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ipv4: Create and use fib_compute_spec_dst() helper.

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The specific destination is the host we direct unicast replies to.
Usually this is the original packet source address, but if we are
responding to a multicast or broadcast packet we have to use something
different.

Specifically we must use the source address we would use if we were to
send a packet to the unicast source of the original packet.

The routing cache precomputes this value, but we want to remove that
precomputation because it creates a hard dependency on the expensive
rpfilter source address validation which we'd like to make cheaper.

There are only three places where this matters:

1) ICMP replies.

2) pktinfo CMSG

3) IP options

Now there will be no real users of rt->rt_spec_dst and we can simply
remove it altogether.

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller
2012-06-28 03:59:11 -07:00
parent 70e7341673
commit 35ebf65e85
5 changed files with 49 additions and 16 deletions
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7
net/ipv4/ip_sockglue.c
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@@ -40,6 +40,7 @@
#if IS_ENABLED(CONFIG_IPV6)
#include <net/transp_v6.h>
#endif
#include <net/ip_fib.h>
#include <linux/errqueue.h>
#include <asm/uaccess.h>
@@ -1019,8 +1020,8 @@ e_inval:
* @sk: socket
* @skb: buffer
*
* To support IP_CMSG_PKTINFO option, we store rt_iif and rt_spec_dst
* in skb->cb[] before dst drop.
* To support IP_CMSG_PKTINFO option, we store rt_iif and specific
* destination in skb->cb[] before dst drop.
* This way, receiver doesnt make cache line misses to read rtable.
*/
void ipv4_pktinfo_prepare(struct sk_buff *skb)
@@ -1030,7 +1031,7 @@ void ipv4_pktinfo_prepare(struct sk_buff *skb)
if (rt) {
pktinfo->ipi_ifindex = rt->rt_iif;
pktinfo->ipi_spec_dst.s_addr = rt->rt_spec_dst;
pktinfo->ipi_spec_dst.s_addr = fib_compute_spec_dst(skb);
} else {
pktinfo->ipi_ifindex = 0;
pktinfo->ipi_spec_dst.s_addr = 0;