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Date: Fri, 26 Jan 2024 15:33:50 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Christian Brauner <brauner@kernel.org>
Cc: Tycho Andersen <tycho@tycho.pizza>, linux-kernel@vger.kernel.org,
	linux-api@vger.kernel.org, Tycho Andersen <tandersen@netflix.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group
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Message-ID: <20240126143349.GD7386@redhat.com>
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User-Agent: Mutt/1.5.24 (2015-08-30)

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.