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:   Tue, 1 Aug 2023 20:40:54 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     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>,
        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 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/

> 
> Then you can safely use 
> 
> struct pt_regs *regs = ftrace_get_regs(fregs);
> 
> I think we can just replace the CONFIG_FPROBE ifdefs with
> CONFIG_DYNAMIC_FTRACE_WITH_REGS in kernel/trace/bpf_trace.c
> And that will be the first version of using ftrace_regs in fprobe.

But it is still slow. The FTRACE_WITH_REGS gives us the full pt_regs
and saves all registers including flags, which is a very slow operation
(and noticeable in profilers).

And this still doesn't work on arm64.

Maybe we can add a ftrace_partial_regs(fregs) that returns a
partially filled pt_regs, and the caller that uses this obviously knows
its partial (as it's in the name). But this doesn't quite help out arm64
because unlike x86, struct ftrace_regs does not contain an address
compatibility with pt_regs fields. It would need to do a copy.

 ftrace_partial_regs(fregs, &regs) ?

-- Steve