Re: [PATCH RFC v2 10/25] kmsan: define READ_ONCE_NOCHECK()

From: Alexander Potapenko <glider@google.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	 Linux Memory Management List <linux-mm@kvack.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	 Andrew Morton <akpm@linux-foundation.org>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	 Andy Lutomirski <luto@kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	 Arnd Bergmann <arnd@arndb.de>, Christoph Hellwig <hch@lst.de>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	 Eric Dumazet <edumazet@google.com>,
	Eric Van Hensbergen <ericvh@gmail.com>,
	 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Harry Wentland <harry.wentland@amd.com>,
	 Herbert Xu <herbert@gondor.apana.org.au>,
	Ingo Molnar <mingo@elte.hu>,  Jens Axboe <axboe@kernel.dk>,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	 Martin Schwidefsky <schwidefsky@de.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Michal Simek <monstr@monstr.eu>,  Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	 Steven Rostedt <rostedt@goodmis.org>,
	Takashi Iwai <tiwai@suse.com>, "Theodore Ts'o" <tytso@mit.edu>,
	 Thomas Gleixner <tglx@linutronix.de>,
	Wolfram Sang <wsa@the-dreams.de>,
	 Vasily Gorbik <gor@linux.ibm.com>,
	Ilya Leoshkevich <iii@linux.ibm.com>,
	 Matthew Wilcox <willy@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	 Andrey Konovalov <andreyknvl@google.com>,
	Marco Elver <elver@google.com>
Subject: Re: [PATCH RFC v2 10/25] kmsan: define READ_ONCE_NOCHECK()
Date: Thu, 21 Nov 2019 13:08:58 +0100	[thread overview]
Message-ID: <CAG_fn=UVnTxJEdn8QOMgNQ6dUg0+Ue_86ew3XCMq57-sgQGUaQ@mail.gmail.com> (raw)
In-Reply-To: <20191105142155.GA45550@lakrids.cambridge.arm.com>

On Tue, Nov 5, 2019 at 9:19 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Wed, Oct 30, 2019 at 03:22:22PM +0100, glider@google.com wrote:
> > READ_ONCE_NOCHECK() is already used by KASAN to ignore memory accesses
> > from e.g. stack unwinders.
> > Define READ_ONCE_NOCHECK() for KMSAN so that it returns initialized
> > values. This helps defeat false positives from leftover stack contents.
> >
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > To: Alexander Potapenko <glider@google.com>
> > Cc: Vegard Nossum <vegard.nossum@oracle.com>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: linux-mm@kvack.org
> > ---
> >
> > Change-Id: Ib38369ba038ab3b581d8e45b81036c3304fb79cb
> > ---
> >  include/linux/compiler.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 5e88e7e33abe..e8c86debdb2b 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -270,9 +270,13 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
> >
> >  /*
> >   * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
> > - * to hide memory access from KASAN.
> > + * to hide memory access from KASAN or KMSAN.
> >   */
> > +#ifndef CONFIG_KMSAN
> >  #define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
> > +#else
> > +#define READ_ONCE_NOCHECK(x) KMSAN_INIT_VALUE(__READ_ONCE(x, 0))
> > +#endif
>
> When !CONFIG_KMSAN, we have:
>
> | #define KMSAN_INIT_VALUE(value) (value)
>
> ... so we don't need ifdeffery here, and can simply have:
>
> /*
>  * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
>  * to hide memory access from KASAN or KMSAN.
>  */
> #define READ_ONCE_NOCHECK(x) KMSAN_INIT_VALUE(__READ_ONCE(x, 0))
Agreed, thanks!
> Thanks,
> Mark.
>
> >
> >  static __no_kasan_or_inline
> >  unsigned long read_word_at_a_time(const void *addr)
> > --
> > 2.24.0.rc0.303.g954a862665-goog
> >


-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg