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

From: Muni Sekhar
Date: Thu Sep 03 2015 - 00:25:15 EST

On Wed, Sep 2, 2015 at 9:39 PM, Jeff Epler <jepler@xxxxxxxxxxxxxx> wrote:
> On Wed, Sep 02, 2015 at 08:32:15PM +0530, Muni Sekhar 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:
> [..]
> Most memory allocators require the pointer eventually passed to the
> freeing function is the same pointer as the one returned from the
> allocating function. This is true for libc malloc/free, for instance.
> As far as I know, it is true for the Linux allocators such as kzalloc.
> The bug lies in whatever part of linux makes the invalid kfree call.
> I have not found any documentation that kernel kzalloc/kfree allow
> passing a pointer "from the middle". For instance,
> These routines are used to dynamically request pointer-aligned chunks of
> memory, like malloc and free do in userspace
> If the faulty code that you allude to is in the Linux source then please
> say what it is so that developers can fix it. If it's an out of source
> module or kernel patch then contact the supplier of that code.

The faulty code mentioned above is not in the Linux source, I noticed
this behaviour during testing our own module. Thanks for the
clarification Jeff.

> Jeff
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