Messages in this thread | | | Subject | Re: [PATCH] x86: Make variable_test_bit reference all of *addr | From | "" <> | Date | Thu, 06 Oct 2011 18:37:27 -0700 |
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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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