From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Eric W. Biederman" Date: Mon, 06 Jun 2022 16:06:01 +0000 Subject: Re: [PATCH 05/16] ptrace: Remove dead code from __ptrace_detach Message-Id: <87ee014w0m.fsf@email.froward.int.ebiederm.org> List-Id: References: <871qwq5ucx.fsf_-_@email.froward.int.ebiederm.org> <20220518225355.784371-5-ebiederm@xmission.com> <20220524114250.GB14347@redhat.com> <20220525143303.GB2687@redhat.com> In-Reply-To: <20220525143303.GB2687@redhat.com> (Oleg Nesterov's message of "Wed, 25 May 2022 16:33:03 +0200") MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, rjw@rjwysocki.net, mingo@kernel.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de, bigeasy@linutronix.de, Will Deacon , tj@kernel.org, linux-pm@vger.kernel.org, Peter Zijlstra , Richard Weinberger , Anton Ivanov , Johannes Berg , linux-um@lists.infradead.org, Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org, Kees Cook , Jann Horn , linux-ia64@vger.kernel.org, Robert OCallahan , Kyle Huey , Richard Henderson , Ivan Kokshaysky , Matt Turner , Jason Wessel , Daniel Thompson , Douglas Anderson , Douglas Miller , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras Oleg Nesterov writes: > On 05/24, Oleg Nesterov wrote: >> >> Sorry for delay. >> >> On 05/18, Eric W. Biederman wrote: >> > >> > Ever since commit 28d838cc4dfe ("Fix ptrace self-attach rule") it has >> > been impossible to attach another thread in the same thread group. >> > >> > Remove the code from __ptrace_detach that was trying to support >> > detaching from a thread in the same thread group. >> >> may be I am totally confused, but I think you misunderstood this code >> and thus this patch is very wrong. >> >> The same_thread_group() check does NOT try to check if debugger and >> tracee is in the same thread group, this is indeed impossible. >> >> We need this check to know if the tracee was ptrace_reparented() before >> __ptrace_unlink() or not. >> >> >> > -static int ignoring_children(struct sighand_struct *sigh) >> > -{ >> > - int ret; >> > - spin_lock(&sigh->siglock); >> > - ret = (sigh->action[SIGCHLD-1].sa.sa_handler = SIG_IGN) || >> > - (sigh->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT); >> > - spin_unlock(&sigh->siglock); >> > - return ret; >> > -} >> >> ... >> >> > @@ -565,14 +552,9 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p) >> > >> > dead = !thread_group_leader(p); >> > >> > - if (!dead && thread_group_empty(p)) { >> > - if (!same_thread_group(p->real_parent, tracer)) >> > - dead = do_notify_parent(p, p->exit_signal); >> > - else if (ignoring_children(tracer->sighand)) { >> > - __wake_up_parent(p, tracer); >> > - dead = true; >> > - } >> > - } >> >> So the code above does: >> >> - if !same_thread_group(p->real_parent, tracer), then the tracee was >> ptrace_reparented(), and now we need to notify its natural parent >> to let it know it has a zombie child. >> >> - otherwise, the tracee is our natural child, and it is actually dead. >> however, since we are going to reap this task, we need to wake up our >> sub-threads possibly sleeping on ->wait_chldexit wait_queue_head_t. >> >> See? >> >> > + if (!dead && thread_group_empty(p)) >> > + dead = do_notify_parent(p, p->exit_signal); >> >> No, this looks wrong. Or I missed something? > > Yes, but... > > That said, it seems that we do not need __wake_up_parent() if it was our > natural child? Agreed on both counts. Hmm. I see where the logic comes from. The ignoring_children test and the __wake_up_parent are what do_notify_parent does when the parent ignores children. Hmm. I even see all of this document in the comment above __ptrace_detach. So I am just going to drop this change. > I'll recheck. Eric, I'll continue to read this series tomorrow, can't > concentrate on ptrace today. No worries. This was entirely too close to the merge window so I dropped it all until today. Eric