linux-kernel - Re: [PATCH v3] tracing: precise log info for kretprobe addr err

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Date:   Mon, 25 Jan 2021 19:19:27 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jianlin Lv <Jianlin.Lv@....com>
Cc:     rostedt@...dmis.org, mingo@...hat.com, mhiramat@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] tracing: precise log info for kretprobe addr err

On 01/26, Jianlin Lv wrote:
>
> When trying to create kretprobe with the wrong function symbol in tracefs;
> The error is triggered in the register_trace_kprobe() and recorded as
> FAIL_REG_PROBE issue,
>
> Example:
>   $ cd /sys/kernel/debug/tracing
>   $ echo 'r:myprobe ERROR_SYMBOL_XXX ret=%x0' >> kprobe_events
>     bash: echo: write error: Invalid argument
>   $ cat error_log
>     [142797.347877] trace_kprobe: error: Failed to register probe event
>     Command: r:myprobe ERROR_SYMBOL_XXX ret=%x0
>                        ^
>
> This error can be detected in the parameter parsing stage, the effect of
> applying this patch is as follows:
>
>   $ echo 'r:myprobe ERROR_SYMBOL_XXX ret=%x0' >> kprobe_events
>     bash: echo: write error: Invalid argument
>   $ cat error_log
>     [415.89]trace_kprobe: error: Retprobe address must be an function entry
>     Command: r:myprobe ERROR_SYMBOL_XXX ret=%x0

IOW, the "offset != 0" check removed by this patch is obviously wrong, right?

Agreed, but...

> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -830,7 +830,7 @@ static int trace_kprobe_create(int argc, const char *argv[])
>  			flags |= TPARG_FL_RETURN;
>  		if (kprobe_on_func_entry(NULL, symbol, offset))
>  			flags |= TPARG_FL_FENTRY;
> -		if (offset && is_return && !(flags & TPARG_FL_FENTRY)) {
> +		if (!strchr(symbol, ':') && is_return && !(flags & TPARG_FL_FENTRY)) {

but why did you add the strchr(':') check instead?

I was really puzzled until I found the this email from Masami:
https://lore.kernel.org/lkml/20210120131406.5a992c1e434681750a0cd5d4@kernel.org/

So I leave this to you and Masami, but perhaps you can document this check at
least in the changelog?

Oleg.