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

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Date:	Sun, 31 Dec 2006 14:38:32 +0100
From:	Bernd Petrovitsch <bernd@...mix.at>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	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.

On Thu, 2006-12-28 at 09:54 +0100, Jan Engelhardt wrote:
> On Dec 27 2006 17:10, Pavel Machek wrote:
> 
> >> Was just wondering if the _var_ in kfree(_var_) could be set to
> >> NULL after its freed. It may solve the problem of accessing some
> >> freed memory as the kernel will crash since _var_ was set to NULL.
> >> 
> >> Does this make sense? If yes, then how about renaming kfree to
> >> something else and providing a kfree macro that would do the
> >> following:
> >> 
> >> #define kfree(x) do { \
> >>                       new_kfree(x); \
> >>                       x = NULL; \
> >>                     } while(0)
> >> 
> >> There might be other better ways too.

----  snip  ----
(x) = NULL; \
----  snip  ----
?

> >No, that would be very confusing. Otoh having
> >KFREE() do kfree() and assignment might be acceptable.
> 
> What about setting x to some poison value from <linux/poison.h>?

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).
Personally I think it is pointless to disallow "kfree(NULL)" by using
some poison value and force people to add a "we have to free that
variable" variable to work around it instead of keeping it NULL (which
makes the "kfree($variable)" a no-op).
Former discussions are to be found in the archives ......

	Bernd
-- 
Firmix Software GmbH                   http://www.firmix.at/
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          Embedded Linux Development and Services

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