netdev - Re: [PATCH bpf v2 1/3] bpf_redirect_neigh: Support supplying the nexthop as a helper parameter

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Message-ID: <87v9f422jx.fsf@toke.dk>
Date:   Tue, 20 Oct 2020 20:08:18 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf v2 1/3] bpf_redirect_neigh: Support supplying the
 nexthop as a helper parameter

Jakub Kicinski <kuba@...nel.org> writes:

> On Tue, 20 Oct 2020 12:51:02 +0200 Toke Høiland-Jørgensen wrote:
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index 20fc24c9779a..ba9de7188cd0 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -607,12 +607,21 @@ struct bpf_skb_data_end {
>>  	void *data_end;
>>  };
>>  
>> +struct bpf_nh_params {
>> +	u8 nh_family;
>> +	union {
>> +		__u32 ipv4_nh;
>> +		struct in6_addr ipv6_nh;
>> +	};
>> +};
>
>> @@ -4906,6 +4910,18 @@ struct bpf_fib_lookup {
>>  	__u8	dmac[6];     /* ETH_ALEN */
>>  };
>>  
>> +struct bpf_redir_neigh {
>> +	/* network family for lookup (AF_INET, AF_INET6) */
>> +	__u8 nh_family;
>> +	 /* avoid hole in struct - must be set to 0 */
>> +	__u8 unused[3];
>> +	/* network address of nexthop; skips fib lookup to find gateway */
>> +	union {
>> +		__be32		ipv4_nh;
>> +		__u32		ipv6_nh[4];  /* in6_addr; network order */
>> +	};
>> +};
>
> Isn't this backward? The hole could be named in the internal structure.
> This is a bit of a gray area, but if you name this hole in uAPI and
> programs start referring to it you will never be able to reuse it.
> So you may as well not require it to be zeroed..

Hmm, yeah, suppose you're right. Doesn't the verifier prevent any part
of the memory from being unitialised anyway? I seem to recall having run
into verifier complaints when I didn't initialise struct on the stack...

-Toke