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From:   Alexander Potapenko <glider@google.com>
Date:   Wed, 10 Jun 2020 13:02:04 +0200
Message-ID: <CAG_fn=X-da3V0OC-Bzd2rmkNuZ_bVpH_n7Sp5P_hSGXD4ryyBA@mail.gmail.com>
Subject: Re: [PATCH] mm/page_alloc: silence a KASAN false positive
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Qian Cai <cai@lca.pw>, Andrew Morton <akpm@linux-foundation.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Kees Cook <keescook@chromium.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux-MM <linux-mm@kvack.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jun 10, 2020 at 7:55 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Wed, Jun 10, 2020 at 7:22 AM Qian Cai <cai@lca.pw> wrote:
> >
> > kernel_init_free_pages() will use memset() on s390 to clear all pages
> > from kmalloc_order() which will override KASAN redzones because a
> > redzone was setup from the end of the allocation size to the end of the
> > last page. Silence it by not reporting it there. An example of the
> > report is,
>
> Interesting. The reason why we did not hit it on x86_64 is because
> clear_page is implemented in asm (arch/x86/lib/clear_page_64.S) and
> thus is not instrumented. Arm64 probably does the same. However, on
> s390 clear_page is defined to memset.

Can we define it to __memset() instead?
__memset() is supposed to be ignored by KASAN, e.g. KASAN runtime uses
it in the places where we don't care about bugs.