linux-kernel - Re: [PATCH v2 2/2] kprobes/trace: Fix kprobe selftest for newer gcc

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Message-Id: <20161213103039.d8402249aed7d974c7f1aaee@kernel.org>
Date:   Tue, 13 Dec 2016 10:30:39 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Marcin Nowakowski <marcin.nowakowski@...tec.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] kprobes/trace: Fix kprobe selftest for newer gcc

On Fri, 9 Dec 2016 15:19:38 +0100
Marcin Nowakowski <marcin.nowakowski@...tec.com> wrote:

> Commit 265a5b7ee3eb ("kprobes/trace: Fix kprobe selftest for gcc 4.6")
> has added __used attribute to kprobe_trace_selftest_target to ensure
> that the method is listed in kallsyms table.
> 
> However, even though the method remains in the kernel image, the actual
> call is optimised away as there are no side efects and the return value
> is never checked.

Nice catch :)

> 
> Add a return value check and a 'noinline' attribute to ensure that an
> inlined copy of the method is not used by the caller. Also add checks
> that verify that the kprobe was really hit, as at the moment the tests
> show positive results despite the test method being optimised away.
> 
> Finally, add __init annotations to find_trace_probe_file() and
> kprobe_trace_selftest_target() as they are only called from within an
> __init method.

Right.

Looks good to me.

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

Thanks!

> 
> Signed-off-by: Marcin Nowakowski <marcin.nowakowski@...tec.com>
> ---
>  kernel/trace/trace_kprobe.c | 28 +++++++++++++++++++++++-----
>  1 file changed, 23 insertions(+), 5 deletions(-)
> 
>  v2: no changes
> 
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index a2af1bc..a133ecd 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -1361,18 +1361,18 @@ fs_initcall(init_kprobe_trace);
>  
>  
>  #ifdef CONFIG_FTRACE_STARTUP_TEST
> -
>  /*
>   * The "__used" keeps gcc from removing the function symbol
> - * from the kallsyms table.
> + * from the kallsyms table. 'noinline' makes sure that there
> + * isn't an inlined version used by the test method below
>   */
> -static __used int kprobe_trace_selftest_target(int a1, int a2, int a3,
> -					       int a4, int a5, int a6)
> +static __used __init noinline int
> +kprobe_trace_selftest_target(int a1, int a2, int a3, int a4, int a5, int a6)
>  {
>  	return a1 + a2 + a3 + a4 + a5 + a6;
>  }
>  
> -static struct trace_event_file *
> +static struct __init trace_event_file *
>  find_trace_probe_file(struct trace_kprobe *tk, struct trace_array *tr)
>  {
>  	struct trace_event_file *file;
> @@ -1450,12 +1450,25 @@ static __init int kprobe_trace_self_tests_init(void)
>  
>  	ret = target(1, 2, 3, 4, 5, 6);
>  
> +	/*
> +	 * Not expecting an error here, the check is only to prevent the
> +	 * optimizer from removing the call to target() as otherwise there
> +	 * are no side-effects and the call is never performed.
> +	 */
> +	if (ret != 21)
> +		warn++;
> +
>  	/* Disable trace points before removing it */
>  	tk = find_trace_kprobe("testprobe", KPROBE_EVENT_SYSTEM);
>  	if (WARN_ON_ONCE(tk == NULL)) {
>  		pr_warn("error on getting test probe.\n");
>  		warn++;
>  	} else {
> +		if (trace_kprobe_nhit(tk) != 1) {
> +			pr_warn("incorrect number of testprobe hits\n");
> +			warn++;
> +		}
> +
>  		file = find_trace_probe_file(tk, top_trace_array());
>  		if (WARN_ON_ONCE(file == NULL)) {
>  			pr_warn("error on getting probe file.\n");
> @@ -1469,6 +1482,11 @@ static __init int kprobe_trace_self_tests_init(void)
>  		pr_warn("error on getting 2nd test probe.\n");
>  		warn++;
>  	} else {
> +		if (trace_kprobe_nhit(tk) != 1) {
> +			pr_warn("incorrect number of testprobe2 hits\n");
> +			warn++;
> +		}
> +
>  		file = find_trace_probe_file(tk, top_trace_array());
>  		if (WARN_ON_ONCE(file == NULL)) {
>  			pr_warn("error on getting probe file.\n");
> -- 
> 2.7.4
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>