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

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
> 
> 
> Serge E. Hallyn wrote:
> > Quoting Oren Laadan (orenl at cs.columbia.edu):
> >> -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)

Hmm, yeah, I guess...  I misunderstood what seq was doing.  In
fact, now I just don't understand what it's doing at all...
Weird.

> >> +	} 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;
> >> -
> > 
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