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    SubjectRe: [PATCH] x86: Make variable_test_bit reference all of *addr
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    This is concerning... the kernel relies heavily on asm volatile being a universal memory consumer.  If that is suddenly broken, we are f*** in many, many, MANY places in the kernel all of a sudden!

    Josh Stone <jistone@redhat.com> wrote:

    >This casts the addr that's fed to asm into a struct-array pointer, so
    >gcc knows that more than just the first long is needed. Since there's
    >no fixed size for all callers, an arbitrary size is chosen just to
    >ensure that it's probably good enough.
    >
    >I noticed this warning on i686, with gcc-4.6.1-9.fc15:
    >
    > CC arch/x86/kernel/kprobes.o
    >In file included from include/linux/bitops.h:22:0,
    > from include/linux/kernel.h:17,
    > from [...]/arch/x86/include/asm/percpu.h:44,
    > from [...]/arch/x86/include/asm/current.h:5,
    > from [...]/arch/x86/include/asm/processor.h:15,
    > from [...]/arch/x86/include/asm/atomic.h:6,
    > from include/linux/atomic.h:4,
    > from include/linux/mutex.h:18,
    > from include/linux/notifier.h:13,
    > from include/linux/kprobes.h:34,
    > from arch/x86/kernel/kprobes.c:43:
    >[...]/arch/x86/include/asm/bitops.h: In function ‘can_boost.part.1’:
    >[...]/arch/x86/include/asm/bitops.h:319:2: warning: use of memory input
    >without lvalue in asm operand 1 is deprecated [enabled by default]
    >
    >In investigating the impact of this warning, I discovered that only the
    >first long of the 32-byte twobyte_is_boostable[] was making into the
    >object file.
    >
    >Jakub advised that variable_test_bit is incorrectly telling gcc that
    >its
    >asm only uses a single long from the addr pointer, and he suggested the
    >struct-array cast to broaden the memory reference.
    >
    >Signed-off-by: Josh Stone <jistone@redhat.com>
    >Cc: Jakub Jelinek <jakub@redhat.com>
    >
    >---
    >
    >An alternate fix would be to make kprobes' twobyte_is_boostable[]
    >volatile, which forces gcc to keep it around. I feel that's treating
    >the symptom though, rather than the cause in variable_test_bit().
    >
    >IMO this is also a good candidate for -stable, for fixing the obviously
    >bad
    >data behavior, but I'll let others judge...
    >
    >---
    > arch/x86/include/asm/bitops.h | 3 ++-
    > 1 files changed, 2 insertions(+), 1 deletions(-)
    >
    >diff --git a/arch/x86/include/asm/bitops.h
    >b/arch/x86/include/asm/bitops.h
    >index 1775d6e..0565371 100644
    >--- a/arch/x86/include/asm/bitops.h
    >+++ b/arch/x86/include/asm/bitops.h
    >@@ -319,7 +319,8 @@ static inline int variable_test_bit(int nr,
    >volatile const unsigned long *addr)
    > asm volatile("bt %2,%1\n\t"
    > "sbb %0,%0"
    > : "=r" (oldbit)
    >- : "m" (*(unsigned long *)addr), "Ir" (nr));
    >+ : "m" (*(struct { unsigned long _[0x10000]; } *)addr),
    >+ "Ir" (nr));
    >
    > return oldbit;
    > }
    >--
    >1.7.6.4

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