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author | Vineet Gupta <vgupta@synopsys.com> | 2015-07-03 05:56:22 (GMT) |
---|---|---|
committer | Vineet Gupta <vgupta@synopsys.com> | 2015-07-09 12:06:32 (GMT) |
commit | 80f420842ff42ad61f84584716d74ef635f13892 (patch) | |
tree | 7896f2d00bbac0afef8a1e8f6a3b12d05df27124 /arch | |
parent | e2fc61f384c9224b55990a644bbcf68c25e20203 (diff) | |
download | linux-80f420842ff42ad61f84584716d74ef635f13892.tar.xz |
ARC: Make ARC bitops "safer" (add anti-optimization)
ARCompact/ARCv2 ISA provide that any instructions which deals with
bitpos/count operand ASL, LSL, BSET, BCLR, BMSK .... will only consider
lower 5 bits. i.e. auto-clamp the pos to 0-31.
ARC Linux bitops exploited this fact by NOT explicitly masking out upper
bits for @nr operand in general, saving a bunch of AND/BMSK instructions
in generated code around bitops.
While this micro-optimization has worked well over years it is NOT safe
as shifting a number with a value, greater than native size is
"undefined" per "C" spec.
So as it turns outm EZChip ran into this eventually, in their massive
muti-core SMP build with 64 cpus. There was a test_bit() inside a loop
from 63 to 0 and gcc was weirdly optimizing away the first iteration
(so it was really adhering to standard by implementing undefined behaviour
vs. removing all the iterations which were phony i.e. (1 << [63..32])
| for i = 63 to 0
| X = ( 1 << i )
| if X == 0
| continue
So fix the code to do the explicit masking at the expense of generating
additional instructions. Fortunately, this can be mitigated to a large
extent as gcc has SHIFT_COUNT_TRUNCATED which allows combiner to fold
masking into shift operation itself. It is currently not enabled in ARC
gcc backend, but could be done after a bit of testing.
Fixes STAR 9000866918 ("unsafe "undefined behavior" code in kernel")
Reported-by: Noam Camus <noamc@ezchip.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Diffstat (limited to 'arch')
-rw-r--r-- | arch/arc/include/asm/bitops.h | 35 |
1 files changed, 9 insertions, 26 deletions
diff --git a/arch/arc/include/asm/bitops.h b/arch/arc/include/asm/bitops.h index 99fe118..57c1f33 100644 --- a/arch/arc/include/asm/bitops.h +++ b/arch/arc/include/asm/bitops.h @@ -50,8 +50,7 @@ static inline void op##_bit(unsigned long nr, volatile unsigned long *m)\ * done for const @nr, but no code is generated due to gcc \ * const prop. \ */ \ - if (__builtin_constant_p(nr)) \ - nr &= 0x1f; \ + nr &= 0x1f; \ \ __asm__ __volatile__( \ "1: llock %0, [%1] \n" \ @@ -82,8 +81,7 @@ static inline int test_and_##op##_bit(unsigned long nr, volatile unsigned long * \ m += nr >> 5; \ \ - if (__builtin_constant_p(nr)) \ - nr &= 0x1f; \ + nr &= 0x1f; \ \ /* \ * Explicit full memory barrier needed before/after as \ @@ -129,16 +127,13 @@ static inline void op##_bit(unsigned long nr, volatile unsigned long *m)\ unsigned long temp, flags; \ m += nr >> 5; \ \ - if (__builtin_constant_p(nr)) \ - nr &= 0x1f; \ - \ /* \ * spin lock/unlock provide the needed smp_mb() before/after \ */ \ bitops_lock(flags); \ \ temp = *m; \ - *m = temp c_op (1UL << nr); \ + *m = temp c_op (1UL << (nr & 0x1f)); \ \ bitops_unlock(flags); \ } @@ -149,17 +144,14 @@ static inline int test_and_##op##_bit(unsigned long nr, volatile unsigned long * unsigned long old, flags; \ m += nr >> 5; \ \ - if (__builtin_constant_p(nr)) \ - nr &= 0x1f; \ - \ bitops_lock(flags); \ \ old = *m; \ - *m = old c_op (1 << nr); \ + *m = old c_op (1UL << (nr & 0x1f)); \ \ bitops_unlock(flags); \ \ - return (old & (1 << nr)) != 0; \ + return (old & (1UL << (nr & 0x1f))) != 0; \ } #endif /* CONFIG_ARC_HAS_LLSC */ @@ -174,11 +166,8 @@ static inline void __##op##_bit(unsigned long nr, volatile unsigned long *m) \ unsigned long temp; \ m += nr >> 5; \ \ - if (__builtin_constant_p(nr)) \ - nr &= 0x1f; \ - \ temp = *m; \ - *m = temp c_op (1UL << nr); \ + *m = temp c_op (1UL << (nr & 0x1f)); \ } #define __TEST_N_BIT_OP(op, c_op, asm_op) \ @@ -187,13 +176,10 @@ static inline int __test_and_##op##_bit(unsigned long nr, volatile unsigned long unsigned long old; \ m += nr >> 5; \ \ - if (__builtin_constant_p(nr)) \ - nr &= 0x1f; \ - \ old = *m; \ - *m = old c_op (1 << nr); \ + *m = old c_op (1UL << (nr & 0x1f)); \ \ - return (old & (1 << nr)) != 0; \ + return (old & (1UL << (nr & 0x1f))) != 0; \ } #define BIT_OPS(op, c_op, asm_op) \ @@ -224,10 +210,7 @@ test_bit(unsigned int nr, const volatile unsigned long *addr) addr += nr >> 5; - if (__builtin_constant_p(nr)) - nr &= 0x1f; - - mask = 1 << nr; + mask = 1UL << (nr & 0x1f); return ((mask & *addr) != 0); } |