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Date:   Thu, 21 Apr 2022 01:21:24 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, dccp@...r.kernel.org
Subject: [PATCH net-next 1/3] ipv4: Don't reset ->flowi4_scope in
 ip_rt_fix_tos().

All callers already initialise ->flowi4_scope with RT_SCOPE_UNIVERSE,
either by manual field assignment, memset(0) of the whole structure or
implicit structure initialisation of on-stack variables
(RT_SCOPE_UNIVERSE actually equals 0).

Therefore, we don't need to always initialise ->flowi4_scope in
ip_rt_fix_tos(). We only need to reduce the scope to RT_SCOPE_LINK when
the special RTO_ONLINK flag is present in the tos.

This will allow some code simplification, like removing
ip_rt_fix_tos(). Also, the long term idea is to remove RTO_ONLINK
entirely by properly initialising ->flowi4_scope, instead of
overloading ->flowi4_tos with a special flag. Eventually, this will
allow to convert ->flowi4_tos to dscp_t.

Signed-off-by: Guillaume Nault <gnault@...hat.com>
---
It's important for the correctness of this patch that all callers
initialise ->flowi4_scope to 0 (in one way or another). Auditing all of
them is long, although each case is pretty trivial.

If it helps, I can send a patch series that converts implicit
initialisation of ->flowi4_scope with an explicit assignment to
RT_SCOPE_UNIVERSE. This would also have the advantage of making it
clear to future readers that ->flowi4_scope _has_ to be initialised. I
haven't sent such patch series to not overwhelm reviewers with trivial
and not technically-required changes (there are 40+ places to modify,
scattered over 30+ different files). But if anyone prefers explicit
initialisation everywhere, then just let me know and I'll send such
patches.
---
 net/ipv4/route.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index e839d424b861..d8f82c0ac132 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -503,8 +503,8 @@ static void ip_rt_fix_tos(struct flowi4 *fl4)
 	__u8 tos = RT_FL_TOS(fl4);
 
 	fl4->flowi4_tos = tos & IPTOS_RT_MASK;
-	fl4->flowi4_scope = tos & RTO_ONLINK ?
-			    RT_SCOPE_LINK : RT_SCOPE_UNIVERSE;
+	if (tos & RTO_ONLINK)
+		fl4->flowi4_scope = RT_SCOPE_LINK;
 }
 
 static void __build_flow_key(const struct net *net, struct flowi4 *fl4,
-- 
2.21.3

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