[RFC][PATCH 3/4] IPC: use the target ID specified in procfs

[Nadia.Derbey-6ktuUTfB/bM@public.gmane.org]

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[PATCH 03/04]

This patch makes use of the target id specified by a previous write into
/proc/self/next_id as the id to use to allocate the next IPC object.

Signed-off-by: Nadia Derbey <Nadia.Derbey-6ktuUTfB/bM@public.gmane.org>

---
 include/linux/sysids.h |    7 +++++++
 ipc/util.c             |   39 +++++++++++++++++++++++++++++++--------
 kernel/nextid.c        |    2 +-
 3 files changed, 39 insertions(+), 9 deletions(-)

Index: linux-2.6.25-rc3-mm1/include/linux/sysids.h
===================================================================
--- linux-2.6.25-rc3-mm1.orig/include/linux/sysids.h	2008-03-28 08:19:49.000000000 +0100
+++ linux-2.6.25-rc3-mm1/include/linux/sysids.h	2008-03-28 08:21:02.000000000 +0100
@@ -37,7 +37,14 @@ struct sys_id {
 	long *blocks[0];
 };
 
+#define next_ipcid(tsk) ((tsk)->next_id					\
+				? ((tsk)->next_id->nids			\
+					? ID_AT((tsk)->next_id, 0)	\
+					: -1)				\
+				: -1)
+
 extern ssize_t get_nextid(struct task_struct *, char *);
 extern int set_nextid(struct task_struct *, char *);
+extern void id_blocks_free(struct sys_id *);
 
 #endif /* _LINUX_SYSIDS_H */
Index: linux-2.6.25-rc3-mm1/kernel/nextid.c
===================================================================
--- linux-2.6.25-rc3-mm1.orig/kernel/nextid.c	2008-03-28 08:20:52.000000000 +0100
+++ linux-2.6.25-rc3-mm1/kernel/nextid.c	2008-03-28 08:21:02.000000000 +0100
@@ -49,7 +49,7 @@ out_undo_partial_alloc:
 	return NULL;
 }
 
-static void id_blocks_free(struct sys_id *ids)
+void id_blocks_free(struct sys_id *ids)
 {
 	if (ids == NULL)
 		return;
Index: linux-2.6.25-rc3-mm1/ipc/util.c
===================================================================
--- linux-2.6.25-rc3-mm1.orig/ipc/util.c	2008-03-28 08:19:49.000000000 +0100
+++ linux-2.6.25-rc3-mm1/ipc/util.c	2008-03-28 08:21:02.000000000 +0100
@@ -260,6 +260,7 @@ int ipc_get_maxid(struct ipc_ids *ids)
 int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
 {
 	int id, err;
+	int next_id;
 
 	if (size > IPCMNI)
 		size = IPCMNI;
@@ -267,20 +268,42 @@ int ipc_addid(struct ipc_ids* ids, struc
 	if (ids->in_use >= size)
 		return -ENOSPC;
 
-	err = idr_get_new(&ids->ipcs_idr, new, &id);
-	if (err)
-		return err;
+	next_id = next_ipcid(current);
+	if (next_id >= 0) {
+		/* There is a target id specified, try to use it */
+		int new_lid = next_id % SEQ_MULTIPLIER;
+
+		if (next_id !=
+		    (new_lid + (next_id / SEQ_MULTIPLIER) * SEQ_MULTIPLIER))
+			return -EINVAL;
+
+		err = idr_get_new_above(&ids->ipcs_idr, new, new_lid, &id);
+		if (err)
+			return err;
+		if (id != new_lid) {
+			idr_remove(&ids->ipcs_idr, id);
+			return -EBUSY;
+		}
+
+		new->id = next_id;
+		new->seq = next_id / SEQ_MULTIPLIER;
+		id_blocks_free(current->next_id);
+	} else {
+		err = idr_get_new(&ids->ipcs_idr, new, &id);
+		if (err)
+			return err;
+
+		new->seq = ids->seq++;
+		if (ids->seq > ids->seq_max)
+			ids->seq = 0;
+		new->id = ipc_buildid(id, new->seq);
+	}
 
 	ids->in_use++;
 
 	new->cuid = new->uid = current->euid;
 	new->gid = new->cgid = current->egid;
 
-	new->seq = ids->seq++;
-	if(ids->seq > ids->seq_max)
-		ids->seq = 0;
-
-	new->id = ipc_buildid(id, new->seq);
 	spin_lock_init(&new->lock);
 	new->deleted = 0;
 	rcu_read_lock();

--