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

[Manfred Spraul <manfred@colorfullife.com>]

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@colorfullife.com>
---

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:
+		 * 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),
-- 
2.17.1