netdev - Re: [v3,2/4] tools: bpftool: add net detach command to detach XDP on interface

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Date:   Thu, 8 Aug 2019 17:05:11 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Y Song <ys114321@...il.com>
Cc:     Maciej Fijalkowski <maciejromanfijalkowski@...il.com>,
        "Daniel T. Lee" <danieltimlee@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [v3,2/4] tools: bpftool: add net detach command to detach XDP
 on interface

On Thu, 8 Aug 2019 12:52:11 -0700, Y Song wrote:
> > Ah ok. In this scenario if driver has a native xdp support we would be invoking
> > its ndo_bpf even if there's no prog currently attached and it wouldn't return
> > error value.
> >
> > Looking at dev_xdp_uninstall, setting driver's prog to NULL is being done only
> > when prog is attached. Maybe we should consider querying the driver in
> > dev_change_xdp_fd regardless of passed fd value? E.g. don't query only when
> > prog >= 0.
> >
> > I don't recall whether this was brought up previously.  
> 
> Thanks for explanation. I think return an error is better in
> such error cases. Otherwise, people mistakenly write wrong
> device name and they may think xdp is detached and it is
> actually not.
> 
> But this probably should not prevent
> this patch as it is more like a kernel issue.

Agreed, we'd probably need a flag here, similar to IF_NOEXIST to keep
backward compat.