Re: [RFC v14-rc2][PATCH 1/7] ipc: allow allocation of an ipc object with desired identifier

[Oren Laadan <orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org>]

Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org):
>> -int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
>> +int
>> +ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size, int req_id)
>>  {
>>  	uid_t euid;
>>  	gid_t egid;
>> +	int lid = 0;
>>  	int id, err;
>>
>>  	if (size > IPCMNI)
>> @@ -268,28 +270,41 @@ int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
>>  	if (ids->in_use >= size)
>>  		return -ENOSPC;
>>
>> +	if (req_id >= 0)
>> +		lid = ipcid_to_idx(req_id);
>> +
>>  	spin_lock_init(&new->lock);
>>  	new->deleted = 0;
>>  	rcu_read_lock();
>>  	spin_lock(&new->lock);
>>
>> -	err = idr_get_new(&ids->ipcs_idr, new, &id);
>> +	err = idr_get_new_above(&ids->ipcs_idr, new, lid, &id);
>>  	if (err) {
>>  		spin_unlock(&new->lock);
>>  		rcu_read_unlock();
>>  		return err;
>>  	}
>>
>> +	if (req_id >= 0) {
>> +		if (id != lid) {
>> +			idr_remove(&ids->ipcs_idr, id);
>> +			spin_unlock(&new->lock);
>> +			rcu_read_unlock();
>> +			return -EBUSY;
>> +		}
>> +		new->seq = req_id / SEQ_MULTIPLIER;
> 
> Should this be
> 	
> 	new->seq = req_id % ids->seq_max;
> 
> ?

This is how the user-visible IPC id is constructed:

	static inline int ipc_buildid(int id, int seq)
	{
	        return SEQ_MULTIPLIER * seq + id;
	}

and I want to get the original 'seq' ....  (the 'id' in this function is
an in-kernel identifier)

> 
>> +	} else {
>> +		new->seq = ids->seq++;
>> +		if (ids->seq > ids->seq_max)
>> +			ids->seq = 0;
>> +	}
>> +
>>  	ids->in_use++;
>>
>>  	current_euid_egid(&euid, &egid);
>>  	new->cuid = new->uid = euid;
>>  	new->gid = new->cgid = egid;
>>
>> -	new->seq = ids->seq++;
>> -	if(ids->seq > ids->seq_max)
>> -		ids->seq = 0;
>> -
>