netdev - Re: [PATCH v2 bpf 1/2] bpf: net: Avoid copying sk_user_data of reuseport_array during sk

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Message-ID: <20200709212716.fvuas7m5dvlotwnj@kafai-mbp>
Date:   Thu, 9 Jul 2020 14:27:16 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        <kernel-team@...com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 bpf 1/2] bpf: net: Avoid copying sk_user_data of
 reuseport_array during sk_clone

On Thu, Jul 09, 2020 at 10:07:59PM +0200, Daniel Borkmann wrote:
> On 7/9/20 8:11 AM, Martin KaFai Lau wrote:
> > It makes little sense for copying sk_user_data of reuseport_array during
> > sk_clone_lock().  This patch reuses the SK_USER_DATA_NOCOPY bit introduced in
> > commit f1ff5ce2cd5e ("net, sk_msg: Clear sk_user_data pointer on clone if tagged").
> > It is used to mark the sk_user_data is not supposed to be copied to its clone.
> > 
> > Although the cloned sk's sk_user_data will not be used/freed in
> > bpf_sk_reuseport_detach(), this change can still allow the cloned
> > sk's sk_user_data to be used by some other means.
> > 
> > Freeing the reuseport_array's sk_user_data does not require a rcu grace
> > period.  Thus, the existing rcu_assign_sk_user_data_nocopy() is not
> > used.
> 
> nit: Would have been nice though to add a nonrcu API for this nevertheless
> instead of open coding.
Agreed.  I will create a follow-up patch to bpf-next later.

I had created (READ|WRITE)_ONCE_SK_USER_DATA() earlier but then noticed
there is no use on the READ_ONCE in that particular call, so ditched the
idea.  I think it does not matter much and should just use READ_ONCE also.