Re: [PATCH iptables-nft] iptables-nft: must withdraw PAYLOAD flag after parsing

[Phil Sutter <phil@nwl.cc>]

On Mon, Sep 19, 2022 at 11:31:09PM +0200, Florian Westphal wrote:
> Florian Westphal <fw@strlen.de> wrote:
> > else, next payload is stacked via 'CTX_PREV_PAYLOAD'.
> > 
> > Example breakage:
> > 
> > ip saddr 1.2.3.4 meta l4proto tcp
> > ... is dumped as
> > -s 6.0.0.0 -p tcp
> > 
> > iptables-nft -s 1.2.3.4 -p tcp is dumped correctly, because
> > the expressions are ordered like:
> > meta l4proto tcp ip saddr 1.2.3.4
> > 
> > ... and 'meta l4proto' will clear the PAYLOAD flag.
> > 
> > Fixes: 250dce876d92 ("nft-shared: support native tcp port delinearize")
> > Signed-off-by: Florian Westphal <fw@strlen.de>
> > ---
> >  iptables/nft-shared.c                         |  2 ++
> >  .../ipt-restore/0018-multi-payload_0          | 27 +++++++++++++++++++
> >  2 files changed, 29 insertions(+)
> >  create mode 100755 iptables/tests/shell/testcases/ipt-restore/0018-multi-payload_0
> > 
> > diff --git a/iptables/nft-shared.c b/iptables/nft-shared.c
> > index 71e2f18dab92..66e09e8fd533 100644
> > --- a/iptables/nft-shared.c
> > +++ b/iptables/nft-shared.c
> > @@ -986,6 +986,8 @@ static void nft_parse_cmp(struct nft_xt_ctx *ctx, struct nftnl_expr *e)
> >  			nft_parse_transport(ctx, e, ctx->cs);
> >  			break;
> >  		}
> > +
> > +		ctx->flags &= ~NFT_XT_CTX_PAYLOAD;
> >  	}
> 
> This isn't ideal either since this breaks dissection of '1-42' ranges
> that use two compare operands, i.e.:
> 
> cmp reg1 gte 1
> cmp reg1 lte 42
> 
> ...as first cmp 'hides' reg1 again.
> 
> I'd propose to rework this context stuff:
> no more payload/meta/whatever flags, instead 'mirror' the raw data
> registers.
> 
> Other ideas/suggestions?

When do we use multiple flags? I see we need NFT_XT_CTX_BITWISE in
addition to NFT_XT_CTX_META. Do we need e.g. NFT_XT_CTX_META and
NFT_XT_CTX_PAYLOAD at the same time? My idea would be to use an enum for
the LHS expression which is overwritten by each consecutive LHS
expression and a bitfield for the "on top" stuff.

Cheers, Phil