Re: [RFC, PATCH 1/3] introduce SYS_CLONE_MASK

[Oleg Nesterov <oleg@tv-sign.ru>]

On 04/08, Eric W. Biederman wrote:
> Oleg Nesterov <oleg@tv-sign.ru> writes:
> 
> > For review only.
> >
> > To implement for-in-kerenl-use-only CLONE_ flags, we need to filter out them
> > in sys_clone().
> 
> Nack
> 
> The current clone_flags field is for user space consumption and we
> have proposed users for all or almost all of the remaining bits.

OK.

> If we are going to have kernel only flags please use an additional
> argument to do_fork and copy_process.

Yes, we can do this. But we have a number of architectures which use
sys_clone() to implement kernel_thread(). It would be nice to have an
architecture neutral kernel_thread() implementation as you proposed.
We should change all of them if we want to add a new parameter to
do_fork().

Perhaps it is better to add reparent_kthread() (next patch) to kthread()
and forget about CLONE_KERNEL_THREAD.

Anyway, re-parenting to swapper breaks pstree, it doesn't show kernel
threads. And if ->parent == /sbin/init, we can't remove us from ->children
(unless we forbid sub-thread-of-init exec). So the only safe change is
set ->exit_state = -1.

> Your current scheme also has the bad side that if user space supplied
> a kernel flag it is hard to detect it and return -EINVAL.  Which
> limits future expansion.  Silently dropping clone flags is a real
> pain, if you are trying to detect if a new flag has been implemented.

Yes. But that is what we are doing now. copy_process() just ignores
unknown flags.

Oleg.