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

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Message-ID: <20190116232304.llq3gpmr2yyrufby@ast-mbp>
Date:   Wed, 16 Jan 2019 15:23:06 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        jakub.kicinski@...ronome.com, netdev@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH bpf-next 1/9] bpf: introduce bpf_spin_lock

On Wed, Jan 16, 2019 at 11:48:15PM +0100, Daniel Borkmann wrote:
> 
> I think if I'm not mistaken there should still be a possibility for causing a
> deadlock, namely if in the middle of the critical section I'm using an LD_ABS
> or LD_IND instruction with oob index such that I cause an implicit return 0
> while lock is held. At least I don't see this being caught, probably also for
> such case a test_verifier snippet would be good.

good catch. My earlier implementation was reusing check_reference_leak()
that is called for both bpf_exit and bpf_ld_abs, but then I realized we cannot
call bpf_exit from callee when lock is held and moved that check before
prepare_func_exit() forgetting about ldabs. argh. Will fix.

> Wouldn't we also need to mark queued spinlock functions as notrace such that
> e.g. from kprobe one cannot attach to these causing a deadlock?

there is recursion check already, so I'm not sure that is necessary, but
will add it since it doesn't hurt and safer indeed.