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Date:   Wed, 29 May 2019 12:01:16 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
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Subject: Re: [PATCH 3/3] asm-generic, x86: Add bitops instrumentation for
 KASAN
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References: <20190528163258.260144-1-elver@google.com>
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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.