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

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Message-ID: <20201020120128.338595e9@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 20 Oct 2020 12:01:28 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
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

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)