Re: [RFC, PATCH] ipc/util.c: use idr_alloc_cyclic() for ipc allocations

[Waiman Long]

On 10/02/2018 12:19 PM, Manfred Spraul wrote:
> A bit related to the patch series that increases IPC_MNI:
>
> (User space) id reuse create the risk of data corruption:
>
> Process A: calls ipc function
> Process A: sleeps just at the beginning of the syscall
> Process B: Frees the ipc object (i.e.: calls ...ctl(IPC_RMID)
> Process B: Creates a new ipc object (i.e.: calls ...get())
> 	<If new object and old object have the same id>
> Process A: is woken up, and accesses the new object
>
> To reduce the probability that the new and the old object
> have the same id, the current implementation adds a
> sequence number to the index of the object in the idr tree.
>
> To further reduce the probability for a reuse, switch from
> idr_alloc to idr_alloc_cyclic.
>
> The patch cycles over at least RADIX_TREE_MAP_SIZE, i.e.
> if there is only a small number of objects, the accesses
> continue to be direct.
>
> As an option, this could be made dependent on the extended
> mode: In extended mode, cycle over e.g. at least 16k ids.
>
> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> ---
>
> Open questions:
> - Is there a significant performance advantage, especially
>   there are many ipc ids?
> - Over how many ids should the code cycle always?
> - Further review remarks?
>
>  ipc/util.c | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/ipc/util.c b/ipc/util.c
> index 0af05752969f..6f83841f6761 100644
> --- a/ipc/util.c
> +++ b/ipc/util.c
> @@ -216,10 +216,30 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
>  	 */
>  
>  	if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
> +		int idr_max;
> +
>  		new->seq = ids->seq++;
>  		if (ids->seq > IPCID_SEQ_MAX)
>  			ids->seq = 0;
> -		idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT);
> +
> +		/*
> +		 * If a user space visible id is reused, then this creates a
> +		 * risk for data corruption. To reduce the probability that
> +		 * a number is reduced, two approaches are used:
  reduced -> reused?

> +		 * 1) the idr index is allocated cyclically.
> +		 * 2) the use space id is build by concatenating the
> +		 *    internal idr index with a sequence number
> +		 * To avoid that both numbers have the same cycle time, try
> +		 * to set the size for the cyclic alloc to an odd number.
> +		 */
> +		idr_max = ids->in_use*2+1;
> +		if (idr_max < RADIX_TREE_MAP_SIZE-1)
> +			idr_max = RADIX_TREE_MAP_SIZE-1;
> +		if (idr_max > IPCMNI)
> +			idr_max = IPCMNI;
> +
> +		idx = idr_alloc_cyclic(&ids->ipcs_idr, new, 0, idr_max,
> +					GFP_NOWAIT);
>  	} else {
>  		new->seq = ipcid_to_seqx(next_id);
>  		idx = idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id),


Each of IPC components have their own sysctl parameters limiting the max
number of objects that can be allocated. With cyclic allocation, you
will have to make sure that idr_max is not larger than the corresponding
IPC sysctl parameters. That may require moving the limits to the
corresponding ipc_ids structure so that it can be used in ipc_idr_alloc().

What is the point of comparing idr_max against RADIX_TREE_MAP_SIZE-1? Is
it for performance reason.

Cheers,
Longman