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

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Message-ID: <Y3uIVknDWTS0bMTT@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Nov 2022 15:16:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     KP Singh <kpsingh@...nel.org>
Cc:     Chris Mason <clm@...a.com>, Steven Rostedt <rostedt@...dmis.org>,
        Mark Rutland <mark.rutland@....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>,
        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>
Subject: Re: [RFC 0/1] BPF tracing for arm64 using fprobe

On Mon, Nov 21, 2022 at 02:47:10PM +0100, KP Singh wrote:

> > > How do I know that a function return was modified by BPF? If I'm debugging
> 
> You can list the BPF programs that are loaded in the kernel with
> 
> # bpftool prog list

Only when you have access to the machine; most cases it's people sending
random splats by email.

> Also, the BPF programs show up in call stacks when you are debugging.

Only when it splats inside the BPF part, not when it splats after
because BPF changed semantics of a function.