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

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Date:   Tue, 16 Oct 2018 10:09:34 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Enke Chen <enkechen@...co.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian@...uner.io>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Dave Martin <Dave.Martin@....com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Roman Gushchin <guro@...com>,
        Marcos Paulo de Souza <marcos.souza.org@...il.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        "Victor Kamensky \(kamensky\)" <kamensky@...co.com>,
        xe-linux-external@...co.com, Stefan Strogin <sstrogin@...co.com>,
        Eugene Syromiatnikov <esyr@...hat.com>
Subject: Re: [PATCH] kernel/signal: Signal-based pre-coredump notification

Oleg Nesterov <oleg@...hat.com> writes:

> On 10/15, Enke Chen wrote:
>>
>> > I don't understand why we need valid_predump_signal() at all.
>>
>> Most of the signals have well-defined semantics, and would not be appropriate
>> for this purpose.
>
> you are going to change the rules anyway.

I will just add that CLD_XXX is only valid with SIGCHLD as they are
signal specific si_codes.  In conjunction with another signal like
SIGUSR it will have another meaning.  I would really appreciate it
if new code does not further complicate siginfo_layout.

>> That is why it is limited to only SIGCHLD, SIGUSR1, SIGUSR2.
>
> Which do not queue. So the parent won't get the 2nd signal if 2 children
> crash at the same time.

We do best effort queueing but we don't guarantee anything.  So yes
this makes signals a very louzy interface for sending this kind of
information.

>> >>  		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?
>>
>> proc_coredump_connector(current) is located here, they should stay together.
>
> Why?
>
> Once again, other threads are still alive. So if the parent restarts the service
> after it recieves -predump_signal, the new process can "race" with the old thread.

Yes.  It isn't until do_coredump calls coredump_wait that all of the
threads are killed.

Eric