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Message-ID: <20070821160506.GA3125@sergelap.austin.ibm.com>
Date: Tue, 21 Aug 2007 11:05:06 -0500
From: "Serge E. Hallyn" <serue@...ibm.com>
To: Oleg Nesterov <oleg@...sign.ru>
Cc: Daniel Pittman <daniel@...space.net>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Ingo Molnar <mingo@...e.hu>, Kirill Korotaev <dev@...ru>,
Pavel Emelyanov <xemul@...ru>,
Roland McGrath <roland@...hat.com>,
"Serge E. Hallyn" <serue@...ibm.com>,
Sukadev Bhattiprolu <sukadev@...ibm.com>,
containers@...ts.osdl.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC,PATCH] fix /sbin/init signal handling
Quoting Oleg Nesterov (oleg@...sign.ru):
> (Not for inclusion yet, against 2.6.23-rc2, untested)
>
> Currently, /sbin/init is protected from unhandled signals by the
> "current == child_reaper(current)" check in get_signal_to_deliver().
> This is not enough, we have multiple problems:
>
> - this doesn't work for multi-threaded inits, and we can't
> fix this by simply making this check group-wide.
>
> - /sbin/init and kernel threads are not protected from
> handle_stop_signal(). Minor problem, but not good and
> allows to "steal" SIGCONT or change ->signal->flags.
>
> - /sbin/init is not protected from __group_complete_signal(),
> sig_fatal() can set SIGNAL_GROUP_EXIT and block exec(), kill
> sub-threads, set ->group_stop_count, etc.
>
> Also, with support for multiple pid namespaces, we need an ability to
> actually kill the sub-namespace's init from the parent namespace. In
> this case it is not possible (without painful and intrusive changes)
> to make the "should we honor this signal" decision on the receiver's
> side.
>
> Hopefully this patch (adds 43 bytes to kernel/signal.o) can solve
> these problems.
>
> Notes:
>
> - Blocked signals are never ignored, so init still can receive
> a pending blocked signal after sigprocmask(SIG_UNBLOCK).
> Easy to fix, but probably we can ignore this issue.
>
> - this patch allows us to simplify de_thread() playing games
> with pid_ns->child_reaper.
>
> (Side note: the current behaviour of things like force_sig_info_fault()
> is not very good, init should not ignore these signals and go to the
> endless loop. Exit + panic is imho better, easy to chamge)
>
> Oleg.
>
> --- t/kernel/signal.c~INITSIGS 2007-08-19 14:39:35.000000000 +0400
> +++ t/kernel/signal.c 2007-08-19 19:00:27.000000000 +0400
> @@ -39,11 +39,35 @@
>
> static struct kmem_cache *sigqueue_cachep;
>
> +static int sig_init_ignore(struct task_struct *tsk)
> +{
> + // Currently this check is a bit racy with exec(),
> + // we can _simplify_ de_thread and close the race.
> + if (likely(!is_init(tsk->group_leader)))
> + return 0;
> +
> + // ---------------- Multiple pid namespaces ----------------
> + // if (current is from tsk's parent pid_ns && !in_interrupt())
> + // return 0;
> +
> + return 1;
> +}
> +
> +static int sig_task_ignore(struct task_struct *tsk, int sig)
> +{
> + void __user * handler = tsk->sighand->action[sig-1].sa.sa_handler;
> +
> + if (handler == SIG_IGN)
> + return 1;
> +
> + if (handler != SIG_DFL)
> + return 0;
> +
> + return sig_kernel_ignore(sig) || sig_init_ignore(tsk);
> +}
Looks good. AFAICS init gets exactly those signals for which it
installed a signal handler.
>
> static int sig_ignored(struct task_struct *t, int sig)
> {
> - void __user * handler;
> -
> /*
> * Tracers always want to know about signals..
> */
> @@ -58,10 +82,7 @@ static int sig_ignored(struct task_struc
> if (sigismember(&t->blocked, sig))
> return 0;
>
> - /* Is it explicitly or implicitly ignored? */
> - handler = t->sighand->action[sig-1].sa.sa_handler;
> - return handler == SIG_IGN ||
> - (handler == SIG_DFL && sig_kernel_ignore(sig));
> + return sig_task_ignore(t, sig);
> }
Looks good.
>
> /*
> @@ -569,6 +590,9 @@ static void handle_stop_signal(int sig,
> */
> return;
>
> + if (sig_init_ignore(p))
> + return;
> +
> if (sig_kernel_stop(sig)) {
> /*
> * This is a stop signal. Remove SIGCONT from all queues.
> @@ -1841,14 +1865,6 @@ relock:
> if (sig_kernel_ignore(signr)) /* Default is nothing. */
> continue;
>
> - /*
> - * Init of a pid space gets no signals it doesn't want from
> - * within that pid space. It can of course get signals from
> - * its parent pid space.
> - */
> - if (current == child_reaper(current))
> - continue;
> -
Ok, so the idea is that this will now be caught when the signal is sent,
using sig_ignored(), (i.e at send_sigqueue, send_group_sigqueue,
specific_send_sig_info, and __group_send_sig_info) and so doesn't need
to be checked here?
I was hoping that meant that sig_init_ignore() would always be called
with current as the sending process, but I guess that's not the case?
At least in get_signal_to_deliver() we might resend a signal, though
I guess we assume the signal comes from current->parent, so maybe we
can pass that as an argument...
> if (sig_kernel_stop(signr)) {
> /*
> * The default action is to stop all threads in
> @@ -2300,13 +2316,10 @@ int do_sigaction(int sig, struct k_sigac
> k = ¤t->sighand->action[sig-1];
>
> spin_lock_irq(¤t->sighand->siglock);
> - if (signal_pending(current)) {
> - /*
> - * If there might be a fatal signal pending on multiple
> - * threads, make sure we take it before changing the action.
> - */
> + if (current->signal->flags & SIGNAL_GROUP_EXIT) {
> spin_unlock_irq(¤t->sighand->siglock);
> - return -ERESTARTNOINTR;
> + /* The return value doesn't matter, SIGKILL is pending */
> + return -EINTR;
> }
Looks right, based on the original comment.
>
> if (oact)
> @@ -2327,8 +2340,7 @@ int do_sigaction(int sig, struct k_sigac
> * (for example, SIGCHLD), shall cause the pending signal to
> * be discarded, whether or not it is blocked"
> */
> - if (act->sa.sa_handler == SIG_IGN ||
> - (act->sa.sa_handler == SIG_DFL && sig_kernel_ignore(sig))) {
> + if (sig_task_ignore(current, sig)) {
> struct task_struct *t = current;
> sigemptyset(&mask);
> sigaddset(&mask, sig);
Haven't tested, but the patch reads good.
thanks,
-serge
-
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