netdev - Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin

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Date:   Mon, 28 Jan 2019 09:43:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, jakub.kicinski@...ronome.com,
        netdev@...r.kernel.org, kernel-team@...com, mingo@...hat.com,
        will.deacon@....com, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        jannh@...gle.com
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock

On Fri, Jan 25, 2019 at 03:42:43PM -0800, Alexei Starovoitov wrote:
> On Fri, Jan 25, 2019 at 10:10:57AM +0100, Peter Zijlstra wrote:

> > What about the progs that run from SoftIRQ ? Since that bpf_prog_active
> > thing isn't inside BPF_PROG_RUN() what is to stop say:
> > 
> >    reuseport_select_sock()
> >      ...
> >        BPF_PROG_RUN()
> >          bpf_spin_lock()
> >         <IRQ>
> > 	  ...
> > 	  BPF_PROG_RUN()
> > 	    bpf_spin_lock() // forever more
> > 
> > 	</IRQ>
> > 
> > Unless you stick that bpf_prog_active stuff inside BPF_PROG_RUN itself,
> > I don't see how you can fundamentally avoid this happening (now or in
> > the future).

> But your issue above is valid.

> We don't use bpf_prog_active for networking progs, since we allow
> for one level of nesting due to the classic SKF_AD_PAY_OFFSET legacy.
> Also we allow tracing progs to nest with networking progs.
> People using this actively.
> Typically it's not an issue, since in networking there is no
> arbitrary nesting (unlike kprobe/nmi in tracing),
> but for bpf_spin_lock it can be, since the same map can be shared
> by networking and tracing progs and above deadlock would be possible:
> (first BPF_PROG_RUN will be from networking prog, then kprobe+bpf's
> BPF_PROG_RUN accessing the same map with bpf_spin_lock)
> 
> So for now I'm going to allow bpf_spin_lock in networking progs only,
> since there is no arbitrary nesting there.

Isn't that still broken? AFAIU networking progs can happen in task
context (TX) and SoftIRQ context (RX), which can nest.

> And once we figure out the safety concerns for kprobe/tracepoint progs
> we can enable bpf_spin_lock there too.
> NMI bpf progs will never have bpf_spin_lock.

kprobe is like NMI, since it pokes an INT3 instruction which can trigger
in the middle of IRQ-disabled or even in NMIs. Similar arguments can be
made for tracepoints, they can happen 'anywhere'.