Re: [PATCH] clone: fix CLONE_PIDFD support

[Christian Brauner]

On Sun, Jul 14, 2019 at 03:02:06PM +0300, Dmitry V. Levin wrote:
> The introduction of clone3 syscall accidentally broke CLONE_PIDFD
> support in traditional clone syscall on compat x86 and those
> architectures that use do_fork to implement clone syscall.
> 
> This bug was found by strace test suite.
> 
> Link: https://strace.io/logs/strace/2019-07-12
> Fixes: 7f192e3cd316 ("fork: add clone3")
> Bisected-and-tested-by: Anatoly Pugachev <matorola@xxxxxxxxx>
> Signed-off-by: Dmitry V. Levin <ldv@xxxxxxxxxxxx>

Good catch! Thank you Dmitry.

One change request below.

> ---
>  arch/x86/ia32/sys_ia32.c | 1 +
>  kernel/fork.c            | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/arch/x86/ia32/sys_ia32.c b/arch/x86/ia32/sys_ia32.c
> index 64a6c952091e..98754baf411a 100644
> --- a/arch/x86/ia32/sys_ia32.c
> +++ b/arch/x86/ia32/sys_ia32.c
> @@ -239,6 +239,7 @@ COMPAT_SYSCALL_DEFINE5(x86_clone, unsigned long, clone_flags,
>  {
>  	struct kernel_clone_args args = {
>  		.flags		= (clone_flags & ~CSIGNAL),
> +		.pidfd		= parent_tidptr,
>  		.child_tid	= child_tidptr,
>  		.parent_tid	= parent_tidptr,
>  		.exit_signal	= (clone_flags & CSIGNAL),
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 8f3e2d97d771..2c3cbad807b6 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2417,6 +2417,7 @@ long do_fork(unsigned long clone_flags,
>  {
>  	struct kernel_clone_args args = {
>  		.flags		= (clone_flags & ~CSIGNAL),
> +		.pidfd		= parent_tidptr,
>  		.child_tid	= child_tidptr,
>  		.parent_tid	= parent_tidptr,
>  		.exit_signal	= (clone_flags & CSIGNAL),
> -- 

Both of these legacy clone helpers need to make CLONE_PIDFD and
CLONE_PARENT_SETTID incompatible, i.e. could you please add a helper to
kernel/fork.c:

bool legacy_clone_args_valid(const struct kernel_clone_args *kargs)
{
	/* clone(CLONE_PIDFD) uses parent_tidptr to return a pidfd */
	if ((kargs->flags & CLONE_PIDFD) && (kargs->flags & CLONE_PARENT_SETTID))
		return false;
}

and export it and use it in ia32 too?

Then resend and I'll put it into my tree that already has a few other
fixes about to be sent.

Christian