[RFC] [PATCH] ipc/util.c: Use binary search for max_idx

[Manfred Spraul]

If semctl(), msgctl() and shmctl() are called with IPC_INFO, SEM_INFO,
MSG_INFO or SHM_INFO, then the return value is the index of the highest
used entry in the kernel's internal array recording information about
all SysV objects of the requested type for the current namespace.
(This information can be used with repeated ..._STAT or ..._STAT_ANY
operations to obtain information about all SysV objects on the system.)

If the current highest used entry is destroyed, then the new highest
used entry is determined by looping over all possible values.
With the introduction of IPCMNI_EXTEND_SHIFT, this could be a
loop over 16 million entries.

As there is no get_last() function for idr structures:
Implement a "get_last()" using a binary search.

As far as I see, ipc is the only user that needs get_last(), thus
implement it in ipc/util.c and not in a central location.

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
 ipc/util.c | 44 +++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 5 deletions(-)

diff --git a/ipc/util.c b/ipc/util.c
index cfa0045e748d..0121bf6b2617 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -64,6 +64,7 @@
 #include <linux/memory.h>
 #include <linux/ipc_namespace.h>
 #include <linux/rhashtable.h>
+#include <linux/log2.h>
 
 #include <asm/unistd.h>
 
@@ -450,6 +451,40 @@ static void ipc_kht_remove(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
 				       ipc_kht_params);
 }
 
+/**
+ * ipc_get_maxusedidx - get highest in-use index
+ * @ids: ipc identifier set
+ * @limit: highest possible index.
+ *
+ * The function determines the highest in use index value.
+ * ipc_ids.rwsem needs to be owned by the caller.
+ * If no ipc object is allocated, then -1 is returned.
+ */
+static int ipc_get_maxusedidx(struct ipc_ids *ids, int limit)
+{
+	void *val;
+	int tmpidx;
+	int i;
+	int retval;
+
+	i = ilog2(limit+1);
+
+	retval = 0;
+	for (; i >= 0; i--) {
+		tmpidx = retval | (1<<i);
+		/*
+		 * "0" is a possible index value, thus search using
+		 * e.g. 15,7,3,1,0 instead of 16,8,4,2,1.
+		 */
+		tmpidx = tmpidx-1;
+		val = idr_get_next(&ids->ipcs_idr, &tmpidx);
+		if (val)
+			retval |= (1<<i);
+	}
+	retval--;
+	return retval;
+}
+
 /**
  * ipc_rmid - remove an ipc identifier
  * @ids: ipc identifier set
@@ -468,11 +503,10 @@ void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
 	ipcp->deleted = true;
 
 	if (unlikely(idx == ids->max_idx)) {
-		do {
-			idx--;
-			if (idx == -1)
-				break;
-		} while (!idr_find(&ids->ipcs_idr, idx));
+
+		idx = ids->max_idx-1;
+		if (idx >= 0)
+			idx = ipc_get_maxusedidx(ids, idx);
 		ids->max_idx = idx;
 	}
 }
-- 
2.29.2