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

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Message-Id: <8CDB95C6-91FB-4F6D-82E1-61F6132A2DE1@netronome.com>
Date:   Thu, 11 Apr 2019 07:02:44 +0100
From:   Jiong Wang <jiong.wang@...ronome.com>
To:     Jakub Kicinski <jakub.kicinski@...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 11 Apr 2019, at 04:13, Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:
> 
> 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?

No. for 32-bit load, I think the type won’t be ptr type. This override is
gated by !SCALAR_VALUE.

Regards,
Jiong

> 
> 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;
>> 		}
>