Re: [PATCH bpf-next v2 4/5] bpf/verifier: relax MUL range computation check

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-04-17 at 13:23 +0100, Cupertino Miranda wrote:

[...]

>  static int is_safe_to_compute_dst_reg_range(struct bpf_insn *insn,
> +					    struct bpf_reg_state dst_reg,
>  					    struct bpf_reg_state src_reg)

Nit: there is no need to pass {dst,src}_reg by value,
     struct bpf_reg_state is 120 bytes in size
    (but maybe compiler handles this).

>  {
> -	bool src_known;
> +	bool src_known, dst_known;
>  	u64 insn_bitness = (BPF_CLASS(insn->code) == BPF_ALU64) ? 64 : 32;
>  	bool alu32 = (BPF_CLASS(insn->code) != BPF_ALU64);
>  	u8 opcode = BPF_OP(insn->code);
>  
> -	bool valid_known = true;
> -	src_known = is_const_reg_and_valid(src_reg, alu32, &valid_known);
> +	bool valid_known_src = true;
> +	bool valid_known_dst = true;
> +	src_known = is_const_reg_and_valid(src_reg, alu32, &valid_known_src);
> +	dst_known = is_const_reg_and_valid(dst_reg, alu32, &valid_known_dst);
>  
>  	/* Taint dst register if offset had invalid bounds
>  	 * derived from e.g. dead branches.
>  	 */
> -	if (valid_known == false)
> +	if (valid_known_src == false)
>  		return UNCOMPUTABLE_RANGE;
>  
>  	switch (opcode) {
> @@ -13457,10 +13460,12 @@ static int is_safe_to_compute_dst_reg_range(struct bpf_insn *insn,
>  	case BPF_OR:
>  		return COMPUTABLE_RANGE;
>  
> -	/* Compute range for the following only if the src_reg is known.
> +	/* Compute range for MUL if at least one of its registers is known.
>  	 */
>  	case BPF_MUL:
> -		return src_known ? COMPUTABLE_RANGE : UNCOMPUTABLE_RANGE;
> +		if (src_known || (dst_known && valid_known_dst))
> +			return COMPUTABLE_RANGE;
> +		break;

Is it even necessary to restrict src or dst to be known?
adjust_scalar_min_max_vals() logic for multiplication looks as follows:

	case BPF_MUL:
		dst_reg->var_off = tnum_mul(dst_reg->var_off, src_reg.var_off);
		scalar32_min_max_mul(dst_reg, &src_reg);
		scalar_min_max_mul(dst_reg, &src_reg);
		break;

Where tnum_mul() refers to a paper, and that paper does not restrict
abstract multiplication algorithm to constant values on either side.
The scalar_min_max_mul() and scalar32_min_max_mul() are similar:
- if both src and dst are positive
- if overflow is not possible
- adjust dst->min *= src->min
- adjust dst->max *= src->max

I think this should work just fine if neither of src or dst is a known constant.
What do you think?

[...]