Subject: Re: [PATCH net-next 1/3] ipv4: Don't reset ->flowi4_scope in ip_rt_fix_tos().

From: Guillaume Nault <[email-masked]>
Date: Fri, 22 Apr 2022 12:53:45 +0200

On Thu, Apr 21, 2022 at 08:30:52PM -0600, David Ahern wrote:
> On 4/20/22 5:21 PM, Guillaume Nault wrote:
> > 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 <[email-masked]>
> > ---
> > 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.
> 
> There are a handful of places that open code the initialization of the
> flow struct. I *think* I found all of them in 40867d74c374.

By open code, do you mean "doesn't use flowi4_init_output() nor
ip_tunnel_init_flow()"? If so, I think there are many more.

To be sure we're on the same page, here's a small part of the diff for
my "explicit flowi4_scope initialisation" patch series:

 drivers/infiniband/core/addr.c                          | 1 +
 drivers/infiniband/sw/rxe/rxe_net.c                     | 1 +
 drivers/net/amt.c                                       | 3 +++
 drivers/net/ethernet/broadcom/bnxt/bnxt_tc.c            | 1 +
 drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun.c     | 3 +++
 drivers/net/ethernet/netronome/nfp/flower/action.c      | 1 +
 drivers/net/ethernet/netronome/nfp/flower/tunnel_conf.c | 7 +++++--
 drivers/net/geneve.c                                    | 9 +++++++--
 drivers/net/gtp.c                                       | 1 +
 drivers/net/ipvlan/ipvlan_core.c                        | 1 +
 drivers/net/vrf.c                                       | 1 +
 drivers/net/vxlan/vxlan_core.c                          | 1 +
 drivers/net/wireguard/socket.c                          | 1 +
 include/net/ip_tunnels.h                                | 1 +
 include/net/route.h                                     | 2 ++
 net/core/filter.c                                       | 1 +
 net/core/lwt_bpf.c                                      | 1 +
 net/dccp/ipv4.c                                         | 1 +
 net/ipv4/icmp.c                                         | 3 +++
 net/ipv4/netfilter.c                                    | 1 +
 net/ipv4/netfilter/nf_reject_ipv4.c                     | 1 +
 net/ipv4/route.c                                        | 1 +
 net/ipv4/xfrm4_policy.c                                 | 1 +
 net/netfilter/ipvs/ip_vs_xmit.c                         | 1 +
 net/netfilter/nf_conntrack_h323_main.c                  | 3 +++
 net/netfilter/nf_conntrack_sip.c                        | 1 +
 net/netfilter/nft_flow_offload.c                        | 1 +
 net/netfilter/nft_rt.c                                  | 1 +
 net/netfilter/xt_TCPMSS.c                               | 2 ++
 net/sctp/protocol.c                                     | 1 +
 net/smc/smc_ib.c                                        | 1 +
 net/tipc/udp_media.c                                    | 1 +
 net/xfrm/xfrm_policy.c                                  | 1 +
 33 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/drivers/infiniband/core/addr.c b/drivers/infiniband/core/addr.c
index f253295795f0..5b6e0003eead 100644
--- a/drivers/infiniband/core/addr.c
+++ b/drivers/infiniband/core/addr.c
@@ -399,6 +399,7 @@ static int addr4_resolve(struct sockaddr *src_sock,
        memset(&fl4, 0, sizeof(fl4));
        fl4.daddr = dst_ip;
        fl4.saddr = src_ip;
+       fl4.flowi4_scope = RT_SCOPE_UNIVERSE;
        fl4.flowi4_oif = addr->bound_dev_if;
        rt = ip_route_output_key(addr->net, &fl4);
        ret = PTR_ERR_OR_ZERO(rt);
diff --git a/drivers/infiniband/sw/rxe/rxe_net.c b/drivers/infiniband/sw/rxe/rxe_net.c
index c53f4529f098..cf6dc89a8785 100644
--- a/drivers/infiniband/sw/rxe/rxe_net.c
+++ b/drivers/infiniband/sw/rxe/rxe_net.c
@@ -31,6 +31,7 @@ static struct dst_entry *rxe_find_route4(struct net_device *ndev,
        fl.flowi4_oif = ndev->ifindex;
        memcpy(&fl.saddr, saddr, sizeof(*saddr));
        memcpy(&fl.daddr, daddr, sizeof(*daddr));
+       fl.flowi4_scope = RT_SCOPE_UNIVERSE;
        fl.flowi4_proto = IPPROTO_UDP;
 
        rt = ip_route_output_key(&init_net, &fl);
diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index 10455c9b9da0..3e957ff64715 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c
@@ -990,6 +990,7 @@ static bool amt_send_membership_update(struct amt_dev *amt,
        fl4.daddr              = amt->remote_ip;
        fl4.saddr              = amt->local_ip;
        fl4.flowi4_tos         = AMT_TOS;
+       fl4.flowi4_scope       = RT_SCOPE_UNIVERSE;
        fl4.flowi4_proto       = IPPROTO_UDP;
        rt = ip_route_output_key(amt->net, &fl4);
        if (IS_ERR(rt)) {
@@ -1048,6 +1049,7 @@ static void amt_send_multicast_data(struct amt_dev *amt,
        fl4.flowi4_oif         = amt->stream_dev->ifindex;
        fl4.daddr              = tunnel->ip4;
        fl4.saddr              = amt->local_ip;
+       fl4.flowi4_scope       = RT_SCOPE_UNIVERSE;
        fl4.flowi4_proto       = IPPROTO_UDP;
        rt = ip_route_output_key(amt->net, &fl4);
        if (IS_ERR(rt)) {
@@ -1103,6 +1105,7 @@ static bool amt_send_membership_query(struct amt_dev *amt,
        fl4.daddr              = tunnel->ip4;
        fl4.saddr              = amt->local_ip;
        fl4.flowi4_tos         = AMT_TOS;
+       fl4.flowi4_scope       = RT_SCOPE_UNIVERSE;
        fl4.flowi4_proto       = IPPROTO_UDP;
        rt = ip_route_output_key(amt->net, &fl4);
        if (IS_ERR(rt)) {
...

> > ---
> >  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,
> 
> Reviewed-by: David Ahern <[email-masked]>
>