linux-kernel - Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

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Message-ID: <CAADnVQ+WPw0pfGAk+z=hCVrSmCBkKuh8GJm-5bkq5Ow7Md3sGA@mail.gmail.com>
Date:   Tue, 1 Aug 2023 16:44:24 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Florent Revest <revest@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

On Tue, Aug 1, 2023 at 4:09 PM Steven Rostedt <rostedt@...dmis.org> wrote>
> Then I recommend that you give up using fprobes and just stick with kprobes
> as that's guaranteed to give you full pt_regs (at the overhead of doing
> things like filing in flags and such). And currently for arm64, fprobes can
> only work with ftrace_regs, without the full pt_regs.

bpf doesn't attach to fprobes directly. That was never requested.
But Jiri's work to support multi attach
https://lore.kernel.org/bpf/20220316122419.933957-1-jolsa@kernel.org/
was a joint effort with Masami that relied on fprobe multi attach api.
register_fprobe_ips() in particular, because the promise you guys
give us that callback will get pt_regs as
described in Documentation/trace/fprobe.rst.
>From bpf side we don't care that such pt_regs is 100% filled in or
only partial as long as this pt_regs pointer is valid for perf_event_output
and stack walking that consume pt_regs.
I believe that was and still is the case for both x86 and arm64.

The way I understood Masami's intent is to change that promise and
fprobe callback will receive ftrace_regs that is incompatible with
pt_regs and that's obviously bad.
What you're suggesting "give up on using fprobe" is not up to us.
We're not using them. We care about register_fprobe_ips() and what
callback receives. Whatever internal changes to fprobe you're doing
are ok as long as the callback receives valid pt_regs (even partially filled).