Re: [PATCH 5/7] bpf: Add BPF_FETCH field / create atomic_fetch_add instruction

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

On Mon, Nov 23, 2020 at 05:32:00PM +0000, Brendan Jackman wrote:
> @@ -3644,8 +3649,21 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
>  		return err;
>  
>  	/* check whether we can write into the same memory */
> -	return check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> -				BPF_SIZE(insn->code), BPF_WRITE, -1, true);
> +	err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> +			       BPF_SIZE(insn->code), BPF_WRITE, -1, true);
> +	if (err)
> +		return err;
> +
> +	if (!(insn->imm & BPF_FETCH))
> +		return 0;
> +
> +	/* check and record load of old value into src reg  */
> +	err = check_reg_arg(env, insn->src_reg, DST_OP);
> +	if (err)
> +		return err;
> +	regs[insn->src_reg].type = SCALAR_VALUE;

check_reg_arg() will call mark_reg_unknown() which will set type to SCALAR_VALUE.
What is the point of another assignment?