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

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Date:   Wed, 2 Aug 2023 16:48:53 +0200
From:   Florent Revest <revest@...omium.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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>,
        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 Wed, Aug 2, 2023 at 3:56 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Tue, 1 Aug 2023 20:40:54 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
>
> > On Wed, 2 Aug 2023 09:21:46 +0900
> > Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> >
> > > > Then use kprobes. When I asked Masami what the difference between fprobes
> > > > and kprobes was, he told me that it would be that it would no longer rely
> > > > on the slower FTRACE_WITH_REGS. But currently, it still does.
> > >
> > > kprobes needs to keep using pt_regs because software-breakpoint exception
> > > handler gets that. And fprobe is used for bpf multi-kprobe interface,
> > > but I think it can be optional.
> > >
> > > So until user-land tool supports the ftrace_regs, you can just disable
> > > using fprobes if CONFIG_DYNAMIC_FTRACE_WITH_REGS=n
> >
> > I'm confused. I asked about the difference between kprobes on ftrace
> > and fprobes, and you said it was to get rid of the requirement of
> > FTRACE_WITH_REGS.
> >
> >  https://lore.kernel.org/all/20230120205535.98998636329ca4d5f8325bc3@kernel.org/
>
> Yes, it is for enabling fprobe (and fprobe-event) on more architectures.
> I don't think it's possible to change everything at once. So, it will be
> changed step by step. At the first step, I will replace pt_regs with
> ftrace_regs, and make bpf_trace.c and fprobe_event depends on
> FTRACE_WITH_REGS.

Just a small note that, strictly speaking,
CONFIG_DYNAMIC_FTRACE_WITH_REGS=y is not enough. fprobe_init() would
also need a way to set FTRACE_OPS_FL_SAVE_REGS conditionally. (you
could be on an arch that supports saving either regs or args and if
you don't set FTRACE_OPS_FL_SAVE_REGS you'd go through the args
trampoline and get a ftrace_regs that doesn't hold a pt_regs)