netdev - Re: [PATCH/RFC v2 bpf-next 06/19] bpf: mark lo32 writes that should be zero extended into hi32

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Date:   Wed, 10 Apr 2019 20:13:12 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     alexei.starovoitov@...il.com, daniel@...earbox.net,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH/RFC v2 bpf-next 06/19] bpf: mark lo32 writes that should
 be zero extended into hi32

On Wed, 10 Apr 2019 20:50:20 +0100, Jiong Wang wrote:
> @@ -2169,6 +2188,12 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
>  						    value_regno);
>  				if (reg_type_may_be_null(reg_type))
>  					regs[value_regno].id = ++env->id_gen;
> +				/* A load of ctx field could have different
> +				 * actual load size with the one encoded in the
> +				 * insn. When the dst is PTR, it is for sure not
> +				 * a sub-register.
> +				 */
> +				regs[value_regno].subreg_def = DEF_NOT_SUBREG;

Can't the rewrite generate a 32bit load?  E.g. reading skb->len will be
LDX | W, we still gotta clear the top bits in that case, no?

I can't find the explanation of this case with a quick scan of the code
and cover letter..

>  			}
>  			regs[value_regno].type = reg_type;
>  		}