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Subject: Re: [PATCH] kernel/signal: Signal-based pre-coredump notification
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Khalid Aziz <khalid.aziz@oracle.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Helge Deller <deller@gmx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian@brauner.io>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Roman Gushchin <guro@fb.com>,
        Marcos Paulo de Souza <marcos.souza.org@gmail.com>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        "Victor Kamensky (kamensky)" <kamensky@cisco.com>,
        xe-linux-external@cisco.com, Stefan Strogin <sstrogin@cisco.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Enke Chen <enkechen@cisco.com>
References: <e7ed306f-8992-9d00-bcab-5131159e8d89@cisco.com>
 <20181015120521.GA10146@redhat.com>
From:   Enke Chen <enkechen@cisco.com>
Message-ID: <fd182817-4656-13f1-5071-b91392dfba96@cisco.com>
Date:   Mon, 15 Oct 2018 11:54:44 -0700
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Hi, Oleg:

On 10/15/18 5:05 AM, Oleg Nesterov wrote:
> On 10/12, Enke Chen wrote:
>>
>> For simplicity and consistency, this patch provides an implementation
>> for signal-based fault notification prior to the coredump of a child
>> process. A new prctl command, PR_SET_PREDUMP_SIG, is defined that can
>> be used by an application to express its interest and to specify the
>> signal (SIGCHLD or SIGUSR1 or SIGUSR2) for such a notification. A new
>> signal code (si_code), CLD_PREDUMP, is also defined for SIGCHLD.
> 
> To be honest, I can't say I like this new feature...

The requirement for predump notification is real. IMO signal notification
is simpler than "connector" or "signal + connector".

> 
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -696,6 +696,10 @@ struct task_struct {
>>  	int				exit_signal;
>>  	/* The signal sent when the parent dies: */
>>  	int				pdeath_signal;
>> +
>> +	/* The signal sent prior to a child's coredump: */
>> +	int				predump_signal;
>> +
> 
> At least, I think predump_signal should live in signal_struct, not
> task_struct.

It makes sense as "signal handling" must be consistent in a process.
I was following the wrong example. I will make the change.

> 
> (pdeath_signal too, but it is too late to change (fix) this awkward API).
> 
>> +static void do_notify_parent_predump(struct task_struct *tsk)
>> +{
>> +	struct sighand_struct *sighand;
>> +	struct task_struct *parent;
>> +	struct kernel_siginfo info;
>> +	unsigned long flags;
>> +	int sig;
>> +
>> +	parent = tsk->real_parent;
> 
> So, debuggere won't be notified, only real_parent...
> 
>> +	sig = parent->predump_signal;
> 
> probably ->predump_signal should be cleared on exec?
> 
>> +	/* Check again with tasklist_lock" locked by the caller */
>> +	if (!valid_predump_signal(sig))
>> +		return;
> 
> I don't understand why we need valid_predump_signal() at all.
> 
>>  bool get_signal(struct ksignal *ksig)
>>  {
>>  	struct sighand_struct *sighand = current->sighand;
>> @@ -2497,6 +2535,19 @@ bool get_signal(struct ksignal *ksig)
>>  		current->flags |= PF_SIGNALED;
>>  
>>  		if (sig_kernel_coredump(signr)) {
>> +			/*
>> +			 * Notify the parent prior to the coredump if the
>> +			 * parent is interested in such a notificaiton.
>> +			 */
>> +			int p_sig = current->real_parent->predump_signal;
>> +
>> +			if (valid_predump_signal(p_sig)) {
>> +				read_lock(&tasklist_lock);
>> +				do_notify_parent_predump(current);
>> +				read_unlock(&tasklist_lock);
>> +				cond_resched();
> 
> perhaps this should be called by do_coredump() after coredump_wait() kills
> all the sub-threads?
> 
>> +static int prctl_set_predump_signal(struct task_struct *tsk, pid_t pid, int sig)
>> +{
>> +	struct task_struct *p;
>> +	int error;
>> +
>> +	/* 0 is valid for disabling the feature */
>> +	if (sig && !valid_predump_signal(sig))
>> +		return -EINVAL;
>> +
>> +	/* For the current task, the common case */
>> +	if (pid == 0) {
>> +		tsk->predump_signal = sig;
>> +		return 0;
>> +	}
>> +
>> +	error = -ESRCH;
>> +	rcu_read_lock();
>> +	p = find_task_by_vpid(pid);
>> +	if (p) {
>> +		if (!set_predump_signal_perm(p))
>> +			error = -EPERM;
>> +		else {
>> +			error = 0;
>> +			p->predump_signal = sig;
>> +		}
>> +	}
>> +	rcu_read_unlock();
>> +	return error;
>> +}
> 
> Why? I mean, why do we really want to support the pid != 0 case?

I will remove it. Please see my reply to Jann.

Thanks.  -- Enke