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>

diff --git a/arch/arc/include/asm/bitops.h b/arch/arc/include/asm/bitops.h
index 99fe118..57c1f33 100644 (file)
--- 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);
 }