[PATCH v11 2/3] ipc: Conserve sequence numbers in ipcmni_extend mode

[Waiman Long]

The mixing in of a sequence number into the IPC IDs is probably to
avoid ID reuse in userspace as much as possible. With ipcmni_extend
mode, the number of usable sequence numbers is greatly reduced leading
to higher chance of ID reuse.

To address this issue, we need to conserve the sequence number space
as much as possible. Right now, the sequence number is incremented
for every new ID created. In reality, we only need to increment the
sequence number when one or more IDs have been removed previously to
make sure that those IDs will not be reused when a new one is built.
This is being done irrespective of the ipcmni mode.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 include/linux/ipc_namespace.h |  1 +
 ipc/util.c                    | 16 +++++++++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 6ab8c1b..7d5f553 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -16,6 +16,7 @@
 struct ipc_ids {
 	int in_use;
 	unsigned short seq;
+	unsigned short deleted;
 	struct rw_semaphore rwsem;
 	struct idr ipcs_idr;
 	int max_idx;
diff --git a/ipc/util.c b/ipc/util.c
index 07ae117..00000a1 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -115,6 +115,7 @@ static int __init ipc_init(void)
 void ipc_init_ids(struct ipc_ids *ids)
 {
 	ids->in_use = 0;
+	ids->deleted = false;
 	ids->seq = 0;
 	init_rwsem(&ids->rwsem);
 	rhashtable_init(&ids->key_ht, &ipc_kht_params);
@@ -193,6 +194,10 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
  *
  * The caller must own kern_ipc_perm.lock.of the new object.
  * On error, the function returns a (negative) error code.
+ *
+ * To conserve sequence number space, especially with extended ipc_mni,
+ * the sequence number is incremented only when one or more IDs have been
+ * removed previously.
  */
 static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 {
@@ -216,9 +221,13 @@ 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 */
-		new->seq = ids->seq++;
-		if (ids->seq > IPCID_SEQ_MAX)
-			ids->seq = 0;
+		if (ids->deleted) {
+			ids->seq++;
+			if (ids->seq > IPCID_SEQ_MAX)
+				ids->seq = 0;
+			ids->deleted = false;
+		}
+		new->seq = ids->seq;
 		idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT);
 	} else {
 		new->seq = ipcid_to_seqx(next_id);
@@ -436,6 +445,7 @@ void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
 	idr_remove(&ids->ipcs_idr, idx);
 	ipc_kht_remove(ids, ipcp);
 	ids->in_use--;
+	ids->deleted = true;
 	ipcp->deleted = true;
 
 	if (unlikely(idx == ids->max_idx)) {
-- 
1.8.3.1