`_remote_debugging`: raising `Invalid bytes length` on codeobjects with linetable > 4096

[maurycy]

Bug report

Bug description:

I've reproduced the issue reported in https://discuss.python.org/t/tachyon-97-error-rate/107619

The issue is caused by:

cpython/Modules/_remote_debugging/code_objects.c

Lines 389 to 390 in 884ac3e

	linetable = read_py_bytes(unwinder,
	GET_MEMBER(uintptr_t, code_object, unwinder->debug_offsets.code_object.linetable), 4096);

The linetable for HttpCli.tx_browser and HttpCli.run is above 4096:

2026-06-06T17:13:54.694708000+0200 maurycy@gimel /Users/maurycy/work/cpython (main fded34d*?) % uv run linetable_dist.py ~/Desktop/copyparty
objects=3472 median=51 max=19536 >4096=7
   19536  AuthSrv._reload           /Users/maurycy/Desktop/copyparty/copyparty/authsrv.py:1762
   10640  HttpCli.tx_browser        /Users/maurycy/Desktop/copyparty/copyparty/httpcli.py:6749
    9131  HttpCli.run               /Users/maurycy/Desktop/copyparty/copyparty/httpcli.py:335
    6245  <module>                  /Users/maurycy/Desktop/copyparty/copyparty/util.py:1
    5087  Up2k._handle_json         /Users/maurycy/Desktop/copyparty/copyparty/up2k.py:3035
    4146  SvcHub.__init__           /Users/maurycy/Desktop/copyparty/copyparty/svchub.py:136
    4133  HttpCli.handle_plain_upload  /Users/maurycy/Desktop/copyparty/copyparty/httpcli.py:3648
    3753  HttpCli.dump_to_file      /Users/maurycy/Desktop/copyparty/copyparty/httpcli.py:2444

The 4096 limit is too small even for our stdlib:

2026-06-06T17:14:47.179006000+0200 maurycy@gimel /Users/maurycy/work/cpython (main fded34d*?) % uv run linetable_dist.py ~/work/cpython/Lib 
objects=92035 median=49 max=37416 >4096=37
   37416  <module>                  /Users/maurycy/work/cpython/Lib/html/entities.py:1
   20958  <module>                  /Users/maurycy/work/cpython/Lib/test/test_ast/snippets.py:1
   17060  <module>                  /Users/maurycy/work/cpython/Lib/locale.py:1
   11158  <module>                  /Users/maurycy/work/cpython/Lib/test/test_dis.py:1
   10063  SuggestionFormattingTestBase.test_name_error_suggestions_do_not_trigger_for_too_many_locals.<locals>.func  /Users/maurycy/work/cpython/Lib/test/test_traceback.py:4899
    9953  <module>                  /Users/maurycy/work/cpython/Lib/stringprep.py:1
    9400  <module>                  /Users/maurycy/work/cpython/Lib/test/re_tests.py:1
    5892  <module>                  /Users/maurycy/work/cpython/Lib/encodings/aliases.py:1
    5791  <module>                  /Users/maurycy/work/cpython/Lib/test/test_socket.py:1
    5730  <module>                  /Users/maurycy/work/cpython/Lib/encodings/mac_arabic.py:1
    5730  <module>                  /Users/maurycy/work/cpython/Lib/encodings/cp866.py:1
    5730  <module>                  /Users/maurycy/work/cpython/Lib/encodings/cp865.py:1
    5730  <module>                  /Users/maurycy/work/cpython/Lib/encodings/cp863.py:1
    5730  <module>                  /Users/maurycy/work/cpython/Lib/encodings/cp862.py:1
    5730  <module>                  /Users/maurycy/work/cpython/Lib/encodings/cp861.py:1

Reproduction

Using real file from stdlib:

import os, _remote_debugging

src = open("/Users/maurycy/work/cpython/Lib/html/entities.py").read()
src += "\n_remote_debugging.RemoteUnwinder(os.getpid()).get_stack_trace()\n"
exec(compile(src, "entities.py", "exec"))
print("OK")

We get:

2026-06-06T17:17:34.919627000+0200 maurycy@gimel /Users/maurycy/work/cpython (main fded34d*?) % uv run repro.py
Traceback (most recent call last):
  File "/Users/maurycy/src/github.com/maurycy/cpython/repro.py", line 5, in <module>
    exec(compile(src, "entities.py", "exec"))
    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "entities.py", line 2515, in <module>
RuntimeError: Invalid bytes length (37445) at 0xc5091c000

Bonus

(thx Claude)

linetable_dist.py

#!/usr/bin/env python3
import warnings
from pathlib import Path
from statistics import median
from sys import argv
from types import CodeType

warnings.simplefilter("ignore")
SKIP = {"venv", "site-packages", "__pycache__"}


def codes(co):
    yield co
    for c in co.co_consts:
        if isinstance(c, CodeType):
            yield from codes(c)


rows = []
for arg in argv[1:]:
    p = Path(arg).expanduser()
    for f in [p] if p.is_file() else p.rglob("*.py"):
        if SKIP & set(f.parts):
            continue
        try:
            co = compile(f.read_text(errors="replace"), str(f), "exec")
        except Exception:
            continue
        rows += [(len(c.co_linetable), str(f), c.co_qualname, c.co_firstlineno)
                 for c in codes(co)]

rows.sort(reverse=True)
vals = [s for s, *_ in rows]

print(f"objects={len(vals)} median={median(vals):.0f} max={max(vals)} "
      f">4096={sum(v > 4096 for v in vals)}")
for s, fn, qual, lineno in rows[:15]:
    print(f"{s:8d}  {qual:24s}  {fn}:{lineno}")

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

[maurycy]

cc @pablogsal 2^16 is tempting :-)

[pablogsal]

640K ought to be enough for anybody