linux-kernel - Re: [PATCH 3/3] tracing/probes: Fix return value when "(fault)" is injected

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Date:   Thu, 6 Jul 2023 13:40:36 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Dan Carpenter <dan.carpenter@...aro.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] tracing/probes: Fix return value when "(fault)" is
 injected

On Wed, 5 Jul 2023 22:49:56 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sun,  2 Jul 2023 23:47:35 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > 
> > When the "(fault)" is injected, the return value of fetch_store_string*()
> > should be the length of the "(fault)", but an error code is returned.
> > Fix it to return the correct length and update the data_loc according the
> > updated length.
> > This needs to update a ftracetest test case, which expects trace output
> > to appear as '(fault)' instead of '"(fault)"'.
> > 
> 
> Ah, because of patch 2, the ret < 0 makes it return without printing the
> "fault"?

No, actually set_data_loc() updates the 'ret' argument, but it is just
disposed... (not returned to the caller)

-static nokprobe_inline void set_data_loc(int ret, void *dest, void *__dest, void *base, int len)
+static nokprobe_inline int set_data_loc(int ret, void *dest, void *__dest, void *base, int len)
 {
-	if (ret >= 0) {
-		*(u32 *)dest = make_data_loc(ret, __dest - base);
-	} else {
+	if (ret < 0) {
 		strscpy(__dest, FAULT_STRING, len);
 		ret = strlen(__dest) + 1;
 	}
+
+	*(u32 *)dest = make_data_loc(ret, __dest - base);
+	return ret;
 }

So this returns updated 'ret', and also update data_loc to use the
updated 'ret' value (which is the length of the stored data).

> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Thank you!

> 
> -- Steve
> 
> 
> > Fixes: 2e9906f84fc7 ("tracing: Add "(fault)" name injection to kernel probes")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---


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