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    Subject[PATCH 8/8] asm-generic, x86: add comments for atomic instrumentation
    Date
    The comments are factored out from the code changes to make them
    easier to read. Add them separately to explain some non-obvious
    aspects.

    Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
    Cc: Mark Rutland <mark.rutland@arm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: kasan-dev@googlegroups.com
    Cc: linux-mm@kvack.org
    Cc: linux-kernel@vger.kernel.org
    Cc: x86@kernel.org
    ---
    arch/x86/include/asm/atomic.h | 7 +++++++
    include/asm-generic/atomic-instrumented.h | 30 ++++++++++++++++++++++++++++++
    2 files changed, 37 insertions(+)

    diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
    index 92dd59f24eba..b2a2220c7ac2 100644
    --- a/arch/x86/include/asm/atomic.h
    +++ b/arch/x86/include/asm/atomic.h
    @@ -23,6 +23,13 @@
    */
    static __always_inline int arch_atomic_read(const atomic_t *v)
    {
    + /*
    + * Note: READ_ONCE() here leads to double instrumentation as
    + * both READ_ONCE() and atomic_read() contain instrumentation.
    + * This is a deliberate choice. READ_ONCE_NOCHECK() is compiled to a
    + * non-inlined function call that considerably increases binary size
    + * and stack usage under KASAN.
    + */
    return READ_ONCE((v)->counter);
    }

    diff --git a/include/asm-generic/atomic-instrumented.h b/include/asm-generic/atomic-instrumented.h
    index 7f8eb761f896..1134af090976 100644
    --- a/include/asm-generic/atomic-instrumented.h
    +++ b/include/asm-generic/atomic-instrumented.h
    @@ -1,3 +1,15 @@
    +/*
    + * This file provides wrappers with KASAN instrumentation for atomic operations.
    + * To use this functionality an arch's atomic.h file needs to define all
    + * atomic operations with arch_ prefix (e.g. arch_atomic_read()) and include
    + * this file at the end. This file provides atomic_read() that forwards to
    + * arch_atomic_read() for actual atomic operation.
    + * Note: if an arch atomic operation is implemented by means of other atomic
    + * operations (e.g. atomic_read()/atomic_cmpxchg() loop), then it needs to use
    + * arch_ variants (i.e. arch_atomic_read()/arch_atomic_cmpxchg()) to avoid
    + * double instrumentation.
    + */
    +
    #ifndef _LINUX_ATOMIC_INSTRUMENTED_H
    #define _LINUX_ATOMIC_INSTRUMENTED_H

    @@ -339,6 +351,15 @@ static __always_inline bool atomic64_add_negative(long long i, atomic64_t *v)
    return arch_atomic64_add_negative(i, v);
    }

    +/*
    + * In the following macros we need to be careful to not clash with arch_ macros.
    + * arch_xchg() can be defined as an extended statement expression as well,
    + * if we define a __ptr variable, and arch_xchg() also defines __ptr variable,
    + * and we pass __ptr as an argument to arch_xchg(), it will use own __ptr
    + * instead of ours. This leads to unpleasant crashes. To avoid the problem
    + * the following macros declare variables with lots of underscores.
    + */
    +
    #define cmpxchg(ptr, old, new) \
    ({ \
    __typeof__(ptr) ___ptr = (ptr); \
    @@ -374,6 +395,15 @@ static __always_inline bool atomic64_add_negative(long long i, atomic64_t *v)
    arch_cmpxchg64_local(____ptr, (old), (new)); \
    })

    +/*
    + * Originally we had the following code here:
    + * __typeof__(p1) ____p1 = (p1);
    + * kasan_check_write(____p1, 2 * sizeof(*____p1));
    + * arch_cmpxchg_double(____p1, (p2), (o1), (o2), (n1), (n2));
    + * But it leads to compilation failures (see gcc issue 72873).
    + * So for now it's left non-instrumented.
    + * There are few callers of cmpxchg_double(), so it's not critical.
    + */
    #define cmpxchg_double(p1, p2, o1, o2, n1, n2) \
    ({ \
    arch_cmpxchg_double((p1), (p2), (o1), (o2), (n1), (n2)); \
    --
    2.12.2.564.g063fe858b8-goog
    \
     
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