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Date: Sun, 31 Dec 2006 22:37:22 -0800 (PST)
From: Amit Choudhary <amit2030@yahoo.com>
Subject: Re: [PATCH] [DISCUSS] Make the variable NULL after freeing it.
To: Ingo Oeser <ioe-lkml@rameria.de>, Bernd Petrovitsch <bernd@firmix.at>
Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>, Pavel Machek <pavel@ucw.cz>,
       Amit Choudhary <amit2030@yahoo.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>
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--- Ingo Oeser <ioe-lkml@rameria.de> wrote:

> On Sunday, 31. December 2006 14:38, Bernd Petrovitsch wrote:
> > That depends on the decision/definition if (so called) "double free" is
> > an error or not (and "free(NULL)" must work in POSIX-compliant
> > environments).
> 
> A double free of non-NULL is certainly an error.
> So the idea of setting it to NULL is ok, since then you can
> kfree the variable over and over again without any harm.
> 
> It is just complicated to do this side effect free.
> 
> Maybe one should check for builtin-constant and take the address,
> if this is not an builtin-constant.
> 
> sth, like this
> 
> #define kfree_nullify(x) do { \
> 	if (__builtin_constant_p(x)) { \
> 		kfree(x); \
> 	} else { \
> 		typeof(x) *__addr_x = &x; \
> 		kfree(*__addr_x); \
> 		*__addr_x = NULL; \
> 	} \
> } while (0)
> 
> Regards
> 
> Ingo Oeser
> 

This is a nice approach but what if someone does kfree_nullify(x+20).

I decided to keep it simple. If someone is calling kfree_nullify() with anything other than a
simple variable, then they should call kfree().  But definitely an approach that takes care of all
situations is the best but I cannot think of a macro that can handle all situations. The simple
macro that I sent earlier will catch all the other usage at compile time. Please let me know if I
have missed something.

-Amit


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