[Devel] Re: [RFC][PATCH] fork: Don't special case CLONE_NEWPID for process or sessions

[Pavel Emelyanov]

Eric W. Biederman wrote:

Sorry for the late answer, I have just noticed that I forgot to
answer on this patch.

> Given that the kernel supports sys_setsid we don't need a special case
> in fork if we want to set: session == pgrp == pid.
> 
> The historical (although not 2.6) linux behavior has been to start the
> init with session == pgrp == 0 which is effectively what removing this
> special case will do. 

Hm... I overlooked this fact. Looks like the namespace's init will
have them set to 1.

> Is there any reason why we want/need this special case in fork?  Or

Mainly to address the issue I describe below.

> can we remove it and save some code, make copy_process easier to read
> easier to maintain, and possibly a little faster?
> 
> I know it is a little weird belong to a process groups that isn't
> visible in your pid namespace, but it there are no good reasons
> why it shouldn't work.

This is not good to have such a situation as the init will have
the ability to kill the tasks from the namespace he can't see,
e.g. his parent and the processes in that group.

> I think making this change makes the interface more flexible,
> and general.
> 
> Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>
> ---
>  kernel/fork.c |   18 +++++-------------
>  1 files changed, 5 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ddafdfa..b0de799 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1292,20 +1292,12 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>  		if (thread_group_leader(p)) {
>  			if (clone_flags & CLONE_NEWPID) {
>  				p->nsproxy->pid_ns->child_reaper = p;
> -				p->signal->tty = NULL;
> -				set_task_pgrp(p, p->pid);
> -				set_task_session(p, p->pid);
> -				attach_pid(p, PIDTYPE_PGID, pid);
> -				attach_pid(p, PIDTYPE_SID, pid);
> -			} else {
> -				p->signal->tty = current->signal->tty;
> -				set_task_pgrp(p, task_pgrp_nr(current));
> -				set_task_session(p, task_session_nr(current));
> -				attach_pid(p, PIDTYPE_PGID,
> -						task_pgrp(current));
> -				attach_pid(p, PIDTYPE_SID,
> -						task_session(current));
>  			}
> +			p->signal->tty = current->signal->tty;
> +			set_task_pgrp(p, task_pgrp_nr(current));
> +			set_task_session(p, task_session_nr(current));
> +			attach_pid(p, PIDTYPE_PGID, task_pgrp(current));
> +			attach_pid(p, PIDTYPE_SID, task_session(current));
>  
>  			list_add_tail_rcu(&p->tasks, &init_task.tasks);
>  			__get_cpu_var(process_counts)++;

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