netdev - Re: [PATCH bpf] bpf: report verifier bugs as warnings

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Message-ID: <20190403173029.lfwhblklss62yone@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 3 Apr 2019 10:30:31 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Paul Chaignon <paul.chaignon@...nge.com>,
        Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Xiao Han <xiao.han@...nge.com>,
        paul.chaignon@...il.com, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf] bpf: report verifier bugs as warnings

On Wed, Apr 03, 2019 at 04:52:40PM +0100, Edward Cree wrote:
> On 02/04/2019 15:37, Daniel Borkmann wrote:
> > If we really want to have a kernel warn, then lets add a
> > helper macro verbose_and_warn(...) which will trigger a one-time warning, but keeps
> > the verbose log intact as well.
> +1
> 
> Any time the verifier detects that its internal invariants have been broken,
>  logging a warning is the right thing to do, just like any other part of the
>  kernel.

It's not black and white.
As I said I don't think verbose_and_warn() is necessary.

Messages like:
verbose(env, "bpf verifier is misconfigured\n");
are technically 'broken internal invariant', but it shouldn't be a warn.

Whereas this:
        if (WARN_ON(regno >= MAX_BPF_REG)) {
                verbose(env, "mark_reg_known_zero(regs, %u)\n", regno);
                /* Something bad happened, let's kill all regs */
                for (regno = 0; regno < MAX_BPF_REG; regno++)
                        __mark_reg_not_init(regs + regno);
                return;
        }
should stay as-is.
It's a warn, and verbose message, and clean of regs.
Similarly:
        if (WARN_ON_ONCE(ptr_reg)) {
                print_verifier_state(env, state);
                verbose(env, "verifier internal error: unexpected ptr_reg\n");
                return -EINVAL;
        }
is a warn and more than just a verbose message.

verbose_and_warn() doesn't fit these two practical cases of warn + verbose.
Hence I see no reason to combine warn and verbose into single helper.
They're perfectly fine being separate.