netdev - Re: [PATCH net v2] ip6mr: Fix lockdep and sparse RCU warnings

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Message-ID: <68ee33a7-c60e-48b9-b362-c991bdcf675f@nokia.com>
Date: Tue, 1 Oct 2024 16:36:28 +0200
From: Stefan Wiehler <stefan.wiehler@...ia.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, David Ahern
 <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Petr Malat <oss@...at.biz>
Subject: Re: [PATCH net v2] ip6mr: Fix lockdep and sparse RCU warnings

> This could be a lockdep annotation error then, at least for
> RT6_TABLE_DFLT, oh well.

As you have already explained, we can ignore the ip6mr_vif_seq_start() error
path, so the issue boils down to ip6mr_get_table() being called without
entering a RCU read-side critical section from these 4 functions:
ipmr_vif_seq_start(), ip6mr_ioctl(), ip6mr_compat_ioctl() and
ip6mr_get_route(). It is my understanding that in none of these four cases the
RTNL lock is held either; at least according to the RCU-lockdep splat we
clearly see that this is not the case in ip6mr_ioctl()  – but please correct me
if I'm wrong.

> Note that net/ipv4/ipmr.c would have a similar issue.

Yes, looks indeed like that :-/

> Please split your patch in small units, their Fixes: tags are likely
> different, and if some code breaks something,
> fixing the issue will be easier.
> 
> The changelog seemed to only address the first ip6mr_vif_seq_start() part.

If you prefer that I can split the change into 4 commits addressing each of the
4 functions mentioned before.

Kind regards,

Stefan