netdev - Re: [PATCH bpf-next 2/3] bpf: verifier: teach the verifier to reason about the BPF

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Message-ID: <26da90af-51da-3080-e5a0-9db533c72d0d@solarflare.com>
Date:   Thu, 13 Dec 2018 16:51:50 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        <alexei.starovoitov@...il.com>, <daniel@...earbox.net>
CC:     <netdev@...r.kernel.org>, <oss-drivers@...ronome.com>
Subject: Re: [PATCH bpf-next 2/3] bpf: verifier: teach the verifier to reason
 about the BPF_JSET instruction

On 13/12/18 03:41, Jakub Kicinski wrote:
> Some JITs (nfp) try to optimize code on their own.  It could make
> sense in case of BPF_JSET instruction which is currently not interpreted
> by the verifier, meaning for instance that dead could would not be
> detected if it was under BPF_JSET branch.
>
> Teach the verifier basics of BPF_JSET, JIT optimizations will be
> removed shortly.
>
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Reviewed-by: Jiong Wang <jiong.wang@...ronome.com>
> ---
Acked-by: Edward Cree <ecree@...arflare.com>
>  kernel/bpf/verifier.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 8b511a4fe84a..50bb45aa4f26 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3788,6 +3788,12 @@ static int is_branch_taken(struct bpf_reg_state *reg, u64 val, u8 opcode)
>  		if (tnum_is_const(reg->var_off))
>  			return !tnum_equals_const(reg->var_off, val);
>  		break;
> +	case BPF_JSET:
> +		if ((~reg->var_off.mask & reg->var_off.value) & val)
> +			return 1;
> +		if (!((reg->var_off.mask | reg->var_off.value) & val))
> +			return 0;
> +		break;
>  	case BPF_JGT:
>  		if (reg->umin_value > val)
>  			return 1;
> @@ -3872,6 +3878,13 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
>  		 */
>  		__mark_reg_known(false_reg, val);
>  		break;
> +	case BPF_JSET:
> +		false_reg->var_off = tnum_and(false_reg->var_off,
> +					      tnum_const(~val));
> +		if (is_power_of_2(val))
> +			true_reg->var_off = tnum_or(true_reg->var_off,
> +						    tnum_const(val));
> +		break;
>  	case BPF_JGT:
>  		false_reg->umax_value = min(false_reg->umax_value, val);
>  		true_reg->umin_value = max(true_reg->umin_value, val + 1);
> @@ -3944,6 +3957,13 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
>  		 */
>  		__mark_reg_known(false_reg, val);
>  		break;
> +	case BPF_JSET:
> +		false_reg->var_off = tnum_and(false_reg->var_off,
> +					      tnum_const(~val));
> +		if (is_power_of_2(val))
> +			true_reg->var_off = tnum_or(true_reg->var_off,
> +						    tnum_const(val));
> +		break;
>  	case BPF_JGT:
>  		true_reg->umax_value = min(true_reg->umax_value, val - 1);
>  		false_reg->umin_value = max(false_reg->umin_value, val);