Re: [PATCH v3 2/5] tracing: Use __free() in trace_probe for cleanup

Masami Hiramatsu
2025-01-08T02:03:25Z

On Tue, 7 Jan 2025 20:34:32 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 8 Jan 2025 09:38:43 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
>
> > > I don't get this? You are telling the compiler not to free tmp, because you
> > > decided to free it yourself? Why not just remove the kfree() here altogether?
> >
> > In the for-loop block, the __free() work only when we exit the loop, not
> > each iteration. In each iteration, kstrdup() is assigned to the 'tmp',
> > so we need to kfree() each time.
>
> Really? It doesn't trigger for each iteration? That's rather unintuitive. :-/
> And sounds buggy, as wouldn't that then cause a memory leak?

Ahh, sorry, it was my misunderstood. I made a test code and confirmed that
kfree() is called in each iteration. Previously I checked but I confused the result.

----------
#include <stdio.h>

void count_func(int *p)
{
printf("Scope out: %d\n", *p);
}

int main(void)
{
for (int i = 0; i < 10; i++) {
int j __attribute((cleanup(count_func))) = 0;

j++;
}
return 0;
}
----------

$ ./loop_cleanup
Scope out: 1
Scope out: 1
Scope out: 1
Scope out: 1
Scope out: 1
Scope out: 1
Scope out: 1
Scope out: 1
Scope out: 1
Scope out: 1

Let me fix that.

Thanks,

>
> I would say not to use __free() for tmp at all. Because now it's just
> getting confusing.
>
> -- Steve
>
>
> >
> > Hmm, maybe this is a sign that I should not use __free() for the 'tmp',
> > or I should call kfree(tmp) right before kstrdup(), like below.
> >
> > for (i = 0; i < argc; i++) {
> > char *tmp __free(kfree) = NULL;
> > ...
> > kfree(tmp);
> > tmp = kstrdup(argv[i], GFP_KERNEL);
> > }
> >
> > Does this make sense?
>

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