Could you test this patch?

Now, there remains the question of CLONE_CHILD_SETTID and
CLONE_CHILD_CLEARTID.  Since we’re passing NULL for ‘ctid’, I expect
that these flags have no effect at all.

Conversely, libc uses these flags to update the thread ID in the child
process (x86_64/arch-fork.h):

--8<---------------cut here---------------start------------->8---
#define ARCH_FORK() \
  INLINE_SYSCALL (clone, 4,                                                   \
                  CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD, 0,     \
                  NULL, &THREAD_SELF->tid)
--8<---------------cut here---------------end--------------->8---

This is certainly useful, but we’d have troubles doing it from the FFI…
It may that this is fine if the process doesn’t use threads.

Ludo’.