Re: [PATCH 3/3] asm-generic, x86: Add bitops instrumentation for KASAN

From: Peter Zijlstra <peterz@infradead.org>
To: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Alexander Potapenko <glider@google.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>
Subject: Re: [PATCH 3/3] asm-generic, x86: Add bitops instrumentation for KASAN
Date: Wed, 29 May 2019 12:30:10 +0200	[thread overview]
Message-ID: <20190529103010.GP2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CANpmjNMvwAny54udYCHfBw1+aphrQmiiTJxqDq7q=h+6fvpO4w@mail.gmail.com>

On Wed, May 29, 2019 at 12:16:31PM +0200, Marco Elver wrote:
> On Wed, 29 May 2019 at 12:01, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, May 29, 2019 at 11:20:17AM +0200, Marco Elver wrote:
> > > For the default, we decided to err on the conservative side for now,
> > > since it seems that e.g. x86 operates only on the byte the bit is on.
> >
> > This is not correct, see for instance set_bit():
> >
> > static __always_inline void
> > set_bit(long nr, volatile unsigned long *addr)
> > {
> >         if (IS_IMMEDIATE(nr)) {
> >                 asm volatile(LOCK_PREFIX "orb %1,%0"
> >                         : CONST_MASK_ADDR(nr, addr)
> >                         : "iq" ((u8)CONST_MASK(nr))
> >                         : "memory");
> >         } else {
> >                 asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
> >                         : : RLONG_ADDR(addr), "Ir" (nr) : "memory");
> >         }
> > }
> >
> > That results in:
> >
> >         LOCK BTSQ nr, (addr)
> >
> > when @nr is not an immediate.
> 
> Thanks for the clarification. Given that arm64 already instruments
> bitops access to whole words, and x86 may also do so for some bitops,
> it seems fine to instrument word-sized accesses by default. Is that
> reasonable?

Eminently -- the API is defined such; for bonus points KASAN should also
do alignment checks on atomic ops. Future hardware will #AC on unaligned
[*] LOCK prefix instructions.

(*) not entirely accurate, it will only trap when crossing a line.
    https://lkml.kernel.org/r/1556134382-58814-1-git-send-email-fenghua.yu@intel.com