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

[Cupertino Miranda <cupertino.miranda@oracle.com>]

Eduard Zingerman writes:

> 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?
>
With the refactor this looked like an armless change. Indeed if we agree
that the algorithm covers all scenarios, then why not.
I did not study the paper or the scalar_min_max_mul function nearly
enough to know for sure.
>
> [...]