linux-kernel - Re: kfree a pointer "from the middle" causing protection faults

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Message-ID: <CAKdAkRTFhSKLB40snpC_10adAPdqPkB_tu2Kg=cvo60H=O8=ww@mail.gmail.com>
Date:	Wed, 2 Sep 2015 09:29:57 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Muni Sekhar <munisekharrms@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>
Subject: Re: kfree a pointer "from the middle" causing protection faults

On Wed, Sep 2, 2015 at 8:02 AM, Muni Sekhar <munisekharrms@...il.com> wrote:
>  [ Please keep me in CC as I'm not subscribed to the list]
>
> Hello,
>
>
> I am getting protection faults in different kernel modules if I try to
> free a pointer "from the middle" for example, look at the following
> code:
>
>
> u8 *buf;
>
>
> buf = kzalloc( 100 , GFP_KERNEL );
>
> …
>
> buf = buf + 50;
>
> …
>
> Kfree(buf);
>
>
>
>
> I would like to know, why the above code is causing protection faults
> in other kernel modules?

Because you can not make a "hole" in returned memory like that, you
can only free entire block allocated by k*alloc and you need to pass
the original pointer in.

Thanks.

-- 
Dmitry
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