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

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Message-Id: <20230911231559.67dc02ec42995042b7e92c58@kernel.org>
Date:   Mon, 11 Sep 2023 23:15:59 +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 Mon, 11 Sep 2023 09:55:09 +0200
Sven Schnelle <svens@...ux.ibm.com> wrote:

> Masami Hiramatsu (Google) <mhiramat@...nel.org> writes:
> 
> >> > IOW, it is ftrace save regs/restore regs code issue. I need to check how the
> >> > function_graph implements it.
> >> 
> >> gpr2-gpr14 are always saved in ftrace_caller/ftrace_regs_caller(),
> >> regardless of the FTRACE_WITH_REGS flags. The only difference is that
> >> without the FTRACE_WITH_REGS flag the program status word (psw) is not
> >> saved because collecting that is a rather expensive operation.
> >
> > Thanks for checking that! So s390 will recover those saved registers
> > even if FTRACE_WITH_REGS flag is not set? (I wonder what is the requirement
> > of the ftrace_regs when returning from ftrace_call() without
> > FTRACE_WITH_REGS?)
> 
> Yes, it will recover these in all cases.

Thanks for the confirmation!

> 
> >> 
> >> I used the following commands to test rethook (is that the correct
> >> testcase?)
> >> 
> >> #!/bin/bash
> >> cd /sys/kernel/tracing
> >> 
> >> echo 'r:icmp_rcv icmp_rcv' >kprobe_events
> >> echo 1 >events/kprobes/icmp_rcv/enable
> >> ping -c 1 127.0.0.1
> >> cat trace
> >
> > No, the kprobe will path pt_regs to rethook.
> > Cna you run
> >
> > echo "f:icmp_rcv%return icmp_rcv" >> dynamic_events
> 
> Ah, ok. Seems to work as well:
> 
>   ping-481     [001] ..s2.    53.918480: icmp_rcv: (ip_protocol_deliver_rcu+0x42/0x218 <- icmp_rcv)
>   ping-481     [001] ..s2.    53.918491: icmp_rcv: (ip_protocol_deliver_rcu+0x42/0x218 <- icmp_rcv)

Nice!
OK, then s390 is safe to use ftrace_regs :) 

Thanks!


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