linux-kernel - Re: [PATCH 3/4] tracing/eprobes: Fix reading of string fields

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Date:   Sat, 20 Aug 2022 21:27:33 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tzvetomir Stoyanov <tz.stoyanov@...il.com>,
        Tom Zanussi <zanussi@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 3/4] tracing/eprobes: Fix reading of string fields

On Fri, 19 Aug 2022 21:40:38 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> Currently when an event probe (eprobe) hooks to a string field, it does
> not display it as a string, but instead as a number. This makes the field
> rather useless. Handle the different kinds of strings, dynamic, static,
> relational/dynamic etc.
> 
> Now when a string field is used, the ":string" type can be used to display
> it:
> 
>   echo "e:sw sched/sched_switch comm=$next_comm:string" > dynamic_events

Really nice!

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

Thank you!

> 
> Cc: stable@...r.kernel.org
> Fixes: 7491e2c44278 ("tracing: Add a probe that attaches to trace events")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/trace_eprobe.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/kernel/trace/trace_eprobe.c b/kernel/trace/trace_eprobe.c
> index 550671985fd1..a1d3423ab74f 100644
> --- a/kernel/trace/trace_eprobe.c
> +++ b/kernel/trace/trace_eprobe.c
> @@ -311,6 +311,27 @@ static unsigned long get_event_field(struct fetch_insn *code, void *rec)
>  
>  	addr = rec + field->offset;
>  
> +	if (is_string_field(field)) {
> +		switch (field->filter_type) {
> +		case FILTER_DYN_STRING:
> +			val = (unsigned long)(rec + (*(unsigned int *)addr & 0xffff));
> +			break;
> +		case FILTER_RDYN_STRING:
> +			val = (unsigned long)(addr + (*(unsigned int *)addr & 0xffff));
> +			break;
> +		case FILTER_STATIC_STRING:
> +			val = (unsigned long)addr;
> +			break;
> +		case FILTER_PTR_STRING:
> +			val = (unsigned long)(*(char *)addr);
> +			break;
> +		default:
> +			WARN_ON_ONCE(1);
> +			return 0;
> +		}
> +		return val;
> +	}
> +
>  	switch (field->size) {
>  	case 1:
>  		if (field->is_signed)
> -- 
> 2.35.1


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