Re: nftables; key update with symbolic values/immediates

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi Florian,

On Tue, May 23, 2023 at 07:29:31PM +0200, Florian Westphal wrote:
> Hello.
> 
> Consider following example:
> 
> table ip t {
> 	set s {
> 		type ipv4_addr . ipv4_addr . inet_service
> 		size 65535
> 		flags dynamic, timeout
> 		timeout 3h
> 	}
> 
> 	chain c1 {
> 		update @s { ip saddr . 10.180.0.4 . 80 }
> 	}
> 
> 	chain c2 {
> 		ip saddr . 1.2.3.4 . 80 @s goto c1
> 	}
> }
> 
> This doesn't work:
> :13:14-20: Error: Can't parse symbolic invalid expressions
> ip saddr . 1.2.3.4 . 80 @s goto c1
> 
> Problem is that expr_evaluate_relational() first evaluates
> the lhs, so by the time concat evaluation encounters '1.2.3.4'
> symbol there is nothing that would hint at what datatype that is.
> 
> For this specific case, its possible to first evaluate the rhs, i.e.:
> 
> --- a/src/evaluate.c
> +++ b/src/evaluate.c
> @@ -2336,8 +2336,15 @@ static int expr_evaluate_relational(struct eval_ctx *ctx, struct expr **expr)
>         struct expr *range;
>         int ret;
>  
> +       right = rel->right;
> +       if (right->etype == EXPR_SYMBOL &&
> +           right->symtype == SYMBOL_SET &&
> +           expr_evaluate(ctx, &rel->right) < 0)
> +               return -1;
> +
> 
> This populates ectx->key and thus allows to infer the symbolic data
> type:
> 
> 1485                 if (key) {
> 1486                         tmp = key->dtype;
> 1487                         dsize = key->len;
> 1488                         bo = key->byteorder;
> 1489                         off--;
> 1490                 } else if (dtype == NULL || off == 0) {
> 1491                         tmp = datatype_lookup(TYPE_INVALID);
> 
> line 1486ff.  With unmodified nft, this hits the 'dtype == NULL' path
> and decoding "1.2.3.4" fails.
> 
> What do you think?  If you think this is fine I can work on this,
> above patch makes nft parse the example above, it needs more work on
> delinarization path.

If you have a use-case for this, go ahead.

Is there any other issue you can forecast on the delinearize path? Or
is it just that the code to handle this is missing?

Thanks.