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: <20220728163104.usdkmsxjyqwaitxu@kafai-mbp.dhcp.thefacebook.com>
Date:   Thu, 28 Jul 2022 09:31:04 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Stanislav Fomichev <sdf@...gle.com>, bpf@...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,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH bpf-next 02/14] bpf: net: Avoid sock_setsockopt() taking
 sk lock when called from bpf

On Wed, Jul 27, 2022 at 06:49:03PM -0700, Jakub Kicinski wrote:
> On Wed, 27 Jul 2022 17:45:46 -0700 Martin KaFai Lau wrote:
> > > bool setsockopt_capable(struct user_namespace *ns, int cap)
> > > {
> > >        if (!in_task()) {
> > >              /* Running in irq/softirq -> setsockopt invoked by bpf program.
> > >               * [not sure, is it safe to assume no regular path leads
> > > to setsockopt from sirq?]
> > >               */
> > >              return true;
> > >        }
> > > 
> > >        /* Running in process context, task has bpf_ctx set -> invoked
> > > by bpf program. */
> > >        if (current->bpf_ctx != NULL)
> > >              return true;
> > > 
> > >        return ns_capable(ns, cap);
> > > }
> > > 
> > > And then do /ns_capable/setsockopt_capable/ in net/core/sock.c
> > > 
> > > But that might be more fragile than passing the flag, idk.  
> > I think it should work.  From a quick look, all bpf_setsockopt usage has
> > bpf_ctx.  The one from bpf_tcp_ca (struct_ops) and bpf_iter is trampoline
> > which also has bpf_ctx.  Not sure about the future use cases.
> > 
> > To be honest, I am not sure if I have missed cases and also have similar questions
> > your have in the above sample code.  This may deserve a separate patch
> > set for discussion.  Using a bit in sockptr is mostly free now.
> > WDYT ?
> 
> Sorry to chime in but I vote against @in_bpf. I had to search the git
> history recently to figure out what SK_USER_DATA_BPF means. It's not
> going to be obvious to a networking person what semantics to attribute
> to "in bpf".
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.