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Subject: Re: [PATCH v4] 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>,
        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>, x86@kernel.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>,
        Enke Chen <enkechen@cisco.com>
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 <84460a77-a111-404e-4bad-88104a6e246e@cisco.com>
 <20181026082812.GA10581@redhat.com>
From:   Enke Chen <enkechen@cisco.com>
Message-ID: <21f678a8-4001-df36-c26e-e96cf203b1b1@cisco.com>
Date:   Fri, 26 Oct 2018 15:23:47 -0700
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Hi, Olge:

This is really a good idea given that "parent" is declared as RCU-protected.
Just a bit odd, though, that the "parent" has not been accessed this way in
the code base.

So just to confirm: the revised code would look like the following:

static void do_notify_parent_predump(void)
{
        struct task_struct *parent;
        int sig;

        rcu_read_lock();
        parent = rcu_dereference(current->parent);
        sig = parent->signal->predump_signal;
        if (sig != 0)
                do_send_sig_info(sig, SEND_SIG_NOINFO, parent, PIDTYPE_TGID);
        rcu_read_unlock();
}

Thank you so much for your help during this review. I would like to ack your
contribution in the "Reviewed-by:" field.

-- Enke

On 10/26/18 1:28 AM, Oleg Nesterov wrote:
> On 10/25, Enke Chen wrote:
>>
>> +static void do_notify_parent_predump(void)
>> +{
>> +	struct task_struct *parent;
>> +	int sig;
>> +
>> +	read_lock(&tasklist_lock);
>> +	parent = current->parent;
>> +	sig = parent->signal->predump_signal;
>> +	if (sig != 0)
>> +		do_send_sig_info(sig, SEND_SIG_NOINFO, parent, PIDTYPE_TGID);
>> +	read_unlock(&tasklist_lock);
> 
> Ah. It is strange I didn't think about this before... Please, do not take
> tasklist_lock, use rcu_read_lock() instead. do_send_sig_info() uses the
> rcu-friendly lock_task_sighand(), so rcu_dereference(parent) should work
> fine.
> 
> Oleg.
>