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

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Message-ID: <19519442-7c40-5115-de04-e0616931fa4b@iogearbox.net>
Date:   Tue, 20 Oct 2020 21:47:24 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jakub Kicinski <kuba@...nel.org>,
        Toke Høiland-Jørgensen 
        <toke@...hat.com>
Cc:     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

On 10/20/20 9:01 PM, Jakub Kicinski wrote:
> On Tue, 20 Oct 2020 20:08:18 +0200 Toke Høiland-Jørgensen wrote:
>>> 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...
> 
> Good point, in which case we have a convenient way to zero the hole
> after nh_family but no convenient way to zero the empty address space
> for IPv4 :) (even though that one only needs to be zeroed for the
> verifier)

Technically, it's uninitialized, so zero or any other garbage from BPF stack's
previous use of the program. We could use couple of __u8 :8 after nh_family to
have an unnamed placeholder (like in __bpf_md_ptr()), or we might as well just
switch to __u32 nh_family and avoid the hole that way (also gets rid of the extra
check) ... given we have the liberty to extend later anyway if ever needed.