Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification

[Enke Chen]

Hi, Oleg:

Thanks for your review. Please see my replies inline.

On 10/23/18 2:23 AM, Oleg Nesterov wrote:
> On 10/22, Enke Chen wrote:
>>
>> As the coredump of a process may take time, in certain time-sensitive
>> applications it is necessary for a parent process (e.g., a process
>> manager) to be notified of a child's imminent death before the coredump
>> so that the parent process can act sooner, such as re-spawning an
>> application process, or initiating a control-plane fail-over.
> 
> Personally I still do not like this feature, but I won't argue.
> 
>> --- a/fs/coredump.c
>> +++ b/fs/coredump.c
>> @@ -546,6 +546,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>>  	struct cred *cred;
>>  	int retval = 0;
>>  	int ispipe;
>> +	bool notify;
>>  	struct files_struct *displaced;
>>  	/* require nonrelative corefile path and be extra careful */
>>  	bool need_suid_safe = false;
>> @@ -590,6 +591,15 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>>  	if (retval < 0)
>>  		goto fail_creds;
>>
>> +	/*
>> +	 * Send the pre-coredump signal to the parent if requested.
>> +	 */
>> +	read_lock(&tasklist_lock);
>> +	notify = do_notify_parent_predump(current);
>> +	read_unlock(&tasklist_lock);
>> +	if (notify)
>> +		cond_resched();
> 
> Hmm. I do not understand why do we need cond_resched(). And even if we need it,
> why we can't call it unconditionally?

Remember the goal is to allow the parent (e.g., a process manager) to take early
action. The "yield" before doing coredump will help.

The yield is made conditional because the notification is conditional.
Is that ok?

> 
> I'd also suggest to move read_lock/unlock(tasklist) into do_notify_parent_predump()
> and remove the "task_struct *tsk" argument, tsk is always current.
> 
> Yes, do_notify_parent() and do_notify_parent_cldstop() are called with tasklist_lock
> held, but there are good reasons for that.

Sure I will make the suggested changes. This function is only called in one place.

> 
> 
>> +static inline int valid_predump_signal(int sig)
>> +{
>> +	return (sig == SIGCHLD) || (sig == SIGUSR1) || (sig == SIGUSR2);
>> +}
> 
> I still do not understand why do we need to restrict predump_signal.
> 
> PR_SET_PREDUMP_SIG can only change the caller's ->predump_signal, so to me
> even PR_SET_PREDUMP_SIG(SIGKILL) is fine.

I will remove it to reduce the code size and give more flexibility to the application.

> 
> And once again, SIGCHLD/SIGUSR do not queue, this means that PR_SET_PREDUMP_SIG
> is pointless if you have 2 or more children.

Hmm, could you point me to the code where SIGCHLD/SIGUSR is treated differently
w.r.t. queuing?  That does not sound right to me.

> 
>> +bool do_notify_parent_predump(struct task_struct *tsk)
>> +{
>> +	struct sighand_struct *sighand;
>> +	struct kernel_siginfo info;
>> +	struct task_struct *parent;
>> +	unsigned long flags;
>> +	pid_t pid;
>> +	int sig;
>> +
>> +	parent = tsk->parent;
>> +	sighand = parent->sighand;
>> +	pid = task_tgid_vnr(tsk);
>> +
>> +	spin_lock_irqsave(&sighand->siglock, flags);
>> +	sig = parent->signal->predump_signal;
>> +	if (!valid_predump_signal(sig)) {
>> +		spin_unlock_irqrestore(&sighand->siglock, flags);
>> +		return false;
>> +	}
> 
> Why do we need to check parent->signal->predump_signal under ->siglock?
> This complicates the code for no reason, afaics.
> 
>> +	clear_siginfo(&info);
>> +	info.si_pid = pid;
>> +	info.si_signo = sig;
>> +	if (sig == SIGCHLD)
>> +		info.si_code = CLD_PREDUMP;
>> +
>> +	__group_send_sig_info(sig, &info, parent);
>> +	__wake_up_parent(tsk, parent);
> 
> Why __wake_up_parent() ?

not needed, and will remove.

> 
> do_notify_parent() does this to wake up the parent sleeping in do_wait(), to
> report the event. But predump_signal has nothing to do with wait().
> 
> Now. This version sends the signal to ->parent, not ->real_parent. OK, but this
> means that real_parent won't be notified if its child is traced.
> > 
>> +	case PR_SET_PREDUMP_SIG:
>> +		if (arg3 || arg4 || arg5)
>> +			return -EINVAL;
>> +
>> +		/* 0 is valid for disabling the feature */
>> +		if (arg2 && !valid_predump_signal((int)arg2))
>> +			return -EINVAL;
>> +		me->signal->predump_signal = (int)arg2;
>> +		break;
> 
> Again, I do not understand why do we need valid_predump_signal(). But even
> if we need it, I don't understand why should we check it twice. IOW, why
> do_notify_parent_predump() can't simply check ->predump_signal != 0?
> 
> Whatever we do, PR_SET_PREDUMP_SIG should validate arg2 anyway. Who else can
> change ->predump_signal  after that?

Ok, will relax.

> 
>> +	case PR_GET_PREDUMP_SIG:
>> +		if (arg3 || arg4 || arg5)
>> +			return -EINVAL;
>> +		error = put_user(me->signal->predump_signal,
>> +				 (int __user *)arg2);
> 
> To me it would be better to simply return ->predump_signal, iow
> 
> 		error = me->signal->predump_signal;
> 		break;
> 
> but I won't insist, this is subjective and cosmetic.

Vast majority of system calls returns 0 or -1. So does PR_GET_PDEATHSIG.
I would like to keep them consistent.

Thanks again.

-- Enke