linux-kernel - Re: [PATCH v2 1/4] pidfd: support PIDFD_NONBLOCK in pidfd

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Date:   Thu, 3 Sep 2020 17:25:29 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Christian Brauner <christian@...uner.io>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Aleksa Sarai <cyphar@...har.com>,
        linux-kselftest@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Jens Axboe <axboe@...nel.dk>, linux-api@...r.kernel.org
Subject: Re: [PATCH v2 1/4] pidfd: support PIDFD_NONBLOCK in pidfd_open()

On Thu, Sep 03, 2020 at 04:58:09PM +0200, Oleg Nesterov wrote:
> Christian, off-topic question...
> 
> On 09/02, Christian Brauner wrote:
> >
> > -static int pidfd_create(struct pid *pid)
> > +static int pidfd_create(struct pid *pid, unsigned int flags)
> >  {
> >  	int fd;
> >
> >  	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
> > -			      O_RDWR | O_CLOEXEC);
> > +			      flags | O_RDWR | O_CLOEXEC);
> 
> I just noticed this comment above pidfd_create:
> 
> 	 * Note, that this function can only be called after the fd table has
> 	 * been unshared to avoid leaking the pidfd to the new process.
> 
> what does it mean?
> 
> Of course, if fd table is shared then pidfd can "leak" to another process,
> but this is true for any file and sys_pidfd_open() doesn't do any check?

It's the same comment we added in kernel/fork.c to make callers aware
that they can leak a pidfd to another process unintentionally. Sure,
this is true of any fd but since pidfds were a new type of handle and on
another process at that we felt that this was important to spell out. The
"can only" should've arguably been "should probably".

> 
> 
> 
> In fact I think this helper buys nothing but adds the unnecessary get/put_pid,
> we can kill it and change pidfd_open() to do
> 
> 	SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> 	{
> 		int fd;
> 		struct pid *p;
> 
> 		if (flags & ~PIDFD_NONBLOCK)
> 			return -EINVAL;
> 
> 		if (pid <= 0)
> 			return -EINVAL;
> 
> 		p = find_get_pid(pid);
> 		if (!p)
> 			return -ESRCH;
> 
> 		fd = -EINVAL;
> 		if (pid_has_task(p, PIDTYPE_TGID)) {
> 			fd = anon_inode_getfd("[pidfd]", &pidfd_fops, pid,
> 						flags | O_RDWR | O_CLOEXEC);
> 		}
> 		if (fd < 0)
> 			put_pid(p);
> 		return fd;
> 	}

Sure, I'd totally take a patch like that!

> 
> but this is cosmetic and off-topic too.

No, much appreciated. Good-looking code is important. :)

Christian