[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>

1 files changed, 12 insertions, 10 deletions

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,