linux-kernel - Re: [PATCH 1/2] pid: add pidfd

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Message-ID: <20190515153515.GA20783@redhat.com>
Date:   Wed, 15 May 2019 17:35:15 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     jannh@...gle.com, viro@...iv.linux.org.uk,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        arnd@...db.de, dhowells@...hat.com, akpm@...ux-foundation.org,
        cyphar@...har.com, ebiederm@...ssion.com,
        elena.reshetova@...el.com, keescook@...omium.org,
        luto@...capital.net, luto@...nel.org, tglx@...utronix.de,
        linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-ia64@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] pid: add pidfd_open()

On 05/15, Oleg Nesterov wrote:
>
> On 05/15, Christian Brauner wrote:
> >
> > +SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> > +{
> > +	int fd, ret;
> > +	struct pid *p;
> > +	struct task_struct *tsk;
> > +
> > +	if (flags)
> > +		return -EINVAL;
> > +
> > +	if (pid <= 0)
> > +		return -EINVAL;
> > +
> > +	p = find_get_pid(pid);
> > +	if (!p)
> > +		return -ESRCH;
> > +
> > +	rcu_read_lock();
> > +	tsk = pid_task(p, PIDTYPE_PID);
>
> You do not need find_get_pid() before rcu_lock and put_pid() at the end.
> You can just do find_vpid() under rcu_read_lock().

Ah, sorry. Somehow I forgot you need to call pidfd_create(pid), you can't
do this under rcu_read_lock().

So I was wrong, you can't avoid get/put_pid.

Oleg.