> Date: Sun, 21 Mar 2021 16:56:47 +0100
> From: Martin Pieuchot <m...@openbsd.org>
>
> On 21/03/21(Sun) 13:42, Mark Kettenis wrote:
> > > Date: Sat, 20 Mar 2021 13:10:17 +0100
> > > From: Martin Pieuchot <m...@openbsd.org>
> > >
> > > On SP systems, like bluhm@'s armv7 regression machine, the kern/ptrace2
> > > test is failing due to a subtle behavior. Diff below makes it pass.
> > >
> > > http://bluhm.genua.de/regress/results/2021-03-19T15%3A17%3A02Z/logs/sys/kern/ptrace2/make.log
> > >
> > > The failing test does a fork(2) and the parent issues a PT_ATTACH on the
> > > child before it has been scheduled for the first time. Then the parent
> > > goes to sleep in waitpid() and when the child starts executing the check
> > > below overwrites the ptrace(2)-received SIGSTOP by a SIGTRAP.
> > >
> > > This scenario doesn't seem to happen on MP machine because the child
> > > starts to execute itself on a different core right after sys_fork() is
> > > finished.
> > >
> > > What is the purpose of this check? Should it be relaxed or removed?
> >
> > This is part of PT_SET_EVENT_MASK/PTRACE_FORK support:
> >
> > https://github.com/openbsd/src/commit/f38bed7f869bd3503530c554b4860228ea4e8641
> >
> > When reporting of the PTRACE_FORK event is requested, the debugger
> > expects to see a SIGTRAP in both the parent and the child. The code
> > expects that the only way to have PS_TRACED set in the child from the
> > start is when PTRACE_FORK is requested. But the failing test shows
> > there is a race with PT_ATTACH.
>
> Thanks for the explanation.
>
> > I think the solution is to have fork1() only run fork_return() if the
> > FORK_PTRACE flag is set, and use run child_return() otherwise.
>
> Diff below does that and prevent the race, ok?
ok kettenis@
> Index: kern/kern_fork.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_fork.c,v
> retrieving revision 1.234
> diff -u -p -r1.234 kern_fork.c
> --- kern/kern_fork.c 15 Feb 2021 09:35:59 -0000 1.234
> +++ kern/kern_fork.c 21 Mar 2021 15:55:26 -0000
> @@ -95,12 +95,15 @@ fork_return(void *arg)
> int
> sys_fork(struct proc *p, void *v, register_t *retval)
> {
> + void (*func)(void *) = child_return;
> int flags;
>
> flags = FORK_FORK;
> - if (p->p_p->ps_ptmask & PTRACE_FORK)
> + if (p->p_p->ps_ptmask & PTRACE_FORK) {
> flags |= FORK_PTRACE;
> - return fork1(p, flags, fork_return, NULL, retval, NULL);
> + func = fork_return;
> + }
> + return fork1(p, flags, func, NULL, retval, NULL);
> }
>
> int
>
