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

[Christian Brauner]

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.

> ---
>  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