [b"held"])
+    c.remove(5)
+    c.version(65536)
+    c.attach()
+    ok("after Tversion the removed file is gone", c.walk_ok(0, 1, S + [b"held"]) == 2)
+    c.clunk(1)
+    c.walk_ok(0, 1, S)
+    rt, _, _ = c.remove(1)
+    ok("cleanup", rt == Rremove, rt)
+    c.close()
+    ok("server healthy after fid state attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- scratch budget
+
+
+def attack_scratch_budget(path):
+    print("# scratch: the global budget after grow/truncate/rename/remove/failed writes")
+    c = Nine(path, timeout=120)
+    c.session()
+    tag = os.urandom(3).hex().encode()
+    root = b"bg-" + tag
+    c.walk_ok(0, 1, [b"scratch"])
+    c.create(1, root, DMDIR | 0o755, OREAD)
+    c.clunk(1)
+    S = [b"scratch", root]
+    per = SCRATCH_MAX_FILE
+    count = SCRATCH_BUDGET // per
+    t0 = time.time()
+    made = 0
+    for i in range(count):
+        c.walk_ok(0, 1, S)
+        rt, _, _ = c.create(1, b"big%d" % i, 0o644, OWRITE)
+        rt, _, _ = c.wstat(1, mkstat(length=per))
+        c.clunk(1)
+        if rt != Rwstat:
+            break
+        made += 1
+    ok(f"{count} files of {per >> 20} MiB fill the {SCRATCH_BUDGET >> 20} MiB budget exactly", made == count, made)
+    print(f"      filled the budget in {time.time() - t0:.1f}s")
+    c.walk_ok(0, 1, S)
+    c.create(1, b"one-more", 0o644, OWRITE)
+    ok("one more byte is 'no space left on device'", c.err(Twrite, struct.pack("<IQI", 1, 0, 1) + b"x") == "no space left on device")
+    ok("a failed write leaves the file empty", c.stat(1)[1]["length"] == 0)
+    ok("a zero-length write at a huge offset is still fine", c.write(1, (1 << 60), b"")[0] == Rwrite)
+    ok("wstat length 1 is 'no space left on device'", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(length=1))) == "no space left on device")
+    # rename does not charge; truncate releases exactly its size
+    c.walk_ok(0, 2, S + [b"big0"])
+    ok("rename of a full file is fine", c.wstat(2, mkstat(name=b"big0-r"))[0] == Rwstat)
+    ok("still no space after the rename", c.err(Twrite, struct.pack("<IQI", 1, 0, 1) + b"x") == "no space left on device")
+    ok("truncate big0-r to 1 MiB", c.wstat(2, mkstat(length=1 << 20))[0] == Rwstat)
+    c.clunk(2)
+    rt, _, _ = c.wstat(1, mkstat(length=per - (1 << 20)))
+    ok("exactly the released amount is writable again", rt == Rwstat, rt)
+    ok("and not one byte more", c.err(Twrite, struct.pack("<IQI", 1, per - (1 << 20), 1) + b"x") == "no space left on device")
+    ok("nor via wstat", c.err(Twstat, struct.pack("<I", 1) + s16(mkstat(length=per - (1 << 20) + 1))) == "no space left on device")
+    # overwriting existing bytes costs nothing
+    ok("overwrite inside the file is fine", c.write(1, 0, b"y" * 4096)[0] == Rwrite)
+    ok("overwrite at the very end is fine", c.write(1, per - (1 << 20) - 4096, b"y" * 4096)[0] == Rwrite)
+    # OTRUNC of a full file releases; remove of a held file releases only on the last clunk
+    c.walk_ok(0, 3, S + [b"big1"])
+    ok("OTRUNC releases", c.open(3, OWRITE | OTRUNC)[0] == Ropen and c.stat(3)[1]["length"] == 0)
+    ok("the released space is writable", c.wstat(1, mkstat(length=per))[0] == Rwstat)
+    ok("a full file writes into itself", c.write(1, per - 10, b"0123456789")[0] == Rwrite)
+    ok("but not past the per-file cap", c.err(Twrite, struct.pack("<IQI", 1, per - 1, 2) + b"xy") == "no space left on device")
+    c.clunk(3)
+    c.clunk(1)
+    # refill the remaining 63 MiB so the budget is exactly full again
+    c.walk_ok(0, 7, S)
+    c.create(7, b"fill", 0o644, OWRITE)
+    ok("the rest of the budget fills exactly", c.wstat(7, mkstat(length=per - (1 << 20)))[0] == Rwstat)
+    ok("and is full again", c.err(Twrite, struct.pack("<IQI", 7, per - (1 << 20), 1) + b"x") == "no space left on device")
+    c.clunk(7)
+    c.walk_ok(0, 4, S + [b"big2"])
+    c.walk_ok(0, 5, S + [b"big2"])
+    c.open(5, OREAD)
+    c.remove(4)
+    c.walk_ok(0, 6, S)
+    c.create(6, b"after-remove", 0o644, OWRITE)
+    ok("space of a removed-but-held file is not released yet", c.err(Twrite, struct.pack("<IQI", 6, 0, 1) + b"x") == "no space left on device")
+    c.clunk(5)
+    ok("the last clunk releases it", c.write(6, 0, b"x")[0] == Rwrite)
+    c.clunk(6)
+    # cleanup
+    c.walk_ok(0, 1, S)
+    c.open(1, OREAD)
+    names = [n for n, _ in records(c.read_all(1))]
+    c.clunk(1)
+    for nm in names:
+        if c.walk_ok(0, 1, S + [nm]) == 3:
+            c.remove(1)
+    c.walk_ok(0, 1, S)
+    ok("cleanup removed the budget test root", c.remove(1)[0] == Rremove, names)
+    c.walk_ok(0, 1, [b"scratch"])
+    c.create(1, b"post-" + tag, 0o644, OWRITE)
+    ok("the whole budget is back: a 64 MiB file fits", c.wstat(1, mkstat(length=per))[0] == Rwstat)
+    c.remove(1)
+    c.close()
+    ok("server healthy after budget attacks", healthy(path))
+
+
+# --------------------------------------------------------------------------- main
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--server")
+    ap.add_argument("--socket")
+    ap.add_argument("--fast", action="store_true", help="skip the 512 MiB scratch budget fill")
+    args = ap.parse_args()
+    proc = None
+    tmp = None
+    if args.server:
+        tmp = tempfile.mkdtemp(prefix="adv9pcore.")
+        path = os.path.join(tmp, "sock")
+        proc = subprocess.Popen([os.path.abspath(args.server), "--unix", path], stderr=subprocess.PIPE)
+        for _ in range(200):
+            if os.path.exists(path):
+                break
+            time.sleep(0.02)
+    elif args.socket:
+        path = args.socket
+    else:
+        ap.error("--server or --socket")
+    try:
+        attack_vars(path)
+        attack_snapshots(path)
+        attack_static(path)
+        attack_debug_providers(path)
+        attack_dir_offsets(path)
+        attack_ctl(path)
+        attack_fid_states(path)
+        attack_msize24(path)
+        attack_flush(path)
+        attack_fid_table(path)
+        if not args.fast:
+            attack_scratch_budget(path)
+        if proc is not None:
+            ok("server process still running", proc.poll() is None, proc.poll())
+    finally:
+        if proc is not None:
+            proc.send_signal(signal.SIGTERM)
+            try:
+                _, err = proc.communicate(timeout=5)
+            except subprocess.TimeoutExpired:
+                proc.kill()
+                _, err = proc.communicate()
+            lines = [ln for ln in err.decode("utf-8", "replace").splitlines() if "connection ended" not in ln and "read: " not in ln]
+            if lines:
+                print("# server stderr (filtered):")
+                for ln in lines[:40]:
+                    print("      " + ln)
+            if tmp:
+                try:
+                    os.unlink(path)
+                    os.rmdir(tmp)
+                except OSError:
+                    pass
+    print(f"# {base.PASSES} passed, {len(base.FAILS)} failed")
+    for f in base.FAILS:
+        print("#   FAIL " + f)
+    sys.exit(1 if base.FAILS else 0)
+
+
+if __name__ == "__main__":
+    main()