[5.10,098/717] net: evaluate net.ipvX.conf.all.ignore_routes_with_linkdown

From: Vincent Bernat <vincent@bernat.ch>

[ Upstream commit c0c5a60f0f1311bcf08bbe735122096d6326fb5b ]

Introduced in 0eeb075fad73, the "ignore_routes_with_linkdown" sysctl
ignores a route whose interface is down. It is provided as a
per-interface sysctl. However, while a "all" variant is exposed, it
was a noop since it was never evaluated. We use the usual "or" logic
for this kind of sysctls.

Tested with:

    ip link add type veth # veth0 + veth1
    ip link add type veth # veth1 + veth2
    ip link set up dev veth0
    ip link set up dev veth1 # link-status paired with veth0
    ip link set up dev veth2
    ip link set up dev veth3 # link-status paired with veth2

    # First available path
    ip -4 addr add 203.0.113.${uts#H}/24 dev veth0
    ip -6 addr add 2001:db8:1::${uts#H}/64 dev veth0

    # Second available path
    ip -4 addr add 192.0.2.${uts#H}/24 dev veth2
    ip -6 addr add 2001:db8:2::${uts#H}/64 dev veth2

    # More specific route through first path
    ip -4 route add 198.51.100.0/25 via 203.0.113.254 # via veth0
    ip -6 route add 2001:db8:3::/56 via 2001:db8:1::ff # via veth0

    # Less specific route through second path
    ip -4 route add 198.51.100.0/24 via 192.0.2.254 # via veth2
    ip -6 route add 2001:db8:3::/48 via 2001:db8:2::ff # via veth2

    # H1: enable on "all"
    # H2: enable on "veth0"
    for v in ipv4 ipv6; do
      case $uts in
        H1)
          sysctl -qw net.${v}.conf.all.ignore_routes_with_linkdown=1
          ;;
        H2)
          sysctl -qw net.${v}.conf.veth0.ignore_routes_with_linkdown=1
          ;;
      esac
    done

    set -xe
    # When veth0 is up, best route is through veth0
    ip -o route get 198.51.100.1 | grep -Fw veth0
    ip -o route get 2001:db8:3::1 | grep -Fw veth0

    # When veth0 is down, best route should be through veth2 on H1/H2,
    # but on veth0 on H2
    ip link set down dev veth1 # down veth0
    ip route show
    [ $uts != H3 ] || ip -o route get 198.51.100.1 | grep -Fw veth0
    [ $uts != H3 ] || ip -o route get 2001:db8:3::1 | grep -Fw veth0
    [ $uts = H3 ] || ip -o route get 198.51.100.1 | grep -Fw veth2
    [ $uts = H3 ] || ip -o route get 2001:db8:3::1 | grep -Fw veth2

Without this patch, the two last lines would fail on H1 (the one using
the "all" sysctl). With the patch, everything succeeds as expected.

Also document the sysctl in `ip-sysctl.rst`.

Fixes: 0eeb075fad73 ("net: ipv4 sysctl option to ignore routes when nexthop link is down")
Signed-off-by: Vincent Bernat <vincent@bernat.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 Documentation/networking/ip-sysctl.rst | 3 +++
 include/linux/inetdevice.h             | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)