linux-kernel - Re: [PATCH v2 0/7] CLONE_FD: Task exit notification via file descriptor

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Date:	Wed, 8 Apr 2015 19:19:49 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Jonathan Corbet <corbet@....net>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Thiago Macieira <thiago.macieira@...el.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 0/7] CLONE_FD: Task exit notification via file
 descriptor

On Wed, Apr 01, 2015 at 09:24:20AM +0200, Jonathan Corbet wrote:
> On Tue, 31 Mar 2015 15:02:24 -0700
> josh@...htriplett.org wrote:
> 
> > > This would appear to assume that a clonefd_info structure is the only
> > > thing that will ever be read from this descriptor.  It seems to me that
> > > there is the potential for, someday, wanting to be able to read and write
> > > other things as well.  Should this structure be marked with type and
> > > length fields so that other structures could be added in the future?  
> > 
> > I don't think it makes sense for a caller to get an arbitrary structure
> > on read(), and have to figure out what they got and ignore something
> > they don't understand.  Instead, I think it makes more sense for the
> > caller to say "Hey, here's a flag saying I understand the new thing, go
> > ahead and give me the new thing".  So, for instance, if you want to
> > receive SIGSTOP/SIGCONT messages for child processes through this
> > descriptor, we could add a flag for that.
> 
> The flag is fine, but, once we have set that flag saying we want those
> messages, how do we know which type of structure we've gotten?  That's
> the piece of the puzzle I'm missing, sorry if I'm being overly slow.

If you pass a flag saying you can handle a new set of potential
structures, those structures can then include any necessary
disambiguating flags/IDs/etc.  No need for them to match the current
clonefd_info structure if userspace has opted into a new version.

- Josh Triplett
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