[XFRM]: Optimize MTU calculation

Replace the probing based MTU estimation, which usually takes 2-3 iterations
to find a fitting value and may underestimate the MTU, by an exact calculation.

Also fix underestimation of the XFRM trailer_len, which causes unnecessary
reallocations.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 38 insertions, 50 deletions

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index bdc65d8af181..a315d5d22764 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -272,32 +272,34 @@ out:
         return -EINVAL;
 }
 
-static u32 esp4_get_max_size(struct xfrm_state *x, int mtu)
+static u32 esp4_get_mtu(struct xfrm_state *x, int mtu)
 {
         struct esp_data *esp = x->data;
         u32 blksize = ALIGN(crypto_blkcipher_blocksize(esp->conf.tfm), 4);
-        int enclen = 0;
+        u32 align = max_t(u32, blksize, esp->conf.padlen);
+        u32 rem;
+
+        mtu -= x->props.header_len + esp->auth.icv_trunc_len;
+        rem = mtu & (align - 1);
+        mtu &= ~(align - 1);
 
         switch (x->props.mode) {
         case XFRM_MODE_TUNNEL:
-                mtu = ALIGN(mtu +2, blksize);
                 break;
         default:
         case XFRM_MODE_TRANSPORT:
                 /* The worst case */
-                mtu = ALIGN(mtu + 2, 4) + blksize - 4;
+                mtu -= blksize - 4;
+                mtu += min_t(u32, blksize - 4, rem);
                 break;
         case XFRM_MODE_BEET:
                 /* The worst case. */
-                enclen = IPV4_BEET_PHMAXLEN;
-                mtu = ALIGN(mtu + enclen + 2, blksize);
+                mtu -= IPV4_BEET_PHMAXLEN;
+                mtu += min_t(u32, IPV4_BEET_PHMAXLEN, rem);
                 break;
         }
 
-        if (esp->conf.padlen)
-                mtu = ALIGN(mtu, esp->conf.padlen);
-
-        return mtu + x->props.header_len + esp->auth.icv_trunc_len - enclen;
+        return mtu - 2;
 }
 
 static void esp4_err(struct sk_buff *skb, u32 info)
@@ -340,6 +342,7 @@ static int esp_init_state(struct xfrm_state *x)
 {
         struct esp_data *esp = NULL;
         struct crypto_blkcipher *tfm;
+        u32 align;
 
         /* null auth and encryption can have zero length keys */
         if (x->aalg) {
@@ -421,7 +424,10 @@ static int esp_init_state(struct xfrm_state *x)
                 }
         }
         x->data = esp;
-        x->props.trailer_len = esp4_get_max_size(x, 0) - x->props.header_len;
+        align = ALIGN(crypto_blkcipher_blocksize(esp->conf.tfm), 4);
+        if (esp->conf.padlen)
+                align = max_t(u32, align, esp->conf.padlen);
+        x->props.trailer_len = align + 1 + esp->auth.icv_trunc_len;
         return 0;
 
 error:
@@ -438,7 +444,7 @@ static struct xfrm_type esp_type =
         .proto          = IPPROTO_ESP,
         .init_state     = esp_init_state,
         .destructor     = esp_destroy,
-        .get_max_size   = esp4_get_max_size,
+        .get_mtu        = esp4_get_mtu,
         .input          = esp_input,
         .output         = esp_output
 };
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index 6b76c4c31137..7107bb7e2e62 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -235,22 +235,24 @@ out:
         return ret;
 }
 
-static u32 esp6_get_max_size(struct xfrm_state *x, int mtu)
+static u32 esp6_get_mtu(struct xfrm_state *x, int mtu)
 {
         struct esp_data *esp = x->data;
         u32 blksize = ALIGN(crypto_blkcipher_blocksize(esp->conf.tfm), 4);
+        u32 align = max_t(u32, blksize, esp->conf.padlen);
+        u32 rem;
 
-        if (x->props.mode == XFRM_MODE_TUNNEL) {
-                mtu = ALIGN(mtu + 2, blksize);
-        } else {
-                /* The worst case. */
+        mtu -= x->props.header_len + esp->auth.icv_trunc_len;
+        rem = mtu & (align - 1);
+        mtu &= ~(align - 1);
+
+        if (x->props.mode != XFRM_MODE_TUNNEL) {
                 u32 padsize = ((blksize - 1) & 7) + 1;
-                mtu = ALIGN(mtu + 2, padsize) + blksize - padsize;
+                mtu -= blksize - padsize;
+                mtu += min_t(u32, blksize - padsize, rem);
         }
-        if (esp->conf.padlen)
-                mtu = ALIGN(mtu, esp->conf.padlen);
 
-        return mtu + x->props.header_len + esp->auth.icv_trunc_len;
+        return mtu - 2;
 }
 
 static void esp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
@@ -380,7 +382,7 @@ static struct xfrm_type esp6_type =
         .proto          = IPPROTO_ESP,
         .init_state     = esp6_init_state,
         .destructor     = esp6_destroy,
-        .get_max_size   = esp6_get_max_size,
+        .get_mtu        = esp6_get_mtu,
         .input          = esp6_input,
         .output         = esp6_output,
         .hdr_offset     = xfrm6_find_1stfragopt,
diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 63a20e818164..69a3600afd9d 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -1667,37 +1667,17 @@ void xfrm_state_delete_tunnel(struct xfrm_state *x)
 }
 EXPORT_SYMBOL(xfrm_state_delete_tunnel);
 
-/*
- * This function is NOT optimal.  For example, with ESP it will give an
- * MTU that's usually two bytes short of being optimal.  However, it will
- * usually give an answer that's a multiple of 4 provided the input is
- * also a multiple of 4.
- */
 int xfrm_state_mtu(struct xfrm_state *x, int mtu)
 {
-        int res = mtu;
-
-        res -= x->props.header_len;
-
-        for (;;) {
-                int m = res;
-
-                if (m < 68)
-                        return 68;
-
-                spin_lock_bh(&x->lock);
-                if (x->km.state == XFRM_STATE_VALID &&
-                    x->type && x->type->get_max_size)
-                        m = x->type->get_max_size(x, m);
-                else
-                        m += x->props.header_len;
-                spin_unlock_bh(&x->lock);
-
-                if (m <= mtu)
-                        break;
-                res -= (m - mtu);
-        }
+        int res;
 
+        spin_lock_bh(&x->lock);
+        if (x->km.state == XFRM_STATE_VALID &&
+            x->type && x->type->get_mtu)
+                res = x->type->get_mtu(x, mtu);
+        else
+                res = mtu;
+        spin_unlock_bh(&x->lock);
         return res;
 }