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Date: Wed, 10 Jun 2020 08:28:17 -0400
From: Qian Cai <cai@lca.pw>
Subject: Re: [PATCH] mm/page_alloc: silence a KASAN false positive
Message-ID: <20200610122817.GC954@lca.pw>
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To: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>, 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>, Heiko Carstens <heiko.carstens@de.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>

On Wed, Jun 10, 2020 at 01:02:04PM +0200, Alexander Potapenko wrote:
> 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.

I suppose that could work if s390 maintains perfer this way.