[RFC][PATCH 06/16] Define is_global_init()

[sukadev at us.ibm.com]

Dave Hansen [hansendc at us.ibm.com] wrote:
| On Thu, 2007-05-24 at 13:24 +0400, Pavel Emelianov wrote:
| > > | > +int is_global_init(struct task_struct *tsk)
| > > | > +{
| > > | > + return (task_active_pid_ns(tsk) == &init_pid_ns && tsk->pid == 1);
| > > | 
| > > | This can OOPS if you pass arbitrary task to this call...
| > > | tsk->nsproxy can already be NULL.
| > > 
| > > Hmm. You are right. btw, this could be a bisect issue. Patch 9 of uses
| > > pid_ns from pid->upid_list and removes nsproxy->pid_ns.
| > 
| > Yes, but that patch is not good either.
| > task_pid(tsk) may become NULL as well and this will oops.
| 
| Have you reviewed the call paths to make sure this can actually happen
| in practice?

task_pid() can be NULL when we are tearing down the task structure in
release_task() and in the tiny window between detach_pid() and attach_pid()
in de_thread(). 

I think task_pid() is safe as long as it is called for 'current'. (we should
probably add some comments)

I will double check my code, but I think all my calls to task_pid() and hence,
to task_active_pid_ns() are safe, except for two cases:

        a) is_global_init(). There are a few calls to process other than
           current, but not sure if they are a problem.

           For instance in current code, unhandled_signal() checks
           tsk->pid == 1 and proceeds to derefernce tsk->sighand.

           If task_pid() is NULL because the task was in release_task(),
           then so is tsk->sighand.

        b) the temporary check I added in check_kill_permissions().
           (I need to address Serge's comment here anyway).

To make is_global_init() more efficient and independent of task_pid(),
can we steal a bit from task_struct->flags ? Like PF_KSWAPD, and there
are unused bits :-)

| 
| This just seems like another one of those racing-with-task-exit races.
| Shouldn't be too invasive to solve.

A little invasive approach for the release_task() case could be to remove
the 'struct pid' from the hash table, but leave it attached to the
'task_struct' till the 'task_struct' itself is freed.

Removing from hash table ensures no one finds this process anymore, but
keeping it attached allows those who have already found the 'task_struct'
to also use the 'struct pid' as long as they have the task_struct.

Of course, needs investigation and micro surgery.

| 
| -- Dave