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Date: Tue, 12 Mar 2024 15:02:27 +0100
From: Florian Westphal <fw@strlen.de>
To: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Quan Tian <tianquan23@gmail.com>, netfilter-devel@vger.kernel.org,
	kadlec@netfilter.org, fw@strlen.de
Subject: Re: [PATCH v3 nf-next 2/2] netfilter: nf_tables: support updating
 userdata for nft_table
Message-ID: <20240312140227.GB1529@breakpoint.cc>
References: <20240311141454.31537-1-tianquan23@gmail.com>
 <20240311141454.31537-2-tianquan23@gmail.com>
 <ZfBdWIne-ujSEePS@calendula>
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User-Agent: Mutt/1.10.1 (2018-07-13)

Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> On Mon, Mar 11, 2024 at 10:14:54PM +0800, Quan Tian wrote:
> > The NFTA_TABLE_USERDATA attribute was ignored on updates. The patch adds
> > handling for it to support table comment updates.
> 
> dump path is lockless:
> 
>         if (table->udata) {
>                 if (nla_put(skb, NFTA_TABLE_USERDATA, table->udlen, table->udata))
>                         goto nla_put_failure;
>         }
> 
> there are two things to update at the same time here, table->udata and
> table->udlen.
> 
> This needs to be reworked fully if updates are required.

See first patch in the series, it makes this a single pointer,
but you are right...

> then, update struct nft_table to have:
> 
>         struct nft_userdata __rcu *user;

.. this needs an __rcu annotation.  I'll respond
to patch 1 too.

> BTW, does swap() ensure rcu semantics?

No, this needs to use rcu_replace_pointer() and manual update
of the old one stored in the transaction update.