[New-bugs-announce] [issue42540] Debug pymalloc crash when using os.fork() [regression]

CendioOssman report at bugs.python.org
Wed Dec 2 09:22:16 EST 2020

New submission from CendioOssman <ossman at cendio.se>:

A python equivalent of the classical daemon() call started throwing an error from 3.8 when the debug hooks are active for pymalloc. If the tracing is also active it segfaults.

This simple example triggers it:

import os

def daemon():
    pid = os.fork()
    if pid != 0:


The error given is:

Debug memory block at address p=0xf013d0: API '1'
    0 bytes originally requested
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfd):
        at p-7: 0x00 *** OUCH
        at p-6: 0x00 *** OUCH
        at p-5: 0x00 *** OUCH
        at p-4: 0x00 *** OUCH
        at p-3: 0x00 *** OUCH
        at p-2: 0x00 *** OUCH
        at p-1: 0x00 *** OUCH
    Because memory is corrupted at the start, the count of bytes requested
       may be bogus, and checking the trailing pad bytes may segfault.
    The 8 pad bytes at tail=0xf013d0 are not all FORBIDDENBYTE (0xfd):
        at tail+0: 0x01 *** OUCH
        at tail+1: 0x00 *** OUCH
        at tail+2: 0x00 *** OUCH
        at tail+3: 0x00 *** OUCH
        at tail+4: 0x00 *** OUCH
        at tail+5: 0x00 *** OUCH
        at tail+6: 0x00 *** OUCH
        at tail+7: 0x00 *** OUCH

Enable tracemalloc to get the memory block allocation traceback

Fatal Python error: bad ID: Allocated using API '1', verified using API 'r'
Python runtime state: finalizing (tstate=0xf023b0)

Tested on Fedora, Ubuntu and RHEL with the same behaviour everwhere. Everything up to 3.8 works fine. 3.8 and 3.9 both exhibit the issue.

Since this is a very standard way of starting a daemon it should affect quite a few users. At the very least it makes it annoying to use development mode to catch other issues.

components: Interpreter Core
messages: 382314
nosy: CendioOssman
priority: normal
severity: normal
status: open
title: Debug pymalloc crash when using os.fork() [regression]
type: crash
versions: Python 3.10, Python 3.8, Python 3.9

Python tracker <report at bugs.python.org>

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