Re: Netfilter: suspicious RCU usage in __nft_rule_lookup

[Florian Westphal <fw@strlen.de>]

Matthieu Baerts <matttbe@kernel.org> wrote:
> Hello,
> 
> First, thank you for all the work you did and are still doing around
> Netfilter!
> 
> I'm writing you this email, because when I run the MPTCP test suite with
> a VM running a kernel built with a debug config including
> CONFIG_PROVE_RCU_LIST=y (and CONFIG_RCU_EXPERT=y), I get the following
> warning:
> 
> 
> > 6.12.0-rc3+ #7 Not tainted
> > -----------------------------
> > net/netfilter/nf_tables_api.c:3420 RCU-list traversed in non-reader section!!
> > 
> > other info that might help us debug this:
> > 
> > rcu_scheduler_active = 2, debug_locks = 1
> > 1 lock held by iptables/134:
> >   #0: ffff888008c4fcc8 (&nft_net->commit_mutex){+.+.}-{3:3}, at: nf_tables_valid_genid (include/linux/jiffies.h:101) nf_tables

List is protected by transaction mutex, but we can't switch to plain
for_each_entry as this is also called from rcu-only context.

We either need two functions or pass nft_net + lockdep_is_held() check
as extra arg.