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Date: Fri, 11 Apr 2025 11:48:20 +0200
From: Florian Westphal <fw@strlen.de>
To: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Florian Westphal <fw@strlen.de>, netfilter-devel@vger.kernel.org
Subject: Re: [PATCH nft 2/2] evaluate: restrict allowed subtypes of
 concatenations
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Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> I am in the need for such a context for payload/meta statements.
> 
>         meta mark set ip dscp map ...
>                       ^^^^^^^
> 
> in this case, ip dscp needs to be evaluated as a key for lookups,
> shift can probably be removed for implicit maps.
> 
> While in this case:
> 
>         meta mark set ip dscp
>                       ^^^^^^^
> 
> in this case, ip dscp needs the shift.
> 
> Then, there is:
> 
>         ip dscp set meta mark
>         ^^^^^^^
> 
> (note: this is not yet supported)
> 
> where ip dscp needs to expand to 16-bit because of the kernel
> checksum routine requirements.
> 
> They are all payload expressions, but evaluation needs to be slightly
> different depending on how the expression is used.
> 
> This context should help disentangle evaluation, evaluation is making
> assumption based on subtle hints, I think there is a need for more
> explicit hints.

Agreed.

> We can revisit in a few weeks, otherwise take this.

OK, lets keep this back for now; technically I don't need
to know the recursion depth, I need to know the placement
(lhs / lookup key resp. rhs / element key) to figure out what
restrictions apply.