linux-kernel - Re: [RFC 0/1] BPF tracing for arm64 using fprobe

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Date:   Fri, 18 Nov 2022 16:34:50 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Chris Mason <clm@...a.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Florent Revest <revest@...omium.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...gle.com>, markowsky@...gle.com,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Xu Kuohai <xukuohai@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC 0/1] BPF tracing for arm64 using fprobe

On Thu, Nov 17, 2022 at 05:40:30PM -0500, Steven Rostedt wrote:
> On Thu, 17 Nov 2022 16:55:12 -0500
> Chris Mason <clm@...a.com> wrote:
> 
> > On 11/17/22 12:16 PM, Steven Rostedt wrote:

> > The short answer to your concerns is that you can't replace kernel
> > functions from proprietary BPF programs.  The LSM and TCP congestion
> > control features intentionally have GPL only support functions in the
> > way.  bpf_probe_read_kernel() is also GPL only and massively limits the
> > things that can be done from proprietary code.
> 
> ^^^^^^^^^^^^^^^^^
> 
> That's the part I wanted to hear. But just the fact of replacing a kernel
> function with BPF code seems a bit concerning.

> > This list of helpers is pretty current and details which ones are GPL only:
> > 
> > https://github.com/iovisor/bcc/blob/master/docs/kernel-versions.md#helpers
> > 
> > I know there's a long and glorious history of collaboration around these
> > parts of bpf and ftrace.  I really hope this time around we all come
> > away feeling like the technical discussion made both projects better.
> > Mark and Florent today certainly made me think that was the direction we
> > were headed.
> > 
> > Along these lines, I'm also hoping to avoid diving into old debates and
> > alarmist conclusions about GPL compliance and signed bpf programs. Or,
> 
> Not alarmist, but concern as being able to modify what a kernel function can
> do is not something I take lightly.

FWIW, given that the aim here seems to be to expose all kernel internals to be
overridden arbitrarily, I'm also concerned that there's a huge surface area for
issues with maintainability, robustness/correctness, and security.

I really don't want to be stuck in a position where someone argues that all
kernel internal functions are ABI and need to stay around as-is to be hooked by
eBPF, and I hope that we all agree that there are no guarantees on that front.

Thanks,
Mark.