linux-kernel - Re: [PATCH] include/linux/slab.h: new KFREE() macro.

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Date:	Tue, 9 Jan 2007 16:00:51 -0800 (PST)
From:	Amit Choudhary <amit2030@...oo.com>
To:	Valdis.Kletnieks@...edu
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Hua Zhong <hzhong@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] include/linux/slab.h: new KFREE() macro.

--- Valdis.Kletnieks@...edu wrote:

> On Tue, 09 Jan 2007 11:02:35 PST, Amit Choudhary said:
> > Correct. And doing kfree(x); x=NULL; is not hiding that. These issues can still be debugged by
> > using the slab debugging options. One other benefit of doing this is that if someone tries to
> > access the same memory again using the variable 'x', then he will get an immediate crash.

What did you understand when I wrote that "if you access the same memory again using the variable
'x"?

Using variable 'x' means using variable 'x' and not variable 'y'.

 And
> the
> > problem can be solved immediately, without using the slab debugging options. I do not yet
> > understand how doing this hides the bugs, obfuscates the code, etc. because I haven't seen an
> > example yet, but only blanket statements.
> 
> char *broken() {
> 	char *x, *y;
> 	x = kmalloc(100);
> 	y = x;
> 	kfree(x);
> 	x = NULL;
> 	return y;
> }
> 
> Setting x to NULL doesn't do anything to fix the *real* bug here, because
> the problematic reference is held in y, not x.  

Did I ever say that it fixes that kind of bug?

>So you never get a crash
> because somebody dereferences x.
> 

Dereferencing 'x' means dereferencing 'x' and not dereferencing 'y'.

-Amit

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