[IPSEC]: Document that policy direction is derived from the index.

Here is a patch that adds a helper called xfrm_policy_id2dir to
document the fact that the policy direction can be and is derived
from the index.

This is based on a patch by YOSHIFUJI Hideaki and 210313105@suda.edu.cn.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/xfrm.h

@@ -931,4 +931,9 @@ static inline int xfrm_addr_cmp(xfrm_address_t *a, xfrm_address_t *b,
 	}
 }
 
+static inline int xfrm_policy_id2dir(u32 index)
+{
+	return index & 7;
+}
+
 #endif	/* _NET_XFRM_H */

net/key/af_key.c

@@ -2154,6 +2154,7 @@ static int key_pol_get_resp(struct sock *sk, struct xfrm_policy *xp, struct sadb
 
 static int pfkey_spdget(struct sock *sk, struct sk_buff *skb, struct sadb_msg *hdr, void **ext_hdrs)
 {
+	unsigned int dir;
 	int err;
 	struct sadb_x_policy *pol;
 	struct xfrm_policy *xp;
@@ -2162,7 +2163,11 @@ static int pfkey_spdget(struct sock *sk, struct sk_buff *skb, struct sadb_msg *h
 	if ((pol = ext_hdrs[SADB_X_EXT_POLICY-1]) == NULL)
 		return -EINVAL;
 
-	xp = xfrm_policy_byid(0, pol->sadb_x_policy_id,
+	dir = xfrm_policy_id2dir(pol->sadb_x_policy_id);
+	if (dir >= XFRM_POLICY_MAX)
+		return -EINVAL;
+
+	xp = xfrm_policy_byid(dir, pol->sadb_x_policy_id,
 			      hdr->sadb_msg_type == SADB_X_SPDDELETE2);
 	if (xp == NULL)
 		return -ENOENT;
@@ -2174,9 +2179,9 @@ static int pfkey_spdget(struct sock *sk, struct sk_buff *skb, struct sadb_msg *h
 	if (hdr->sadb_msg_type == SADB_X_SPDDELETE2) {
 		c.data.byid = 1;
 		c.event = XFRM_MSG_DELPOLICY;
-		km_policy_notify(xp, pol->sadb_x_policy_dir-1, &c);
+		km_policy_notify(xp, dir, &c);
 	} else {
-		err = key_pol_get_resp(sk, xp, hdr, pol->sadb_x_policy_dir-1);
+		err = key_pol_get_resp(sk, xp, hdr, dir);
 	}
 
 	xfrm_pol_put(xp);

net/xfrm/xfrm_policy.c

@@ -163,7 +163,7 @@ static void xfrm_policy_timer(unsigned long data)
 	if (xp->dead)
 		goto out;
 
-	dir = xp->index & 7;
+	dir = xfrm_policy_id2dir(xp->index);
 
 	if (xp->lft.hard_add_expires_seconds) {
 		long tmo = xp->lft.hard_add_expires_seconds +
@@ -417,7 +417,7 @@ struct xfrm_policy *xfrm_policy_byid(int dir, u32 id, int delete)
 	struct xfrm_policy *pol, **p;
 
 	write_lock_bh(&xfrm_policy_lock);
-	for (p = &xfrm_policy_list[id & 7]; (pol=*p)!=NULL; p = &pol->next) {
+	for (p = &xfrm_policy_list[dir]; (pol=*p)!=NULL; p = &pol->next) {
 		if (pol->index == id) {
 			xfrm_pol_hold(pol);
 			if (delete)