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

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Message-ID: <yt9dy1hdi1pe.fsf@linux.ibm.com>
Date:   Mon, 11 Sep 2023 09:55:09 +0200
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Masami Hiramatsu (Google) <mhiramat@...nel.org>
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

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.

>> 
>> 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)