ipc: allow boot time extension of IPCMNI from 32k to 16M

The maximum number of unique System V IPC identifiers was limited to
32k.  That limit should be big enough for most use cases.

However, there are some users out there requesting for more, especially
those that are migrating from Solaris which uses 24 bits for unique
identifiers.  To satisfy the need of those users, a new boot time kernel
option "ipcmni_extend" is added to extend the IPCMNI value to 16M.  This
is a 512X increase which should be big enough for users out there that
need a large number of unique IPC identifier.

The use of this new option will change the pattern of the IPC
identifiers returned by functions like shmget(2).  An application that
depends on such pattern may not work properly.  So it should only be
used if the users really need more than 32k of unique IPC numbers.

This new option does have the side effect of reducing the maximum number
of unique sequence numbers from 64k down to 128.  So it is a trade-off.

The computation of a new IPC id is not done in the performance critical
path.  So a little bit of additional overhead shouldn't have any real
performance impact.

Link: http://lkml.kernel.org/r/20190329204930.21620-1-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/ipc/util.c b/ipc/util.c
index 095274a871f8..cf5d1087409e 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -109,7 +109,7 @@ static const struct rhashtable_params ipc_kht_params = {
  * @ids: ipc identifier set
  *
  * Set up the sequence range to use for the ipc identifier range (limited
- * below IPCMNI) then initialise the keys hashtable and ids idr.
+ * below ipc_mni) then initialise the keys hashtable and ids idr.
  */
 void ipc_init_ids(struct ipc_ids *ids)
 {
@@ -225,7 +225,7 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 				0, GFP_NOWAIT);
 	}
 	if (idx >= 0)
-		new->id = SEQ_MULTIPLIER * new->seq + idx;
+		new->id = (new->seq << IPCMNI_SEQ_SHIFT) + idx;
 	return idx;
 }
 
@@ -253,8 +253,8 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	/* 1) Initialize the refcount so that ipc_rcu_putref works */
 	refcount_set(&new->refcount, 1);
 
-	if (limit > IPCMNI)
-		limit = IPCMNI;
+	if (limit > ipc_mni)
+		limit = ipc_mni;
 
 	if (ids->in_use >= limit)
 		return -ENOSPC;
@@ -737,7 +737,7 @@ static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
 	if (total >= ids->in_use)
 		return NULL;
 
-	for (; pos < IPCMNI; pos++) {
+	for (; pos < ipc_mni; pos++) {
 		ipc = idr_find(&ids->ipcs_idr, pos);
 		if (ipc != NULL) {
 			*new_pos = pos + 1;