linux-kernel - Re: [PATCH V2 1/2] tracing/probes: Fix a warning message to show correct maximum length

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Date:   Fri, 10 Feb 2017 11:04:34 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH V2 1/2] tracing/probes: Fix a warning message to show
 correct maximum length

On Fri, 10 Feb 2017 22:21:55 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Since tracing/*probe_events will accept a probe definition
> up to 4096 - 2 ('\n' and '\0') bytes, it must show 4094 instead
> of 4096 in warning message.

Actually, during the testing I found that we don't need the '\n'.

	echo -n 'p:irq do_IRQ a=@...fies_64' > kprobe_events

works just fine. My tests work with 4095 characters. Before and after
my patch.

-- Steve

> 
> Note that there is one possible case of exceed 4094. If user
> prepare 4096 bytes null-terminated string and syscall write
> it with the count == 4095, then it can be accepted. However,
> if user puts a '\n' after that, it must rejected.
> So IMHO, the warning message should indicate shorter one,
> since it is safer.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> ---
>  kernel/trace/trace_probe.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index 2a06f1f..847c1e0 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -678,8 +678,9 @@ ssize_t traceprobe_probes_write(struct file *file, const char __user *buffer,
>  				if (done + size < count) {
>  					if (buf != kbuf)
>  						break;
> +					/* This can accept WRITE_BUFSIZE - 2 ('\n' + '\0') */
>  					pr_warn("Line length is too long: Should be less than %d\n",
> -						WRITE_BUFSIZE);
> +						WRITE_BUFSIZE - 2);
>  					ret = -EINVAL;
>  					goto out;
>  				}