netdev - [PATCH net-next] net: ipv6: respect route prfsrc and fill empty saddr before ECMP hash

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Message-Id: <20251005-ipv6-set-saddr-to-prefsrc-before-hash-to-stabilize-ecmp-v1-1-d43b6ef00035@proton.me>
Date: Sun, 05 Oct 2025 20:49:55 +0000
From: Dmitry Z via B4 Relay <devnull+demetriousz.proton.me@...nel.org>
To: "David S. Miller" <davem@...emloft.net>, 
 David Ahern <dsahern@...nel.org>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Dmitry Z <demetriousz@...ton.me>
Subject: [PATCH net-next] net: ipv6: respect route prfsrc and fill empty
 saddr before ECMP hash

From: Dmitry Z <demetriousz@...ton.me>

In an IPv6 ECMP scenario, if a multi-homed host initiates a connection,
`saddr` may remain empty during the initial call to `rt6_multipath_hash()`.
It gets filled later, once the outgoing interface (OIF) is determined and
`ipv6_dev_get_saddr()` (RFC 6724) selects the proper source address.

In some cases, this can cause the flow to switch paths: the first packets
go via one link, while the rest of the flow is routed over another.

A practical example is a Git-over-SSH session. When running `git fetch`,
the initial control traffic uses TOS 0x48, but data transfer switches to
TOS 0x20. This triggers a new hash computation, and at that time `saddr`
is already populated. As a result, packets with TOS 0x20 may be sent via
a different OIF, because `rt6_multipath_hash()` now produces a different
result.

This issue can happen even if the matched IPv6 route specifies a `src`
(preferred source) address. The actual impact depends on the network
topology. In my setup, the flow was redirected to a different switch and
reached another host, leading to TCP RSTs from the host where the session
was never established.

Possible workarounds:
1. Use netfilter to normalize the DSCP field before route lookup.
   (breaks DSCP/TOS assignment set by the socket)
2. Exclude the source address from the ECMP hash via sysctl knobs.
   (excludes an important part from hash computation)

This patch uses the `fib6_prefsrc.addr` value from the selected route to
populate `saddr` before ECMP hash computation, ensuring consistent path
selection across the flow.

Signed-off-by: Dmitry Z <demetriousz@...ton.me>
---
 net/ipv6/route.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 3299cfa12e21c96ecb5c4dea5f305d5f7ce16084..d2ecf16417a6f0fc6956f0ebff3d8dea593da059 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2270,6 +2270,11 @@ struct rt6_info *ip6_pol_route(struct net *net, struct fib6_table *table,
 	if (res.f6i == net->ipv6.fib6_null_entry)
 		goto out;

+	if (ipv6_addr_any(&fl6->saddr) &&
+	    !ipv6_addr_any(&res.f6i->fib6_prefsrc.addr)) {
+		fl6->saddr = res.f6i->fib6_prefsrc.addr;
+	}
+
 	fib6_select_path(net, &res, fl6, oif, false, skb, strict);

 	/*Search through exception table */

---
base-commit: e5f0a698b34ed76002dc5cff3804a61c80233a7a
change-id: 20251005-ipv6-set-saddr-to-prefsrc-before-hash-to-stabilize-ecmp-6d646ec96ac4

Best regards,
-- 
Dmitry Z <demetriousz@...ton.me>