linux-kernel - Re: [PATCH 3/5] tracing: kprobe-event: Allocate string buffers from heap

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Message-Id: <20250719101701.0df8cde9709bdd9f0075c82a@kernel.org>
Date: Sat, 19 Jul 2025 10:17:01 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] tracing: kprobe-event: Allocate string buffers from
 heap

On Fri, 18 Jul 2025 13:46:27 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 18 Jul 2025 20:34:29 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > 
> > Allocate temporary string buffers for parsing kprobe-events
> > from heap instead of stack.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---
> >  kernel/trace/trace_kprobe.c |   39 +++++++++++++++++++++++++--------------
> >  1 file changed, 25 insertions(+), 14 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> > index 15d7a381a128..793af6000f16 100644
> > --- a/kernel/trace/trace_kprobe.c
> > +++ b/kernel/trace/trace_kprobe.c
> > @@ -861,20 +861,20 @@ static int trace_kprobe_create_internal(int argc, const char *argv[],
> >  	 *  FETCHARG:TYPE : use TYPE instead of unsigned long.
> >  	 */
> >  	struct trace_kprobe *tk __free(free_trace_kprobe) = NULL;
> > +	const char *event = NULL, *group = KPROBE_EVENT_SYSTEM;
> > +	const char **new_argv __free(kfree) = NULL;
> >  	int i, len, new_argc = 0, ret = 0;
> > -	bool is_return = false;
> >  	char *symbol __free(kfree) = NULL;
> > -	char *tmp = NULL;
> > -	const char **new_argv __free(kfree) = NULL;
> > -	const char *event = NULL, *group = KPROBE_EVENT_SYSTEM;
> > +	char *ebuf __free(kfree) = NULL;
> > +	char *gbuf __free(kfree) = NULL;
> > +	char *abuf __free(kfree) = NULL;
> > +	char *dbuf __free(kfree) = NULL;
> >  	enum probe_print_type ptype;
> > +	bool is_return = false;
> >  	int maxactive = 0;
> > -	long offset = 0;
> >  	void *addr = NULL;
> > -	char buf[MAX_EVENT_NAME_LEN];
> > -	char gbuf[MAX_EVENT_NAME_LEN];
> > -	char abuf[MAX_BTF_ARGS_LEN];
> > -	char *dbuf __free(kfree) = NULL;
> > +	char *tmp = NULL;
> > +	long offset = 0;
> >  
> >  	switch (argv[0][0]) {
> >  	case 'r':
> > @@ -893,6 +893,8 @@ static int trace_kprobe_create_internal(int argc, const char *argv[],
> >  		event++;
> >  
> >  	if (isdigit(argv[0][1])) {
> > +		char *buf __free(kfree) = NULL;
> 
> So this gets freed when this if block ends, right?

Yes, because in this block, "buf" is used as a really temporary
buffer.

> 
> > +
> >  		if (!is_return) {
> >  			trace_probe_log_err(1, BAD_MAXACT_TYPE);
> >  			return -EINVAL;
> > @@ -905,7 +907,7 @@ static int trace_kprobe_create_internal(int argc, const char *argv[],
> >  			trace_probe_log_err(1, BAD_MAXACT);
> >  			return -EINVAL;
> >  		}
> > -		memcpy(buf, &argv[0][1], len);
> > +		buf = kmemdup(&argv[0][1], len + 1, GFP_KERNEL);
> >  		buf[len] = '\0';
> >  		ret = kstrtouint(buf, 0, &maxactive);
> >  		if (ret || !maxactive) {
> > @@ -973,6 +975,9 @@ static int trace_kprobe_create_internal(int argc, const char *argv[],
> >  
> >  	trace_probe_log_set_index(0);
> >  	if (event) {
> > +		gbuf = kmalloc(MAX_EVENT_NAME_LEN, GFP_KERNEL);
> > +		if (!gbuf)
> > +			return -ENOMEM;
> >  		ret = traceprobe_parse_event_name(&event, &group, gbuf,
> >  						  event - argv[0]);
> 
> And you can't use the same trick here because
> traceprobe_parse_event_name() assigns "group" to gbuf and is used
> outside this if block.

Yes, that holds the group name used until parsing the probe.

> 
> I notice there's no comment that states this. At the very minimum,
> traceprobe_parse_event_name() should have a kerneldoc comment above its
> definition and state this.

Yeah, that function should be docummented, it is a bit complicated.

> But that's not an issue with this patch
> series. Just an observation. Thus...
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Thank you!

> 
> -- Steve
> 
> 
> >  		if (ret)
> > @@ -981,16 +986,22 @@ static int trace_kprobe_create_internal(int argc, const char *argv[],
> >  
> >  	if (!event) {
> >  		/* Make a new event name */
> > +		ebuf = kmalloc(MAX_EVENT_NAME_LEN, GFP_KERNEL);
> > +		if (!ebuf)
> > +			return -ENOMEM;
> >  		if (symbol)
> > -			snprintf(buf, MAX_EVENT_NAME_LEN, "%c_%s_%ld",
> > +			snprintf(ebuf, MAX_EVENT_NAME_LEN, "%c_%s_%ld",
> >  				 is_return ? 'r' : 'p', symbol, offset);
> >  		else
> > -			snprintf(buf, MAX_EVENT_NAME_LEN, "%c_0x%p",
> > +			snprintf(ebuf, MAX_EVENT_NAME_LEN, "%c_0x%p",
> >  				 is_return ? 'r' : 'p', addr);
> > -		sanitize_event_name(buf);
> > -		event = buf;
> > +		sanitize_event_name(ebuf);
> > +		event = ebuf;
> >  	}
> >  
> > +	abuf = kmalloc(MAX_BTF_ARGS_LEN, GFP_KERNEL);
> > +	if (!abuf)
> > +		return -ENOMEM;
> >  	argc -= 2; argv += 2;
> >  	ctx->funcname = symbol;
> >  	new_argv = traceprobe_expand_meta_args(argc, argv, &new_argc,
> 


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