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

Masami Hiramatsu
2025-07-19T01:17:05Z

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

> On Fri, 18 Jul 2025 20:34:29 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
>
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> >
> > Allocate temporary string buffers for parsing kprobe-events
> > from heap instead of stack.
> >
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.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@goodmis.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@kernel.org>