Re: [PATCH] netlink: Don't try to get further datagrams in nl_route_dup() on NLMSG_DONE

[Paul Holzinger <pholzing@redhat.com>]

On 15/03/2024 16:17, Stefano Brivio wrote:
> On Fri, 15 Mar 2024 15:52:50 +0100
> Stefano Brivio <sbrivio@redhat.com> wrote:
>
>> On Fri, 15 Mar 2024 14:11:40 +0100
>> Paul Holzinger <pholzing@redhat.com> wrote:
>>
>>> On 15/03/2024 12:24, Stefano Brivio wrote:
>>>> Martin reports that, with Fedora Linux kernel version
>>>> kernel-core-6.9.0-0.rc0.20240313gitb0546776ad3f.4.fc41.x86_64,
>>>> including commit 87d381973e49 ("genetlink: fit NLMSG_DONE into same
>>>> read() as families"), pasta doesn't exit once the network namespace
>>>> is gone.
>>>>
>>>> Actually, pasta is completely non-functional, at least with default
>>>> options, because nl_route_dup(), which duplicates routes from the
>>>> parent namespace into the target namespace at start-up, is stuck on
>>>> a second receive operation for RTM_GETROUTE.
>>>>
>>>> However, with that commit, the kernel is now able to fit the whole
>>>> response, including the NLMSG_DONE message, into a single datagram,
>>>> so no further messages will be received.
>>>>
>>>> It turns out that commit 4d6e9d0816e2 ("netlink: Always process all
>>>> responses to a netlink request") accidentally relied on the fact that
>>>> we would always get at least two datagrams as a response to
>>>> RTM_GETROUTE.
>>>>
>>>> That is, the test to check if we expect another datagram, is based
>>>> on the 'status' variable, which is 0 if we just parsed NLMSG_DONE,
>>>> but we'll also expect another datagram if NLMSG_OK on the last
>>>> message is false. But NLMSG_OK with a zero length is always false.
>>>>
>>>> The problem is that we don't distinguish if status is zero because
>>>> we got a NLMSG_DONE message, or because we processed all the
>>>> available datagram bytes.
>>>>
>>>> Introduce an explicit check on NLMSG_DONE. We should probably
>>>> refactor this slightly, for example by introducing a special return
>>>> code from nl_status(), but this is probably the least invasive fix
>>>> for the issue at hand.
>>>>
>>>> Reported-by: Martin Pitt <mpitt@redhat.com>
>>>> Link: https://github.com/containers/podman/issues/22052
>>>> Fixes: 4d6e9d0816e2 ("netlink: Always process all responses to a netlink request")
>>>> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Tested-by: Paul Holzinger <pholzing@redhat.com>
>>>> ---
>>>>    netlink.c | 3 ++-
>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/netlink.c b/netlink.c
>>>> index 9e7cccb..20de9b3 100644
>>>> --- a/netlink.c
>>>> +++ b/netlink.c
>>>> @@ -525,7 +525,8 @@ int nl_route_dup(int s_src, unsigned int ifi_src,
>>>>    		}
>>>>    	}
>>>>    
>>>> -	if (!NLMSG_OK(nh, status) || status > 0) {
>>>> +	if (nh->nlmsg_type != NLMSG_DONE &&
>>>> +	    (!NLMSG_OK(nh, status) || status > 0)) {
>>>>    		/* Process any remaining datagrams in a different
>>>>    		 * buffer so we don't overwrite the first one.
>>>>    		 */
>>> I was about to add my tested-by when I noticed a weird thing, but that
>>> happens only on the new kernel as well:
>>>
>>> On the host $ ip route default via 192.168.122.1 dev enp1s0 proto dhcp
>>> src 192.168.122.92 metric 100 192.168.122.0/24 dev enp1s0 proto kernel
>>> scope link src 192.168.122.92 metric 100 ./pasta --config-net ip route
>>> default via 192.168.122.1 dev enp1s0 proto dhcp metric 100
>>> 192.168.122.0/24 dev enp1s0 proto kernel scope link src 192.168.122.92
>>> 192.168.122.0/24 dev enp1s0 proto kernel scope link metric 100 It seems
>>> we now have the same local route duplicated for some reason?
>> I can reproduce this, it looks rather weird at the moment. Let's say I
>> have two IPv4 routes on the host:
>>
>> $ ip ro sh
>> default via 88.198.0.161 dev eth0 proto dhcp src 88.198.0.164 metric 100
>> 88.198.0.160/27 dev eth0 proto kernel scope link src 88.198.0.164 metric 100
>>
>> if I stop pasta after it re-creates the default route (and not the
>> subnet one, yet), say, at this point (the message with padding confuses
>> strace so we don't get the full decoding, but it's valid):
>>
>> recvfrom(5, [[{nlmsg_len=68, nlmsg_type=RTM_NEWROUTE, nlmsg_flags=NLM_F_MULTI|NLM_F_DUMP_FILTERED, nlmsg_seq=13, nlmsg_pid=28341}, {rtm_family=AF_INET, rtm_dst_len=0, rtm_src_len=0, rtm_tos=0, rtm_table=RT_TABLE_MAIN, rtm_protocol=RTPROT_DHCP, rtm_scope=RT_SCOPE_UNIVERSE, rtm_type=RTN_UNICAST, rtm_flags=0}, [[{nla_len=8, nla_type=RTA_TABLE}, RT_TABLE_MAIN], [{nla_len=8, nla_type=RTA_PRIORITY}, 100], [{nla_len=8, nla_type=RTA_PREFSRC}, inet_addr("88.198.0.164")], [{nla_len=8, nla_type=RTA_GATEWAY}, inet_addr("88.198.0.161")], [{nla_len=8, nla_type=RTA_OIF}, if_nametoindex("eth0")]]], [{nlmsg_len=68, nlmsg_type=RTM_NEWROUTE, nlmsg_flags=NLM_F_MULTI|NLM_F_DUMP_FILTERED, nlmsg_seq=13, nlmsg_pid=28341}, {rtm_family=AF_INET, rtm_dst_len=27, rtm_src_len=0, rtm_tos=0, rtm_table=RT_TABLE_MAIN, rtm_protocol=RTPROT_KERNEL, rtm_scope=RT_SCOPE_LINK, rtm_type=RTN_UNICAST, rtm_flags=0}, [[{nla_len=8, nla_type=RTA_TABLE}, RT_TABLE_MAIN], [{nla_len=8, nla_type=RTA_DST}, inet_addr("88.198.0.160")],
>   [{nla_len=8, nla_type=RTA_PRIORITY}, 100], [{nla_len=8, nla_type=RTA_PREFSRC}, inet_addr("88.198.0.164")], [{nla_len=8, nla_type=RTA_OIF}, if_nametoindex("eth0")]]], [{nlmsg_len=20, nlmsg_type=NLMSG_DONE, nlmsg_flags=NLM_F_MULTI|NLM_F_DUMP_FILTERED, nlmsg_seq=13, nlmsg_pid=28341}, 0]], 65536, 0, NULL, NULL) = 156
>> sendto(7, [{nlmsg_len=68, nlmsg_type=0x18 /* NLMSG_??? */, nlmsg_flags=NLM_F_REQUEST|NLM_F_MULTI|NLM_F_ACK|0x400, nlmsg_seq=14, nlmsg_pid=0}, "\x02\x00\x00\x00\xfe\x10\x00\x01\x00\x00\x00\x00\x08\x00\x0f\x00\xfe\x00\x00\x00\x08\x00\x06\x00\x64\x00\x00\x00\x08\x00\x00\x00"...], 68, 0, NULL, 0) = 68
>>
>> I see that the kernel already created both in the target namespace:
>>
>> # ip ro sh
>> default via 88.198.0.161 dev eth0 proto dhcp metric 100
>> 88.198.0.160/27 dev eth0 proto kernel scope link src 88.198.0.164
> Ah, no, the issue is pre-existing, you can actually see the same on an
> older kernel, without this patch. The subnet route is added
> autonomously by the kernel after we add the address via RTM_NEWADDR
> with a ifa_prefixlen < 32 attribute.
>
> Then we add it "again" via RTM_NEWROUTE.
>
> We could probably avoid the creation of any route on RTM_NEWADDR giving
> 32 or 128 as prefix length, because anyway we'll explicitly copy the
> subnet route later, but that needs a bit more pondering -- I'd say it's
> beyond the scope of this change.
>
Yes, agreed. This fix looks correct and fixes the hang which is the most 
important thing.