linux-kernel - Re: [PATCH] [DISCUSS] Make the variable NULL after freeing it.

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Date:	Sun, 31 Dec 2006 22:37:22 -0800 (PST)
From:	Amit Choudhary <amit2030@...oo.com>
To:	Ingo Oeser <ioe-lkml@...eria.de>,
	Bernd Petrovitsch <bernd@...mix.at>
Cc:	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Pavel Machek <pavel@....cz>,
	Amit Choudhary <amit2030@...oo.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [DISCUSS] Make the variable NULL after freeing it.

--- Ingo Oeser <ioe-lkml@...eria.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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