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SubjectRe: [PATCH 1/2] bpf: add a bpf_override_function helper

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

> On Fri, Nov 10, 2017 at 10:34:59AM +0100, Ingo Molnar wrote:
> >
> > * Josef Bacik <josef@toxicpanda.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@kernel.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

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