netdev - Re: [PATCH/RFC bpf-next 02/16] bpf: refactor propagate

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Date:   Wed, 27 Mar 2019 09:35:05 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     daniel@...earbox.net, bpf@...r.kernel.org, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH/RFC bpf-next 02/16] bpf: refactor propagate_live
 implementation

On Tue, Mar 26, 2019 at 06:05:25PM +0000, Jiong Wang wrote:
> Some code inside current implementation of "propagate_liveness" is a little
> bit verbose.
> 
> This patch refactor them so the code looks more simple and more clear.
> 
> The redundant usage of "vparent->frame[vstate->curframe]" is removed as we
> are here. It is safe to do this because "state_equal" has guaranteed that
> vstate->curframe must be equal with vparent->curframe.
> 
> Signed-off-by: Jiong Wang <jiong.wang@...ronome.com>
> ---
>  kernel/bpf/verifier.c | 44 ++++++++++++++++++++++++++++++--------------
>  1 file changed, 30 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6cc8c38..245bb3c 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -6050,6 +6050,22 @@ static bool states_equal(struct bpf_verifier_env *env,
>  	return true;
>  }
>  
> +static int propagate_liveness_reg(struct bpf_verifier_env *env,
> +				  struct bpf_reg_state *reg,
> +				  struct bpf_reg_state *parent_reg, u8 flag)
> +{
> +	int err;
> +
> +	if (parent_reg->live & flag || !(reg->live & flag))
> +		return 0;
> +
> +	err = mark_reg_read(env, reg, parent_reg);
> +	if (err)
> +		return err;
> +
> +	return 1;
> +}

what is the difference between 1 and 0 ? it doesn't seem to be used.

> +
>  /* A write screens off any subsequent reads; but write marks come from the
>   * straight-line code between a state and its parent.  When we arrive at an
>   * equivalent state (jump target or such) we didn't arrive by the straight-line
> @@ -6061,8 +6077,9 @@ static int propagate_liveness(struct bpf_verifier_env *env,
>  			      const struct bpf_verifier_state *vstate,
>  			      struct bpf_verifier_state *vparent)
>  {
> -	int i, frame, err = 0;
> +	struct bpf_reg_state *regs, *parent_regs;
>  	struct bpf_func_state *state, *parent;
> +	int i, frame, err = 0;
>  
>  	if (vparent->curframe != vstate->curframe) {
>  		WARN(1, "propagate_live: parent frame %d current frame %d\n",
> @@ -6071,16 +6088,13 @@ static int propagate_liveness(struct bpf_verifier_env *env,
>  	}
>  	/* Propagate read liveness of registers... */
>  	BUILD_BUG_ON(BPF_REG_FP + 1 != MAX_BPF_REG);
> +	parent_regs = vparent->frame[vparent->curframe]->regs;
> +	regs = vstate->frame[vstate->curframe]->regs;


may be do:
frame = vstate->curframe;
if (vparent->curframe != frame) { WARN...
parent_regs = vparent->frame[frame]->regs;
regs = vstate->frame[frame]->regs;

?

>  	/* We don't need to worry about FP liveness because it's read-only */
>  	for (i = 0; i < BPF_REG_FP; i++) {
> -		if (vparent->frame[vparent->curframe]->regs[i].live & REG_LIVE_READ)
> -			continue;
> -		if (vstate->frame[vstate->curframe]->regs[i].live & REG_LIVE_READ) {
> -			err = mark_reg_read(env, &vstate->frame[vstate->curframe]->regs[i],
> -					    &vparent->frame[vstate->curframe]->regs[i]);
> -			if (err)
> -				return err;
> -		}
> +		err = propagate_liveness_reg(env, &regs[i], &parent_regs[i]);
> +		if (err < 0)
> +			return err;
>  	}
>  
>  	/* ... and stack slots */
> @@ -6089,11 +6103,13 @@ static int propagate_liveness(struct bpf_verifier_env *env,
>  		parent = vparent->frame[frame];
>  		for (i = 0; i < state->allocated_stack / BPF_REG_SIZE &&
>  			    i < parent->allocated_stack / BPF_REG_SIZE; i++) {
> -			if (parent->stack[i].spilled_ptr.live & REG_LIVE_READ)
> -				continue;
> -			if (state->stack[i].spilled_ptr.live & REG_LIVE_READ)
> -				mark_reg_read(env, &state->stack[i].spilled_ptr,
> -					      &parent->stack[i].spilled_ptr);
> +			struct bpf_reg_state *parent_reg, *reg;
> +
> +			parent_reg = &parent->stack[i].spilled_ptr;
> +			reg = &state->stack[i].spilled_ptr;
> +			err = propagate_liveness_reg(env, reg, parent_reg);
> +			if (err < 0)
> +				return err;
>  		}
>  	}
>  	return err;
> -- 
> 2.7.4
>