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

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Date:   Thu, 4 Apr 2019 00:41:32 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Edward Cree <ecree@...arflare.com>
Cc:     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 04/03/2019 07:30 PM, Alexei Starovoitov wrote:
> 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.

Sure, I think that's okay as well; was mainly thinking to keep some of these
WARN wrt broken internal invariant such that tools like syzkaller will actually
generate a report w/ reproducer if it ever hits these (as opposed to just ignore
them due to ignoring such logs in general).