netdev - Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG

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Message-ID: <02b50aae-f0e9-47a4-8365-a977a85975d3@ovn.org>
Date: Thu, 4 Apr 2024 00:52:15 +0200
From: Ilya Maximets <i.maximets@....org>
To: Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>
Cc: Stefano Brivio <sbrivio@...hat.com>, davem@...emloft.net,
 netdev@...r.kernel.org, pabeni@...hat.com, jiri@...nulli.us,
 idosch@...sch.org, johannes@...solutions.net, fw@...len.de,
 pablo@...filter.org, Martin Pitt <mpitt@...hat.com>,
 Paul Holzinger <pholzing@...hat.com>,
 David Gibson <david@...son.dropbear.id.au>, i.maximets@....org
Subject: Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG_DONE into same
 read() as families

On 3/19/24 18:40, Jakub Kicinski wrote:
> On Tue, 19 Mar 2024 18:17:47 +0100 Eric Dumazet wrote:
>>> Hi Stefano! I was worried this may happen :( I think we should revert
>>> offending commits, but I'd like to take it on case by case basis.
>>> I'd imagine majority of netlink is only exercised by iproute2 and
>>> libmnl-based tools. Does passt hang specifically on genetlink family
>>> dump? Your commit also mentions RTM_GETROUTE. This is not the only
>>> commit which removed DONE:
>>>
>>> $ git log --since='1 month ago' --grep=NLMSG_DONE --no-merges  --oneline
>>>
>>> 9cc4cc329d30 ipv6: use xa_array iterator to implement inet6_dump_addr()
>>> 87d381973e49 genetlink: fit NLMSG_DONE into same read() as families
>>> 4ce5dc9316de inet: switch inet_dump_fib() to RCU protection
>>> 6647b338fc5c netlink: fix netlink_diag_dump() return value  
>>
>> Lets not bring back more RTNL locking please for the handlers that
>> still require it.
> 
> Definitely. My git log copy/paste is pretty inaccurate, these two are
> better examples:
> 
> 5d9b7cb383bb nexthop: Simplify dump error handling
> 02e24903e5a4 netlink: let core handle error cases in dump operations
> 
> I was trying to point out that we merged a handful of DONE "coalescing"
> patches, and if we need to revert - let's only do that for the exact
> commands needed. The comment was raised on my genetlink patch while
> the discussion in the link points to RTM_GETROUTE.
> 
>> The core can generate an NLMSG_DONE by itself, if we decide this needs
>> to be done.
> 
> Exactly.

FWIW, it seems that Libreswan is suffering from the same issue on
RTM_GETROUTE dump.

On 6.9.0-rc1 I see:

/usr/sbin/ipsec auto --config ipsec.conf --ctlsocket pluto.ctl \
                     --start --asynchronous tun-in-1

recvfrom(7, 
[
  [{nlmsg_len=52, nlmsg_type=RTM_NEWROUTE, nlmsg_flags=NLM_F_MULTI, ...],
  ...
  [{nlmsg_len=52, nlmsg_type=RTM_NEWROUTE, nlmsg_flags=NLM_F_MULTI, ...],
  [{nlmsg_len=20, nlmsg_type=NLMSG_DONE, nlmsg_flags=NLM_F_MULTI, ...]
], 40960, 0, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, [12])

recvfrom(7, <-- Stuck here forever


On 6.8.0 the output is following:

recvfrom(7, 
[
  [{nlmsg_len=52, nlmsg_type=RTM_NEWROUTE, nlmsg_flags=NLM_F_MULTI, ...],
  ...
  [{nlmsg_len=52, nlmsg_type=RTM_NEWROUTE, nlmsg_flags=NLM_F_MULTI, ...]
], 40960, 0, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, [12])

recvfrom(7,
  [{nlmsg_len=20, nlmsg_type=NLMSG_DONE, nlmsg_flags=NLM_F_MULTI,}, 0],
  40728, 0, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, [12])
close(7)

So, it seems like it explicitly waits for NLMSG_DONE to be in a separate
message.

I reported the issue to Libreswan:
  https://github.com/libreswan/libreswan/issues/1675
but just wanted to let you know as well.

Found this since it breaks IPsec system tests in Open vSwitch.

Best regards, Ilya Maximets.