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

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