ipc: add sysctl to specify desired next object id

Add 3 new variables and sysctls to tune them (by one "next_id" variable
for messages, semaphores and shared memory respectively).  This variable
can be used to set desired id for next allocated IPC object.  By default
it's equal to -1 and old behaviour is preserved.  If this variable is
non-negative, then desired idr will be extracted from it and used as a
start value to search for free IDR slot.

Notes:

1) this patch doesn't guarantee that the new object will have desired
   id.  So it's up to user space how to handle new object with wrong id.

2) After a sucessful id allocation attempt, "next_id" will be set back
   to -1 (if it was non-negative).

[akpm@linux-foundation.org: checkpatch fixes]
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/ipc/util.c b/ipc/util.c
index 72fd0785ac94..74e1d9c7a98a 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -122,6 +122,7 @@ void ipc_init_ids(struct ipc_ids *ids)
 
         ids->in_use = 0;
         ids->seq = 0;
+        ids->next_id = -1;
         {
                 int seq_limit = INT_MAX/SEQ_MULTIPLIER;
                 if (seq_limit > USHRT_MAX)
@@ -252,6 +253,7 @@ int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
         kuid_t euid;
         kgid_t egid;
         int id, err;
+        int next_id = ids->next_id;
 
         if (size > IPCMNI)
                 size = IPCMNI;
@@ -264,7 +266,8 @@ 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,
+                                (next_id < 0) ? 0 : ipcid_to_idx(next_id), &id);
         if (err) {
                 spin_unlock(&new->lock);
                 rcu_read_unlock();
@@ -277,9 +280,14 @@ int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
         new->cuid = new->uid = euid;
         new->gid = new->cgid = egid;
 
-        new->seq = ids->seq++;
-        if(ids->seq > ids->seq_max)
-                ids->seq = 0;
+        if (next_id < 0) {
+                new->seq = ids->seq++;
+                if (ids->seq > ids->seq_max)
+                        ids->seq = 0;
+        } else {
+                new->seq = ipcid_to_seqx(next_id);
+                ids->next_id = -1;
+        }
 
         new->id = ipc_buildid(id, new->seq);
         return id;