linux-kernel - Re: [PATCH 1/2] bpf: add a bpf_override

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Date:   Sat, 11 Nov 2017 09:14:55 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     rostedt@...dmis.org, mingo@...hat.com, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        ast@...nel.org, kernel-team@...com, daniel@...earbox.net,
        Josef Bacik <jbacik@...com>
Subject: Re: [PATCH 1/2] bpf: add a bpf_override_function helper


* Josef Bacik <josef@...icpanda.com> wrote:

> On Fri, Nov 10, 2017 at 10:34:59AM +0100, Ingo Molnar wrote:
> > 
> > * Josef Bacik <josef@...icpanda.com> wrote:
> > 
> > > @@ -551,6 +578,10 @@ static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func
> > >  		return &bpf_get_stackid_proto;
> > >  	case BPF_FUNC_perf_event_read_value:
> > >  		return &bpf_perf_event_read_value_proto;
> > > +	case BPF_FUNC_override_return:
> > > +		pr_warn_ratelimited("%s[%d] is installing a program with bpf_override_return helper that may cause unexpected behavior!",
> > > +				    current->comm, task_pid_nr(current));
> > > +		return &bpf_override_return_proto;
> > 
> > So if this new functionality is used we'll always print this into the syslog?
> > 
> > The warning is also a bit passive aggressive about informing the user: what 
> > unexpected behavior can happen, what is the worst case?
> > 
> 
> It's modeled after the other warnings bpf will spit out, but with this feature
> you are skipping a function and instead returning some arbitrary value, so
> anything could go wrong if you mess something up.  For instance I screwed up my
> initial test case and made every IO submitted return an error instead of just on
> the one file system I was attempting to test, so all sorts of hilarity ensued.

Ok, then for the x86 bits:

  NAK-ed-by: Ingo Molnar <mingo@...nel.org>

One of the major advantages of having an in-kernel BPF sandbox is to never crash 
the kernel - and allowing BPF programs to just randomly modify the return value of 
kernel functions sounds immensely broken to me.

(And yes, I realize that kprobes are used here as a vehicle, but the point 
remains.)

Thanks,

	Ingo