linux-kernel - Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

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Date: Fri, 26 Jan 2024 15:33:50 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Tycho Andersen <tycho@...ho.pizza>, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Tycho Andersen <tandersen@...flix.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group
 leaders

On 01/26, Christian Brauner wrote:
>
> > --- a/include/uapi/linux/pidfd.h
> > +++ b/include/uapi/linux/pidfd.h
> > @@ -7,6 +7,7 @@
> >  #include <linux/fcntl.h>
> >
> >  /* Flags for pidfd_open().  */
> > -#define PIDFD_NONBLOCK O_NONBLOCK
> > +#define PIDFD_NONBLOCK	O_NONBLOCK
> > +#define PIDFD_THREAD	O_EXCL	// or anything else not used by anon_inode's
>
> I like it!
>
> The only request I would have is to not alias O_EXCL and PIDFD_THREAD.
> Because it doesn't map as clearly as NONBLOCK did.

But it would be nice to have PIDFD_THREAD in file->f_flags. Where else
can we keep it?

I chose O_EXCL because it can only be used at open time, it can never
be used or changed after anon_inode_getfile(), so we can safely do

	pidfd_file->f_flags |= PIDFD_THREAD;

in __pidfd_prepare() and then check in pidfd_poll/pidfd_send_signal.

What do you suggest instead?

Oleg.