linux-kernel - Re: [PATCH v1 3/4] signal: support CLONE_PIDFD with pidfd_send

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Date:   Wed, 17 Apr 2019 17:20:55 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
        jannh@...gle.com, dhowells@...hat.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, serge@...lyn.com, luto@...nel.org,
        arnd@...db.de, ebiederm@...ssion.com, keescook@...omium.org,
        tglx@...utronix.de, mtk.manpages@...il.com,
        akpm@...ux-foundation.org, cyphar@...har.com,
        joel@...lfernandes.org, dancol@...gle.com
Subject: Re: [PATCH v1 3/4] signal: support CLONE_PIDFD with pidfd_send_signal

On 04/17, Christian Brauner wrote:
>
> On Wed, Apr 17, 2019 at 04:01:06PM +0200, Oleg Nesterov wrote:
> > On 04/16, Christian Brauner wrote:
> > >
> > > @@ -3581,12 +3588,12 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> > >  	if (flags)
> > >  		return -EINVAL;
> > >
> > > -	f = fdget_raw(pidfd);
> > > +	f = fdget(pidfd);
> >
> > could you explain this change?
> >
> > I am just curious, I don't understand why should we disallow O_PATH and how
> > this connects to this patch.
>
> Sending a signal through a pidfd is considered to be on a par with a
> "write" to that pidfd.

OK, but how this connects to "support pidfds" ?

> Additionally, we use the fops associated with the fd to detect whether
> it is actually a pidfd or not. This is not possible with O_PATH since
> f_ops will be set to dummy fops.

indeed... I didn't know this, thanks!

But this means that pidfd_send_signal() will return -EBADF with or without
this change; pidfd_to_pid() will return -EBADF even if fdget_raw() suceeds,
right?

To clarify, I am not arguing. I am trying to understand why exactly do we
need this s/fdget_raw/fdget/ change and, why it doesn't come as a separate
patch. Can you add a note into the changelog?

Oleg.