netdev - Re: [PATCH v2 bpf-next 1/7] bpf: implement BPF ring buffer and verifier support for it

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Date:   Thu, 21 May 2020 18:07:22 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, andrii.nakryiko@...il.com,
        kernel-team@...com, "Paul E . McKenney" <paulmck@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH v2 bpf-next 1/7] bpf: implement BPF ring buffer and
 verifier support for it

On Sun, May 17, 2020 at 12:57:21PM -0700, Andrii Nakryiko wrote:
> -	if (off < 0 || size < 0 || (size == 0 && !zero_size_allowed) ||
> -	    off + size > map->value_size) {
> -		verbose(env, "invalid access to map value, value_size=%d off=%d size=%d\n",
> -			map->value_size, off, size);
> -		return -EACCES;
> -	}
> -	return 0;
> +	if (off >= 0 && size_ok && off + size <= mem_size)
> +		return 0;
> +
> +	verbose(env, "invalid access to memory, mem_size=%u off=%d size=%d\n",
> +		mem_size, off, size);
> +	return -EACCES;

iirc invalid access to map value is one of most common verifier errors that
people see when they're use unbounded access. Generalizing it to memory is
technically correct, but it makes the message harder to decipher.
What is 'mem_size' ? Without context it is difficult to guess that
it's actually size of map value element.
Could you make this error message more human friendly depending on
type of pointer?

>  	if (err) {
> -		verbose(env, "R%d min value is outside of the array range\n",
> +		verbose(env, "R%d min value is outside of the memory region\n",
>  			regno);
>  		return err;
>  	}
> @@ -2518,18 +2527,38 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
>  	 * If reg->umax_value + off could overflow, treat that as unbounded too.
>  	 */
>  	if (reg->umax_value >= BPF_MAX_VAR_OFF) {
> -		verbose(env, "R%d unbounded memory access, make sure to bounds check any array access into a map\n",
> +		verbose(env, "R%d unbounded memory access, make sure to bounds check any memory region access\n",
>  			regno);
>  		return -EACCES;
>  	}
> -	err = __check_map_access(env, regno, reg->umax_value + off, size,
> +	err = __check_mem_access(env, reg->umax_value + off, size, mem_size,
>  				 zero_size_allowed);
> -	if (err)
> -		verbose(env, "R%d max value is outside of the array range\n",
> +	if (err) {
> +		verbose(env, "R%d max value is outside of the memory region\n",
>  			regno);

I'm not that worried about above three generalizations of errors,
but if you can make it friendly by describing type of memory region
I think it will be a plus.