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

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Message-ID: <59FA6FFA.1080503@iogearbox.net>
Date:   Thu, 02 Nov 2017 02:08:10 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Josef Bacik <josef@...icpanda.com>, rostedt@...dmis.org,
        mingo@...hat.com, davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ast@...nel.org, kernel-team@...com
CC:     Josef Bacik <jbacik@...com>
Subject: Re: [PATCH 1/2] bpf: add a bpf_override_function helper

On 11/01/2017 06:00 PM, Josef Bacik wrote:
> From: Josef Bacik <jbacik@...com>
>
> Error injection is sloppy and very ad-hoc.  BPF could fill this niche
> perfectly with it's kprobe functionality.  We could make sure errors are
> only triggered in specific call chains that we care about with very
> specific situations.  Accomplish this with the bpf_override_funciton
> helper.  This will modify the probe'd callers return value to the
> specified value and set the PC to an override function that simply
> returns, bypassing the originally probed function.  This gives us a nice
> clean way to implement systematic error injection for all of our code
> paths.
>
> Signed-off-by: Josef Bacik <jbacik@...com>

Looks useful (seccomp/BPF has something similar but on syscall level).
I think given we change kernel behavior, it would be good if we emit
similar warning like in case of bpf_get_probe_write_proto(), such that
from staring at the log (e.g. in case of subsequent crash), it could
help identifying that cause to trigger the bug could have been due to
bpf_override_function().