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Date:   Wed, 18 May 2022 13:48:07 +0200
From:   Florian Westphal <fw@strlen.de>
To:     Phil Sutter <phil@nwl.cc>, Pablo Neira Ayuso <pablo@netfilter.org>,
        netfilter-devel@vger.kernel.org, fw@strlen.de
Subject: Re: [PATCH] netfilter: nf_tables: restrict expression reduction to
 first expression
Message-ID: <20220518114807.GE4316@breakpoint.cc>
References: <20220518100842.1950-1-pablo@netfilter.org>
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Phil Sutter <phil@nwl.cc> wrote:
> > > | reduce = reduce && expr->ops->type->reduce;
> > 
> > Could you elaborate?
> 
> Well, an expression which may set verdict register to NFT_BREAK should
> prevent reduction of later expressions in same rule as it may stop rule
> evaluation at run-time. This is obvious for nft_cmp, but nft_meta is
> also a candidate: NFT_META_IFTYPE causes NFT_BREAK if pkt->skb->dev is
> NULL. The optimizer must not assume later expressions are evaluated.

This all seems fragile to me, with huge potential to add subtle bugs
that will be hard to track down.