Re: [PATCH 1/2] CLONE_PIDFD: do not use the value pointed by parent_tidptr

[Christian Brauner]

On Mon, Jun 24, 2019 at 04:45:31PM +0300, Dmitry V. Levin wrote:
> On Mon, Jun 24, 2019 at 01:59:43PM +0200, Christian Brauner wrote:
> > 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@xxxxxxxxxxxx>
> > > 
> > > Reviewed-by: Christian Brauner <christian@xxxxxxxxxx>
> > > 
> > > 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.
> 
> s/Old kernel that only support/Old kernels that only support/

Fixed.

Thanks!
Christian