netdev - RE: [PATCH bpf-next 02/14] bpf: net: Avoid sock_setsockopt() taking sk lock when called from bpf

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Message-ID: <732a8006394f49d58c586156f3f81281@AcuMS.aculab.com>
Date:   Fri, 29 Jul 2022 10:04:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jakub Kicinski' <kuba@...nel.org>, Martin KaFai Lau <kafai@...com>
CC:     Stanislav Fomichev <sdf@...gle.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        "kernel-team@...com" <kernel-team@...com>,
        Paolo Abeni <pabeni@...hat.com>
Subject: RE: [PATCH bpf-next 02/14] bpf: net: Avoid sock_setsockopt() taking
 sk lock when called from bpf

From: Jakub Kicinski
> Sent: 28 July 2022 17:56
> 
> On Thu, 28 Jul 2022 09:31:04 -0700 Martin KaFai Lau wrote:
> > If I understand the concern correctly, it may not be straight forward to
> > grip the reason behind the testings at in_bpf() [ the in_task() and
> > the current->bpf_ctx test ] ?  Yes, it is a valid point.
> >
> > The optval.is_bpf bit can be directly traced back to the bpf_setsockopt
> > helper and should be easier to reason about.
> 
> I think we're saying the opposite thing. in_bpf() the context checking
> function is fine. There is a clear parallel to in_task() and combined
> with the capability check it should be pretty obvious what the code
> is intending to achieve.
> 
> sockptr_t::in_bpf which randomly implies that the lock is already held
> will be hard to understand for anyone not intimately familiar with the
> BPF code. Naming that bit is_locked seems much clearer.
> 
> Which is what I believe Stan was proposing.

Or make sk_setsockopt() be called after the integer value
has been read and with the lock held.

That saves any (horrid) conditional locking.

Also sockptr_t should probably have been a structure with separate
user and kernel address fields.
Putting the length in there would (probably) save code.

There then might be scope for pre-copying short user buffers
into a kernel buffer while still allowing the requests that
ignore the length copy directly from a user buffer.

	David

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