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[152.2.130.143]) by smtp.gmail.com with ESMTPSA id af79cd13be357-93cca2d361bsm115242285a.45.2026.10.01.20.21.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 01 Oct 2026 20:21:08 -0700 (PDT) From: Hengyu Liang To: idosch@nvidia.com Cc: netdev@vger.kernel.org, davem@davemloft.net, edumazet@kernel.org, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, dsahern@kernel.org, jiayuan.chen@shopee.com, jiayuan.chen@linux.dev, shuah@kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH net 1/2] ipv6: route: do not validate nexthop of routes promoted to reject routes Date: Thu, 1 Oct 2026 23:21:07 -0400 Message-ID: <20261002032107.3289370-1-hengyul@cs.unc.edu> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261001154629.GA1611116@shredder> References: <20261001154629.GA1611116@shredder> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Thu, Oct 01, 2026 at 06:46:29PM +0300, Ido Schimmel wrote: > On Wed, Sep 30, 2026 at 11:22:28AM -0400, hengyul@cs.unc.edu wrote: [...] > > # ip link set dev lo down > > # ip -6 route add 2001:db8::/32 dev lo > > Error: Nexthop device is not up. > > Consistent with IPv4 and expected: [...] > I don't understand what motivated this series. Thanks for the review, and sorry that the motivation was not clear. I found this while comparing the behavior of different kernel versions. It does not come from a user report and I am not aware of anything that depends on the old behavior. These commands succeeded before 21ec92774d15, and its changelog describes the "ip -6 route add 2001:db8::/32 dev lo" case as unchanged apart from the unused nhc_pcpu_rth_output allocation. As the commit also went to the stable trees, I took the new errors for an unintended side effect and tried to restore the old results. I agree that the current behavior is consistent with IPv4. The syzbot ci report on this series also shows that the old behavior was wrong for multipath routes. With patch 1 applied, as before 21ec92774d15, # ip -6 route add 2001:db8:100::/64 \ nexthop via fe80::1 dev eth1 nexthop via fe80::1 dev lo is accepted and installs two separate routes: the loopback nexthop becomes a reject route that does not qualify for ECMP, while ip6_route_multipath_add() still notifies the first route with nhn - 1 siblings. With a netdevsim device, I can reproduce the WARN_ON_ONCE() in nsim_fib6_event_init() with this command on the patched kernel, and it is followed by a NULL pointer dereference in nsim_fib_event_work(). The current code rejects the loopback nexthop instead. So please drop this series. Sorry for the noise.