linux-kernel - Re: [PATCH v3] kernel/signal: Signal-based pre-coredump notification

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Date:   Wed, 24 Oct 2018 15:02:29 -0700
From:   Enke Chen <enkechen@...co.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        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>, x86@...nel.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>,
        Enke Chen <enkechen@...co.com>
Subject: Re: [PATCH v3] kernel/signal: Signal-based pre-coredump notification

Hi, Oleg:

On 10/24/18 7:02 AM, Oleg Nesterov wrote:
> On 10/23, Enke Chen wrote:
>>
>> --- a/fs/coredump.c
>> +++ b/fs/coredump.c
>> @@ -590,6 +590,12 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>>  	if (retval < 0)
>>  		goto fail_creds;
>>
>> +	/*
>> +	 * Send the pre-coredump signal to the parent if requested.
>> +	 */
>> +	do_notify_parent_predump();
>> +	cond_resched();
> 
> I am still not sure cond_resched() makes any sense here...
> 
>> @@ -1553,6 +1553,9 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
>>  	tty_audit_fork(sig);
>>  	sched_autogroup_fork(sig);
>>
>> +	/* Clear the pre-coredump signal for the child */
>> +	sig->predump_signal = 0;
> 
> No need, copy_signal() does zalloc().
> 

Removed.

> 
>> +void do_notify_parent_predump(void)
>> +{
>> +	struct sighand_struct *sighand;
>> +	struct kernel_siginfo info;
>> +	struct task_struct *parent;
>> +	unsigned long flags;
>> +	int sig;
>> +
>> +	read_lock(&tasklist_lock);
>> +	parent = current->parent;
>> +	sig = parent->signal->predump_signal;
>> +	if (sig != 0) {
>> +		clear_siginfo(&info);
>> +		info.si_pid = task_tgid_vnr(current);
>> +		info.si_signo = sig;
>> +		if (sig == SIGCHLD)
>> +			info.si_code = CLD_PREDUMP;
>> +
>> +		sighand = parent->sighand;
>> +		spin_lock_irqsave(&sighand->siglock, flags);
>> +		__group_send_sig_info(sig, &info, parent);
>> +		spin_unlock_irqrestore(&sighand->siglock, flags);
> 
> You can just use do_send_sig_info() and remove sighand/flags/spin_lock_irqsave.

Ok.

> 
> Perhaps the "likely" predump_signal==0 check at the start makes sense to avoid
> read_lock(tasklist).

I am not sure if we should/need to deviate from the convention (locking before
access the parent). In any case it may not matter as the coredump is in the
exceptional code flow.

> 
> And I'd suggest to move it into coredump.c and make it static. It won't have
> another user.

Ok.

Thanks.  -- Enke