linux-kernel - Re: [PATCH v15 09/19] tracing: Add ftrace_fill_perf

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Message-ID: <20241009100502.8007-E-hca@linux.ibm.com>
Date: Wed, 9 Oct 2024 12:05:02 +0200
From: Heiko Carstens <hca@...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>,
        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>, Guo Ren <guoren@...nel.org>,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v15 09/19] tracing: Add ftrace_fill_perf_regs() for perf
 event

On Sun, Sep 15, 2024 at 06:11:14PM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Add ftrace_fill_perf_regs() which should be compatible with the
> perf_fetch_caller_regs(). In other words, the pt_regs returned from the
> ftrace_fill_perf_regs() must satisfy 'user_mode(regs) == false' and can be
> used for stack tracing.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
...
>  arch/s390/include/asm/ftrace.h    |    5 +++++
...
> diff --git a/arch/s390/include/asm/ftrace.h b/arch/s390/include/asm/ftrace.h
> index 9cdd48a46bf7..0d9f6df21f81 100644
> --- a/arch/s390/include/asm/ftrace.h
> +++ b/arch/s390/include/asm/ftrace.h
> @@ -84,6 +84,11 @@ ftrace_regs_get_frame_pointer(struct ftrace_regs *fregs)
>  	return sp[0];	/* return backchain */
>  }
>  
> +#define arch_ftrace_fill_perf_regs(fregs, _regs)	 do {		\
> +		(_regs)->psw.addr = (fregs)->regs.psw.addr;		\
> +		(_regs)->gprs[15] = (fregs)->regs.gprs[15];		\
> +	} while (0)

After reading your commit description and looking at the code I think the
s390 implementation of perf_arch_fetch_caller_regs() is at least
suboptimal: it is not possible to tell if an address belongs to user or
kernel space by looking only at the address (psw.addr); this also requires
to look at psw.mask. It _looks_ like all current callers of
perf_arch_fetch_caller_regs() initialize the passed in pt_regs to zero,
which of course also sets psw.mask to zero, and therefore
user_mode(regs) == false is satisfied.

However I'd rather make that explicit and don't want to rely on
callers. Therefore the above arch_ftrace_fill_perf_regs() should
set the mask to zero

		(_regs)->psw.mask = 0;

I will change perf_arch_fetch_caller_regs() accordingly.