linux-kernel - Re: [patch 2/3] MAP_NOZERO

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Message-ID: <4681E37A.9090708@redhat.com>
Date:	Wed, 27 Jun 2007 00:11:38 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Ulrich Drepper <drepper@...il.com>
CC:	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 2/3] MAP_NOZERO - implement sys_brk2()

Ulrich Drepper wrote:
> On 6/26/07, Rik van Riel <riel@...hat.com> wrote:
>> Since programs can get back free()d memory after a malloc(),
>> with the old contents of the memory intact, surely your
>> MAP_NONZERO behavior could be the default for programs that
>> can get away with it?
>>
>> Maybe we could use some magic ELF header, similar to the
>> way non-executable stack is handled?
> 
> No.  This is an implementation detail of the libc version.  The malloc
> as compiled today is expecting brk-ed memory to be zeroed.  This
> default can of course be changed (it's a simple define) but you cannot
> make this the default behavior for brk.

After going through the first malloc()/free() cycle, surely
the memory will no longer be zeroed on the second malloc() ?

What makes the first brk malloc so special?

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