Re: [PATCH v2 2/2] pid: Remove pidhash

Gargi Sharma
2017-09-30T15:42:29Z

On Wed, Sep 27, 2017 at 9:58 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> If I was not clear...
>
> in short, after this patch the very first idr_alloc_cyclic() is already
> wrong. Because, once again, the new not-fully-initialized pid can be found
> by find_pid_ns().

If the PIDNS_ADDING check fails, I jump to the flag that performs
this
while (++i <= ns->level)
idr_remove(&ns->idr, (pid->numbers + i)->nr);
So when find_pid_ns() is called, it will not find this pid.

>
> perhaps you should chane the previous patch to do
> idr_alloc_cyclic(ptr = NULL) and use idr_replace() in this patch after
> the PIDNS_HASH_ADDING check.

I'm not sure if I understand this. Do we want to do this to make sure
the pid namespace is
initialized before the first process enters into
the namespace? If yes, how does idr_alloc_cyclic(ptr = NULL) help?

>
> And I just noticed you didn't cc Eric Biederman <ebiederm@xmission.com>,
> please do next time.

Sorry for missing out on this. Will do with the next version.

Thanks!
Gargi
>
>
> On 09/27, Oleg Nesterov wrote:
>>
>> On 09/27, Gargi Sharma wrote:
>> >
>> > -#define find_next_offset(map, off) \
>> > - find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
>> > -
>>
>> this should go into the previous patch, but this is minor...
>>
>> > @@ -208,12 +200,10 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>> >
>> > upid = pid->numbers + ns->level;
>> > spin_lock_irq(&pidmap_lock);
>> > - if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
>> > + if (!(ns->pid_allocated & PIDNS_ADDING))
>> > goto out_unlock;
>> > for ( ; upid >= pid->numbers; --upid) {
>> > - hlist_add_head_rcu(&upid->pid_chain,
>> > - &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
>> > - upid->ns->nr_hashed++;
>> > + upid->ns->pid_allocated++;
>>
>> No, this is wrong.
>>
>> It is too late to check PIDNS_HASH_ADDING/PIDNS_ADDING and increment pid_allocated,
>> once we call idr_alloc_cyclic() this pid is already "hashed" in that it can be found
>> by find_pid_ns() with this patch applied.
>>
>> And of course, it is too late to do atomic_set(&pid->count, 1) and initialize
>> pid->tasks[type] lists by the same reason.
>>
>> Oleg.
>