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Date: Sat, 8 Aug 2015 15:09:33 -0700
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
stable@...r.kernel.org, Noam Camus <noamc@...hip.com>,
Vineet Gupta <vgupta@...opsys.com>
Subject: [PATCH 4.1 095/123] ARC: Make ARC bitops "safer" (add anti-optimization)
4.1-stable review patch. If anyone has any objections, please let me know.
------------------
From: Vineet Gupta <vgupta@...opsys.com>
commit 80f420842ff42ad61f84584716d74ef635f13892 upstream.
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@...hip.com>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
arch/arc/include/asm/bitops.h | 35 +++++++++--------------------------
1 file changed, 9 insertions(+), 26 deletions(-)
--- a/arch/arc/include/asm/bitops.h
+++ b/arch/arc/include/asm/bitops.h
@@ -50,8 +50,7 @@ static inline void op##_bit(unsigned lon
* 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(un
\
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 lon
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(un
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
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 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
addr += nr >> 5;
- if (__builtin_constant_p(nr))
- nr &= 0x1f;
-
- mask = 1 << nr;
+ mask = 1UL << (nr & 0x1f);
return ((mask & *addr) != 0);
}
--
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