Re: [PATCH] x86: Make variable_test_bit reference all of *addr

From: hpanvin@xxxxxxxxx
Date: Thu Oct 06 2011 - 21:38:04 EST

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@xxxxxxxxxx> 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 âcan_boost.part.1â:
>[...]/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
>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@xxxxxxxxxx>
>Cc: Jakub Jelinek <jakub@xxxxxxxxxx>
>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
>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
>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;
> }

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