glibc 2.34 tries to use the clone3 system call to create threads.

This change is already included in builds glibc-2.33.9000-44.fc35 and later (only in Koji, not tagged into the buildroot/compose).

Currently, valgrind prints a warning for every clone3 system call, which can be reproduced e.g. with Python:

$ valgrind python3
==3260== Memcheck, a memory error detector
==3260== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==3260== Using Valgrind-3.17.0 and LibVEX; rerun with -h for copyright info
==3260== Command: python3
==3260== 
Python 3.10.0b4 (default, Jul 16 2021, 00:00:00) [GCC 11.1.1 20210623 (Red Hat 11.1.1-6)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import threading
>>> threading.Thread(None, lambda: print("Thread is running")).start()
--3260-- WARNING: unhandled amd64-linux syscall: 435
--3260-- You may be able to write your own handler.
--3260-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
--3260-- Nevertheless we consider this a bug.  Please report
--3260-- it at http://valgrind.org/support/bug_reports.html.
Thread is running
>>> 

The thread is still launched, but the warning is annoying.