[PATCH v6 02/10] ipc: "use key as id" functionality for resource get system call introduced

[Stanislav Kinsbursky <skinsbursky@parallels.com>]

This patch introduces new IPC resource get request flag IPC_PRESET, which
should be interpreted as a request to try to allocate IPC slot with number,
starting from value resented by key. IOW, kernel will try
allocate new segment in specified slot.

Note: if desired slot is not emply, then next free slot will be used.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
---
 include/uapi/linux/ipc.h |    1 +
 ipc/msg.c                |    4 +++-
 ipc/sem.c                |    4 +++-
 ipc/shm.c                |    4 +++-
 ipc/util.c               |   18 +++++++++++++++---
 ipc/util.h               |    3 ++-
 6 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/include/uapi/linux/ipc.h b/include/uapi/linux/ipc.h
index de08dd4..f5f52b6 100644
--- a/include/uapi/linux/ipc.h
+++ b/include/uapi/linux/ipc.h
@@ -24,6 +24,7 @@ struct ipc_perm
 #define IPC_CREAT  00001000   /* create if key is nonexistent */
 #define IPC_EXCL   00002000   /* fail if key exists */
 #define IPC_NOWAIT 00004000   /* return error on wait */
+#define IPC_PRESET 00040000   /* use key as id */
 
 /* these fields are used by the DIPC package so the kernel as standard
    should avoid using them if possible */
diff --git a/ipc/msg.c b/ipc/msg.c
index 2f272fa..2f44946 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -190,6 +190,7 @@ static int newque(struct ipc_namespace *ns, struct ipc_params *params)
 
 	msq->q_perm.mode = msgflg & S_IRWXUGO;
 	msq->q_perm.key = key;
+	msq->q_perm.id = (msgflg & IPC_PRESET) ? key : 0;
 
 	msq->q_perm.security = NULL;
 	retval = security_msg_queue_alloc(msq);
@@ -201,7 +202,8 @@ static int newque(struct ipc_namespace *ns, struct ipc_params *params)
 	/*
 	 * ipc_addid() locks msq
 	 */
-	id = ipc_addid(&msg_ids(ns), &msq->q_perm, ns->msg_ctlmni);
+	id = ipc_addid(&msg_ids(ns), &msq->q_perm, ns->msg_ctlmni,
+		       msgflg & IPC_PRESET);
 	if (id < 0) {
 		security_msg_queue_free(msq);
 		ipc_rcu_putref(msq);
diff --git a/ipc/sem.c b/ipc/sem.c
index 58d31f1..10e9085 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -306,6 +306,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 
 	sma->sem_perm.mode = (semflg & S_IRWXUGO);
 	sma->sem_perm.key = key;
+	sma->sem_perm.id = (semflg & IPC_PRESET) ? key : 0;
 
 	sma->sem_perm.security = NULL;
 	retval = security_sem_alloc(sma);
@@ -314,7 +315,8 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 		return retval;
 	}
 
-	id = ipc_addid(&sem_ids(ns), &sma->sem_perm, ns->sc_semmni);
+	id = ipc_addid(&sem_ids(ns), &sma->sem_perm, ns->sc_semmni,
+		       semflg & IPC_PRESET);
 	if (id < 0) {
 		security_sem_free(sma);
 		ipc_rcu_putref(sma);
diff --git a/ipc/shm.c b/ipc/shm.c
index dff40c9..80b0046 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -480,6 +480,7 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 
 	shp->shm_perm.key = key;
 	shp->shm_perm.mode = (shmflg & S_IRWXUGO);
+	shp->shm_perm.id = (shmflg & IPC_PRESET) ? key : 0;
 	shp->mlock_user = NULL;
 
 	shp->shm_perm.security = NULL;
@@ -510,7 +511,8 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 	if (IS_ERR(file))
 		goto no_file;
 
-	id = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni);
+	id = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni,
+		       shmflg & IPC_PRESET);
 	if (id < 0) {
 		error = id;
 		goto no_id;
diff --git a/ipc/util.c b/ipc/util.c
index 72fd078..503946e 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -238,16 +238,22 @@ int ipc_get_maxid(struct ipc_ids *ids)
  *	@ids: IPC identifier set
  *	@new: new IPC permission set
  *	@size: limit for the number of used ids
+ *	@preset: use passed new->id value as desired id
  *
  *	Add an entry 'new' to the IPC ids idr. The permissions object is
  *	initialised and the first free entry is set up and the id assigned
  *	is returned. The 'new' entry is returned in a locked state on success.
  *	On failure the entry is not locked and a negative err-code is returned.
  *
+ *	If 'preset' is set, then passed new->id is desired to be set for new
+ *	segment. And allocated id is equal to passed value, then ipc ids will
+ *	left unchanged and new->seq will be updated to correspond specified id value.
+ *
  *	Called with ipc_ids.rw_mutex held as a writer.
  */
  
-int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
+int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size,
+	      int preset)
 {
 	kuid_t euid;
 	kgid_t egid;
@@ -264,7 +270,8 @@ int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
 	rcu_read_lock();
 	spin_lock(&new->lock);
 
-	err = idr_get_new(&ids->ipcs_idr, new, &id);
+	err = idr_get_new_above(&ids->ipcs_idr, new,
+				ipcid_to_idx(new->id), &id);
 	if (err) {
 		spin_unlock(&new->lock);
 		rcu_read_unlock();
@@ -277,6 +284,11 @@ int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
 	new->cuid = new->uid = euid;
 	new->gid = new->cgid = egid;
 
+	if (preset && ipcid_to_idx(new->id) == id) {
+		new->seq = ipcid_to_seq(new->id);
+		return id;
+	}
+
 	new->seq = ids->seq++;
 	if(ids->seq > ids->seq_max)
 		ids->seq = 0;
@@ -736,7 +748,7 @@ struct kern_ipc_perm *ipc_lock_check(struct ipc_ids *ids, int id)
 int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
 			struct ipc_ops *ops, struct ipc_params *params)
 {
-	if (params->key == IPC_PRIVATE)
+	if (params->key == IPC_PRIVATE && ((params->flg & IPC_PRESET) == 0))
 		return ipcget_new(ns, ids, ops, params);
 	else
 		return ipcget_public(ns, ids, ops, params);
diff --git a/ipc/util.h b/ipc/util.h
index c8fe2f7..3a9e558 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -92,9 +92,10 @@ void __init ipc_init_proc_interface(const char *path, const char *header,
 #define IPC_SHM_IDS	2
 
 #define ipcid_to_idx(id) ((id) % SEQ_MULTIPLIER)
+#define ipcid_to_seq(id) ((id) / SEQ_MULTIPLIER)
 
 /* must be called with ids->rw_mutex acquired for writing */
-int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
+int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int, int);
 
 /* must be called with ids->rw_mutex acquired for reading */
 int ipc_get_maxid(struct ipc_ids *);