linux-kernel - Re: [PATCH v3 2/5] tracing: Use __free() in trace

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Message-Id: <20250108110322.46e91dd1ed354e2b146b8e5e@kernel.org>
Date: Wed, 8 Jan 2025 11:03:22 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Anil S Keshavamurthy
 <anil.s.keshavamurthy@...el.com>, "David S . Miller" <davem@...emloft.net>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Oleg Nesterov
 <oleg@...hat.com>, Tzvetomir Stoyanov <tz.stoyanov@...il.com>, Naveen N Rao
 <naveen@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, Jason Baron
 <jbaron@...mai.com>, Ard Biesheuvel <ardb@...nel.org>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/5] tracing: Use __free() in trace_probe for cleanup

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

> On Wed, 8 Jan 2025 09:38:43 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.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@...nel.org>