Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220630080834.2742777-1-davidgow@google.com> <20220630080834.2742777-2-davidgow@google.com>
 <CACT4Y+ZahTu0pGNSdZmx=4ZJHt4=mVuhxQnH_7ykDA5_fBJZVQ@mail.gmail.com>
 <20220630125434.GA20153@axis.com> <CA+fCnZe6zk8WQ7FkCsnMPLpDW2+wJcjdcrs5fxJRh+T=FvFDVA@mail.gmail.com>
In-Reply-To: <CA+fCnZe6zk8WQ7FkCsnMPLpDW2+wJcjdcrs5fxJRh+T=FvFDVA@mail.gmail.com>
From:   David Gow <davidgow@google.com>
Date:   Fri, 1 Jul 2022 17:08:27 +0800
Message-ID: <CABVgOSmxnTc31C-gbmbns+8YOkpppK77sdXLzASZ-hspFYDwfA@mail.gmail.com>
Subject: Re: [PATCH v4 2/2] UML: add support for KASAN under x86_64
To:     Andrey Konovalov <andreyknvl@gmail.com>
Cc:     Vincent Whitchurch <vincent.whitchurch@axis.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        Patricia Alfonso <trishalfonso@google.com>,
        Jeff Dike <jdike@addtoit.com>,
        Richard Weinberger <richard@nod.at>,
        "anton.ivanov@cambridgegreys.com" <anton.ivanov@cambridgegreys.com>,
        Brendan Higgins <brendanhiggins@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        "linux-um@lists.infradead.org" <linux-um@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Daniel Latypov <dlatypov@google.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "kunit-dev@googlegroups.com" <kunit-dev@googlegroups.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Jun 30, 2022 at 9:29 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
>
> On Thu, Jun 30, 2022 at 2:54 PM Vincent Whitchurch
> <vincent.whitchurch@axis.com> wrote:
> >
> > On Thu, Jun 30, 2022 at 11:41:04AM +0200, Dmitry Vyukov wrote:
> > > On Thu, 30 Jun 2022 at 10:08, David Gow <davidgow@google.com> wrote:
> > > > diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
> > > > index 1c2d4b29a3d4..a089217e2f0e 100644
> > > > --- a/arch/um/kernel/Makefile
> > > > +++ b/arch/um/kernel/Makefile
> > > > @@ -27,6 +27,9 @@ obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
> > > >  obj-$(CONFIG_STACKTRACE) += stacktrace.o
> > > >  obj-$(CONFIG_GENERIC_PCI_IOMAP) += ioport.o
> > > >
> > > > +KASAN_SANITIZE_stacktrace.o := n
> > > > +KASAN_SANITIZE_sysrq.o := n
> > >
> > > Why are these needed?
> > > It's helpful to leave some comments for any of *_SANITIZE:=n.
> > > Otherwise later it's unclear if it's due to some latent bugs, some
> > > inherent incompatibility, something that can be fixed, etc.
> >
> > I believe I saw the stacktrace code itself triggering KASAN splats and
> > causing recursion when sanitization was not disabled on it.  I noticed
> > that other architectures disabled sanitization of their stacktrace code,
> > eg. ARM in commit 4d576cab16f57e1f87978f ("ARM: 9028/1: disable KASAN in
> > call stack capturing routines"), so I did not investigate it further.
> >
> > (Note that despite the name, sysrq.c is also just stacktrace code.)
>
> Stack trace collection code might trigger KASAN splats when walking
> stack frames, but this can be resolved by using unchecked accesses.
> The main reason to disable instrumentation here is for performance
> reasons, see the upcoming patch for arm64 [1] for some details.
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?id=802b91118d11

Ah -- that does it! Using READ_ONCE_NOCHECK() in dump_trace() gets rid
of the nasty recursive KASAN failures we were getting in the tests.

I'll send out v5 with those files instrumented again.

Thanks!
-- David