linux-kernel - Re: [PATCH v3 1/1] waitid: Add support for waiting for the current process group

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Date:   Wed, 14 Aug 2019 19:02:01 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Rich Felker <dalias@...c.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        libc-alpha@...rceware.org, alistair23@...il.com,
        ebiederm@...ssion.com, arnd@...db.de,
        torvalds@...ux-foundation.org, adhemerval.zanella@...aro.org,
        fweimer@...hat.com, palmer@...ive.com, macro@....com,
        zongbox@...il.com, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, hpa@...or.com
Subject: Re: [PATCH v3 1/1] waitid: Add support for waiting for the current
 process group

On Wed, Aug 14, 2019 at 12:55:01PM -0400, Rich Felker wrote:
> On Wed, Aug 14, 2019 at 06:34:44PM +0200, Christian Brauner wrote:
> > On Wed, Aug 14, 2019 at 06:15:17PM +0200, Christian Brauner wrote:
> > > On Wed, Aug 14, 2019 at 06:09:17PM +0200, Oleg Nesterov wrote:
> > > > On 08/14, Christian Brauner wrote:
> > > > >
> > > > > and a signal could come in between the system call that
> > > > > retrieved the process gorup and the call to waitid that changes the
> > > >                         ^^^^^
> > > > > current process group.
> > > > 
> > > > I noticed this typo only because I spent 2 minutes or more trying to
> > > > understand this sentence ;) But yes, a signal handler or another thread
> > > 
> > > I'll try to rewrite it. :)
> > 
> > Ok, here's what I changed it to:
> > 
> > It was recently discovered that the linux version of waitid is not a
> > superset of the other wait functions because it does not include
> > support for waiting for the current process group. This has two
> > downsides:
> > 1. An extra system call is needed to get the current process group.
> > 2. After the current process group is received and before it is passed
> >    to waitid a signal could arrive causing the current process group to change.
> 
> I don't think "downsides" sufficiently conveys that this is hard
> breakage of a requirement for waitpid. How about something like the
> following?
> 
> "It was recently discovered that the linux version of waitid is not a
> superset of the other wait functions because it does not include
> support for waiting for the current process group. Userspace cannot
> simply emulate this functionality with an additional getpgid syscall
> due to inherent race conditions that violate the async-signal safety
> requirements for waitpid."

I like the rather specific example in there. How about we add that after
this section like so:

It was recently discovered that the linux version of waitid is not a
superset of the other wait functions because it does not include
support for waiting for the current process group. This has two
downsides:
1. An extra system call is needed to get the current process group.
2. After the current process group is received and before it is passed
   to waitid a signal could arrive causing the current process group to change.

Such inherent race-conditions as mentioned in 2. make it impossible for
userspace to emulate this functionaly and thus violate the async-signal
safety requirements for waitpid.

Christian