Re: [PATCH v4 bpf-next 08/14] bpf: introduce the bpf_get_local_storage() helper function

[Daniel Borkmann]

On 07/27/2018 11:52 PM, Roman Gushchin wrote:
[...]
> @@ -2533,6 +2541,16 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
>  	}
>  
>  	regs = cur_regs(env);
> +
> +	/* check that flags argument in get_local_storage(map, flags) is 0,
> +	 * this is required because get_local_storage() can't return an error.
> +	 */
> +	if (func_id == BPF_FUNC_get_local_storage &&
> +	    !tnum_equals_const(regs[BPF_REG_2].var_off, 0)) {
> +		verbose(env, "get_local_storage() doesn't support non-zero flags\n");
> +		return -EINVAL;
> +	}

Hmm, this check is actually not correct. You will still be able to pass non-zero
values in there. arg2_type from the helper is ARG_ANYTHING, so the register type
could for example be one of the pointer types and it will still pass the verifier.
The correct way to check would be to use register_is_null().

> +
>  	/* reset caller saved regs */
>  	for (i = 0; i < CALLER_SAVED_REGS; i++) {
>  		mark_reg_not_init(env, regs, caller_saved[i]);