[PATCH] Signal semantics for /sbin/init

[sukadev at us.ibm.com]

Eric,

Can you send out your modified patch for this - I can port mine on top
and resend ?

Suka

Eric W. Biederman [ebiederm at xmission.com] wrote:
| sukadev at us.ibm.com writes:
| 
| > | This change by making the presence of a signal handler effectively
| > | a permission check allows us to do all of the work before
| > | we enqueue the signal.  Which means that we can now allow
| > | force_sig_info to send signals to init and that panic the kernel
| > | instead of going into an infinite busy loop taking an exception
| > | sending a signal and then retaking the same exception.
| > | 
| > | This change also makes it possible to easily implement the 
| > | the desired semantics of /sbin/init for pid namespaces where
| > | outer processes can kill init but processes inside the pid
| > | namespace can not.
| > | 
| > | Please take a look and tell me what you think.
| >
| > Overall, it looks good, couple of questions below. Will port my
| > patches and test it out.
| >
| > | 
| > | diff --git a/kernel/signal.c b/kernel/signal.c
| > | index 4537bdd..79856eb 100644
| > | --- a/kernel/signal.c
| > | +++ b/kernel/signal.c
| > | @@ -546,6 +546,22 @@ static int check_kill_permission(int sig, struct siginfo
| > *info,
| > |  	return security_task_kill(t, info, sig, 0);
| > |  }
| > | 
| > | +static int sig_available(struct task_struct *tsk, int sig)
| >
| > Hmm. IMHO, 'available' is not immediately obvious/clear.
| 
| Agreed.  It was what I had to work with.  I have since renamed it
| sig_init_drop.  Which is a little better.
| 
| > | +{
| > | +	void __user * handler;
| > | +	int available = 1;
| > | +
| > | +	if (likely(!is_global_init(tsk)))
| > | +		goto out;
| >
| > With multiple pid namespaces, I guess, this would become
| >
| > 	if (!is_container_init(tsk))
| > 		goto out;
| 
| Although I need to make that tsk->group_leader.
| 
| > 	/* from parent namespace, don't ignore */
| > 	if (!task_in_descendant_ns(tsk))
| > 		goto out;
| >
| > If this is correct, I have a question below re: do_sigaction.
| 
| > | +
| > | +	handler = tsk->sighand->action[sig-1].sa.sa_handler;
| > | +	available = (handler != SIG_IGN) &&
| > | +		    (handler != SIG_DFL);
| >
| > Can we use sig_user_defined() for the above checks ? sig_available()
| > looks like an extension of sig_user_defined() for init. 
| 
| Well the SIG_IGN check is actually wrong here.  We should just check
| for SIG_DFL if we want to maintain the most possible compatibility.
| 
| 
| > How about 
| > sig_init_user_defined() or reverse the logic and use sig_init_ignore()
| > like Oleg did ?
| 
| That could work.
| 
| > | +out:
| > | +	return available;
| > | +}
| > | +
| > | +
| > |  /* forward decl */
| > |  static void do_notify_parent_cldstop(struct task_struct *tsk, int why);
| > | 
| > | @@ -948,6 +964,9 @@ __group_send_sig_info(int sig, struct siginfo *info,
| > struct task_struct *p)
| > |  	int ret = 0;
| > | 
| > |  	assert_spin_locked(&p->sighand->siglock);
| > | +	if (!sig_available(p, sig))
| > | +		return ret;
| > | +
| >
| > |  	handle_stop_signal(sig, p);
| > | 
| > |  	/* Short-circuit ignored signals.  */
| > | @@ -1379,6 +1398,11 @@ send_group_sigqueue(int sig, struct sigqueue *q, struct
| > task_struct *p)
| > |  	read_lock(&tasklist_lock);
| > |  	/* Since it_lock is held, p->sighand cannot be NULL. */
| > |  	spin_lock_irqsave(&p->sighand->siglock, flags);
| > | +	if (!sig_available(p, sig)) {
| > | +		ret = 1;
| > | +		goto out;
| > | +	}
| > | +
| > |  	handle_stop_signal(sig, p);
| > | 
| > |  	/* Short-circuit ignored signals.  */
| > 	
| > Hmm. I see now that Oleg's approach would result in the sig_init_ignore()
| > check being done twice (once in handle_stop_signal() and once in the following
| > sig_ignored()).
| >
| > Is that why you don't fold sig_available() into sig_ignored() ? Or, there
| > other more important/correctness issues as well ?
| 
| It is a very slight but subtle point.
| I have modified the definition so that signals with a handler SIG_DFL never
| reach the init process not even to the pending mask.  Essentially
| this means we should drop them before any attempts at processing them.
| 
| So dropping the signals before handle_stop_signal is important.
| 
| As for not folding the signals into sig_ignore.  We need to drop the signal
| even if the signal is masked.  So it looks like an ugly special case.
| 
| In addition by not dropping the signal to init everywhere (in particular
| on the paths where we force a signal) we allow the kernel to kill init
| and panic the system if /sbin/init does something nasty instead of looping
| forever.
| 
| 
| > | @@ -1860,12 +1884,6 @@ relock:
| > |  		if (sig_kernel_ignore(signr)) /* Default is nothing. */
| > |  			continue;
| > | 
| > | -		/*
| > | -		 * Global init gets no signals it doesn't want.
| > | -		 */
| > | -		if (is_global_init(current))
| > | -			continue;
| > | -
| > |  		if (sig_kernel_stop(signr)) {
| > |  			/*
| > |  			 * The default action is to stop all threads in
| > | @@ -2246,8 +2264,10 @@ static int do_tkill(int tgid, int pid, int sig)
| > |  		 */
| > |  		if (!error && sig && p->sighand) {
| > |  			spin_lock_irq(&p->sighand->siglock);
| > | -			handle_stop_signal(sig, p);
| > | -			error = specific_send_sig_info(sig, &info, p);
| > | +			if (sig_available(p, sig)) {
| > | +				handle_stop_signal(sig, p);
| > | +				error = specific_send_sig_info(sig, &info, p);
| > | +			}
| > |  			spin_unlock_irq(&p->sighand->siglock);
| > |  		}
| > |  	}
| > | @@ -2336,7 +2356,8 @@ int do_sigaction(int sig, struct k_sigaction *act,
| > struct k_sigaction *oact)
| > |  		 *   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))) {
| > | +		   (act->sa.sa_handler == SIG_DFL && sig_kernel_ignore(sig)) ||
| > | +		   !sig_available(current, sig)) {
| > |  			struct task_struct *t = current;
| > |  			sigemptyset(&mask);
| > |  			sigaddset(&mask, sig);
| >
| > Not sure about this. Consider that we extend the sig_available() as
| > I mentioned above for container_init(). Then suppose following sequence
| > occurs:
| >
| > 	- container-init receives a fatal signal say SIGUSR1, from parent-ns
| > 	  i.e the signal is pending.
| >
| > 	- before processing the pending signal, the container-init sets
| > 	  the handler to SIG_DFL (which is to terminate). 
| > 	  
| > 	  Will we then discard the pending SIGUSR1 even though it was from
| > 	  from parent ns ?
| 
| Good question. 
| 
| I guess if a signal from a child is already pending we have limited
| responsibility for dropping it, because we have already allowed it
| through...
| 
| My thought had been especially after I saw that part of Oleg's patch
| that it was the right thing to do.
| 
| Given my attempt at well defined semantics for dropping the signal
| from the sender.  The rule would be if the signal makes it to the init
| process we should not treat it specially.
| 
| The historical behavior would have been that if the signal was from a
| child the signal would have been dropped just after this point when it
| was delivered.
| 
| However if someone wants to prevent that case we can use sigwait, to
| remove blocked signals.  Further different rules for signal handling
| for init I think are largely problematic for authors of different
| inits because they are hard to remember.
| 
| Further sysvinit doesn't ever set a signal handler to SIG_DFL except
| after it forks and just before it execs a process so the common case
| will not be affected.
| 
| Therefore I think you are right.  We don't need this case and it
| is likely to be more problematic then useful.
| 
| Eric