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SubjectRe: [PATCH 1/2] CLONE_PIDFD: do not use the value pointed by parent_tidptr
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@altlinux.org>

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

Thank you Dmitry, queueing this up for rc7.

> ---
> kernel/fork.c | 12 ------------
> 1 file changed, 12 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 75675b9bf6df..39a3adaa4ad1 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1822,8 +1822,6 @@ static __latent_entropy struct task_struct *copy_process(
> }
>
> if (clone_flags & CLONE_PIDFD) {
> - int reserved;
> -
> /*
> * - CLONE_PARENT_SETTID is useless for pidfds and also
> * parent_tidptr is used to return pidfds.
> @@ -1834,16 +1832,6 @@ static __latent_entropy struct task_struct *copy_process(
> if (clone_flags &
> (CLONE_DETACHED | CLONE_PARENT_SETTID | CLONE_THREAD))
> return ERR_PTR(-EINVAL);
> -
> - /*
> - * Verify that parent_tidptr is sane so we can potentially
> - * reuse it later.
> - */
> - if (get_user(reserved, parent_tidptr))
> - return ERR_PTR(-EFAULT);
> -
> - if (reserved != 0)
> - return ERR_PTR(-EINVAL);
> }
>
> /*
> --
> ldv

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