linux-kernel - Re: [PATCH v3 33/35] codetag: debug: mark codetags for reserved pages as empty

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Message-ID: <CAJuCfpEoS=ea90EHHc-Kwg3G3_ZWsVgKvhRiZ4SVuGARBe=vnA@mail.gmail.com>
Date: Mon, 12 Feb 2024 16:15:12 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Kees Cook <keescook@...omium.org>
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Subject: Re: [PATCH v3 33/35] codetag: debug: mark codetags for reserved pages
 as empty

On Mon, Feb 12, 2024 at 2:45 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Mon, Feb 12, 2024 at 01:39:19PM -0800, Suren Baghdasaryan wrote:
> > To avoid debug warnings while freeing reserved pages which were not
> > allocated with usual allocators, mark their codetags as empty before
> > freeing.
>
> How do these get their codetags to begin with?

The space for the codetag reference is inside the page_ext and that
reference is set to NULL. So, unless we set the reference as empty
(set it to CODETAG_EMPTY), the free routine will detect that we are
freeing an allocation that has never been accounted for and will issue
a warning. To prevent this warning we use this CODETAG_EMPTY to denote
that this codetag reference is expected to be empty because it was not
allocated in a usual way.

> Regardless:
>
> Reviewed-by: Kees Cook <keescook@...omium.org>
>
> --
> Kees Cook