cifs: Invoke id mapping functions (try #17 repost)

rb tree search and insertion routines.

A SID which needs to be mapped, is looked up in one of the rb trees
depending on whether SID is either owner or group SID.
If found in the tree, a (mapped) id from that node is assigned to
uid or gid as appropriate.  If unmapped, an upcall is attempted to
map the SID to an id.  If upcall is successful, node is marked as
mapped.  If upcall fails, node stays marked as unmapped and a mapping
is attempted again only after an arbitrary time period has passed.

To map a SID, which can be either a Owner SID or a Group SID, key
description starts with the string "os" or "gs" followed by SID converted
to a string. Without "os" or "gs", cifs.upcall does not know whether
SID needs to be mapped to either an uid or a gid.

Nodes in rb tree have fields to prevent multiple upcalls for
a SID.  Searching, adding, and removing nodes is done within global locks.
Whenever a node is either found or inserted in a tree, a reference
is taken on that node.
Shrinker routine prunes a node if it has expired but does not prune
an expired node if its refcount is not zero (i.e. sid/id of that node
is_being/will_be accessed).
Thus a node, if its SID needs to be mapped by making an upcall,
can safely stay and its fields accessed without shrinker pruning it.
A reference (refcount) is put on the node without holding the spinlock
but a reference is get on the node by holding the spinlock.

Every time an existing mapped node is accessed or mapping is attempted,
its timestamp is updated to prevent it from getting erased or a
to prevent multiple unnecessary repeat mapping retries respectively.

For now, cifs.upcall is only used to map a SID to an id (uid or gid) but
it would be used to obtain an SID for an id.
Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsacl.h

@@ -39,6 +39,15 @@
 #define ACCESS_ALLOWED	0
 #define ACCESS_DENIED	1
 
+#define SIDOWNER 1
+#define SIDGROUP 2
+#define SIDLEN 150 /* S- 1 revision- 6 authorities- max 5 sub authorities */
+
+#define SID_ID_MAPPED 0
+#define SID_ID_PENDING 1
+#define SID_MAP_EXPIRE (3600 * HZ) /* map entry expires after one hour */
+#define SID_MAP_RETRY (300 * HZ)   /* wait 5 minutes for next attempt to map */
+
 struct cifs_ntsd {
 	__le16 revision; /* revision level */
 	__le16 type;
@@ -74,6 +83,21 @@ struct cifs_wksid {
 	char sidname[SIDNAMELENGTH];
 } __attribute__((packed));
 
+struct cifs_sid_id {
+	unsigned int refcount; /* increment with spinlock, decrement without */
+	unsigned long id;
+	unsigned long time;
+	unsigned long state;
+	char *sidstr;
+	struct rb_node rbnode;
+	struct cifs_sid sid;
+};
+
+#ifdef __KERNEL__
+extern struct key_type cifs_idmap_key_type;
+extern const struct cred *root_cred;
+#endif /* KERNEL */
+
 extern int match_sid(struct cifs_sid *);
 extern int compare_sids(const struct cifs_sid *, const struct cifs_sid *);