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

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Message-Id: <20250108093843.dc5ea69f297a9fd99cf4c396@kernel.org>
Date: Wed, 8 Jan 2025 09:38:43 +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 10:36:43 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue,  7 Jan 2025 20:50:25 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > @@ -1790,18 +1777,15 @@ int traceprobe_expand_dentry_args(int argc, const char *argv[], char **buf)
> >  				       offsetof(struct file, f_path.dentry),
> >  				       equal ? equal + 1 : tmp);
> >  
> > -		kfree(tmp);
> > +		kfree(no_free_ptr(tmp));
> 
> 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.

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?

> 
> -- Steve
> 
> 
> >  		if (ret >= bufsize - used)
> > -			goto nomem;
> > +			return -ENOMEM;
> >  		argv[i] = tmpbuf + used;
> >  		used += ret + 1;
> >  	}
> >  
> > -	*buf = tmpbuf;
> > +	*buf = no_free_ptr(tmpbuf);
> >  	return 0;
> > -nomem:
> > -	kfree(tmpbuf);
> > -	return -ENOMEM;
> >  }

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