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

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:

quoted

On Sun, Jun 23, 2019 at 02:27:17PM +0300, Dmitry V. Levin wrote:

quoted

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 <redacted>

Reviewed-by: Christian Brauner <christian@brauner.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.

s/Old kernel that only support/Old kernels that only support/

Besides that, fine with me.  Thanks.


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ldv