linux-kernel - Re: [PATCH v4 4/9] fprobe: rethook: Use ftrace

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Message-Id: <20230905223633.23cd4e6e8407c45b934be477@kernel.org>
Date:   Tue, 5 Sep 2023 22:36:33 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Florent Revest <revest@...omium.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>, 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 4/9] fprobe: rethook: Use ftrace_regs in fprobe exit
 handler and rethook

On Tue, 05 Sep 2023 09:17:02 +0200
Sven Schnelle <svens@...ux.ibm.com> wrote:

> Masami Hiramatsu (Google) <mhiramat@...nel.org> writes:
> 
> > I found that this is not enough becuase s390/loongarch already implemented
> > their rethook, and as far as I can see, the s390 ftrace_regs does not save
> > the required registers for rethook. Thus, for such architecture, we need
> > another kconfig flag and keep using the pt_regs for rethook.
> 
> Looking into arch_rethook_trampoline() i think we save all required
> registers - which register do you think are missing? Or is there another
> function i should look at?

Yes, arch_rethook_trampoline() is good. It needs to save all registers.

In this series, I'm trying to change the pt_regs with ftrace_regs which will
reduce trampoline overhead if DYNAMIC_FTRACE_WITH_ARGS=y.

kprobe -> (pt_regs) -> rethook_try_hook()
fprobe -> (ftrace_regs) -> rethook_try_hook_ftrace() # new function

Thus, we need to ensure that the ftrace_regs which is saved in the ftrace
*without* FTRACE_WITH_REGS flags, can be used for hooking the function
return. I saw;

void arch_rethook_prepare(struct rethook_node *rh, struct pt_regs *regs, bool mcount)
{
        rh->ret_addr = regs->gprs[14];
        rh->frame = regs->gprs[15];

        /* Replace the return addr with trampoline addr */
        regs->gprs[14] = (unsigned long)&arch_rethook_trampoline;
}

gprs[15] is a stack pointer, so it is saved in ftrace_regs too, but what about
gprs[14]? (I guess it is a link register)
We need to read the gprs[14] and ensure that is restored to gpr14 when the
ftrace is exit even without FTRACE_WITH_REGS flag.

IOW, it is ftrace save regs/restore regs code issue. I need to check how the
function_graph implements it.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>