linux-kernel - Re: [PATCH 1/2] CLONE_PIDFD: do not use the value pointed by parent

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Date:   Mon, 24 Jun 2019 13:59:43 +0200
From:   Christian Brauner <christian@...uner.io>
To:     "Dmitry V. Levin" <ldv@...linux.org>
Cc:     Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] CLONE_PIDFD: do not use the value pointed by
 parent_tidptr

On Mon, Jun 24, 2019 at 11:49:40AM +0200, Christian Brauner wrote:
> On Sun, Jun 23, 2019 at 02:27:17PM +0300, Dmitry V. Levin wrote:
> > Userspace needs a cheap and reliable way to tell whether CLONE_PIDFD
> > is supported by the kernel or not.
> > 
> > While older kernels without CLONE_PIDFD support just leave unchanged
> > the value pointed by parent_tidptr, current implementation fails with
> > EINVAL if that value is non-zero.
> > 
> > If CLONE_PIDFD is supported and fd 0 is closed, then mandatory pidfd == 0
> > pointed by parent_tidptr also remains unchanged, which effectively
> > means that userspace must either check CLONE_PIDFD support beforehand
> > or ensure that fd 0 is not closed when invoking CLONE_PIDFD.
> > 
> > The check for pidfd == 0 was introduced during v5.2 release cycle
> > by commit b3e583825266 ("clone: add CLONE_PIDFD") to ensure that
> > CLONE_PIDFD could be potentially extended by passing in flags through
> > the return argument.
> > 
> > However, that extension would look horrendous, and with introduction of
> > clone3 syscall in v5.3 there is no need to extend legacy clone syscall
> > this way.
> > 
> > So remove the pidfd == 0 check.  Userspace that needs to be portable
> > to kernels without CLONE_PIDFD support is advised to initialize pidfd
> > with -1 and check the pidfd value returned by CLONE_PIDFD.
> > 
> > Signed-off-by: Dmitry V. Levin <ldv@...linux.org>
> 
> Reviewed-by: Christian Brauner <christian@...uner.io>
> 
> Thank you Dmitry, queueing this up for rc7.

This is now sitting in

https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/commit/?h=fixes&id=43754d05f235dd1b6c7f8ab9f42007770d721f10

I reformulated the commit message a bit and gave it a Fixes tag. Dmitry,
if you want you can take a look and tell me if that's acceptable to you.

Thanks!
Christian