[PATCH 4.19 11/29] powerpc/bitops: Fix possible undefined behaviour with fls() and fls64()

Greg Kroah-Hartman
2021-01-05T09:28:52Z

From: Christophe Leroy <christophe.leroy@csgroup.eu>

[ Upstream commit 1891ef21d92c4801ea082ee8ed478e304ddc6749 ]

fls() and fls64() are using __builtin_ctz() and _builtin_ctzll().
On powerpc, those builtins trivially use ctlzw and ctlzd power
instructions.

Allthough those instructions provide the expected result with
input argument 0, __builtin_ctz() and __builtin_ctzll() are
documented as undefined for value 0.

The easiest fix would be to use fls() and fls64() functions
defined in include/asm-generic/bitops/builtin-fls.h and
include/asm-generic/bitops/fls64.h, but GCC output is not optimal:

00000388 <testfls>:
388: 2c 03 00 00 cmpwi r3,0
38c: 41 82 00 10 beq 39c <testfls+0x14>
390: 7c 63 00 34 cntlzw r3,r3
394: 20 63 00 20 subfic r3,r3,32
398: 4e 80 00 20 blr
39c: 38 60 00 00 li r3,0
3a0: 4e 80 00 20 blr

000003b0 <testfls64>:
3b0: 2c 03 00 00 cmpwi r3,0
3b4: 40 82 00 1c bne 3d0 <testfls64+0x20>
3b8: 2f 84 00 00 cmpwi cr7,r4,0
3bc: 38 60 00 00 li r3,0
3c0: 4d 9e 00 20 beqlr cr7
3c4: 7c 83 00 34 cntlzw r3,r4
3c8: 20 63 00 20 subfic r3,r3,32
3cc: 4e 80 00 20 blr
3d0: 7c 63 00 34 cntlzw r3,r3
3d4: 20 63 00 40 subfic r3,r3,64
3d8: 4e 80 00 20 blr

When the input of fls(x) is a constant, just check x for nullity and
return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction
directly.

For fls64() on PPC64, do the same but with __builtin_clzll() and
cntlzd instruction. On PPC32, lets take the generic fls64() which
will use our fls(). The result is as expected:

00000388 <testfls>:
388: 7c 63 00 34 cntlzw r3,r3
38c: 20 63 00 20 subfic r3,r3,32
390: 4e 80 00 20 blr

000003a0 <testfls64>:
3a0: 2c 03 00 00 cmpwi r3,0
3a4: 40 82 00 10 bne 3b4 <testfls64+0x14>
3a8: 7c 83 00 34 cntlzw r3,r4
3ac: 20 63 00 20 subfic r3,r3,32
3b0: 4e 80 00 20 blr
3b4: 7c 63 00 34 cntlzw r3,r3
3b8: 20 63 00 40 subfic r3,r3,64
3bc: 4e 80 00 20 blr

Fixes: 2fcff790dcb4 ("powerpc: Use builtin functions for fls()/__fls()/fls64()")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/348c2d3f19ffcff8abe50d52513f989c4581d000.1603375524.git.christophe.leroy@csgroup.eu
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/powerpc/include/asm/bitops.h | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/bitops.h b/arch/powerpc/include/asm/bitops.h
index ff71566dadee5..76db1c5000bd6 100644
--- a/arch/powerpc/include/asm/bitops.h
+++ b/arch/powerpc/include/asm/bitops.h
@@ -221,15 +221,34 @@ static __inline__ void __clear_bit_unlock(int nr, volatile unsigned long *addr)
*/
static __inline__ int fls(unsigned int x)
{
- return 32 - __builtin_clz(x);
+ int lz;
+
+ if (__builtin_constant_p(x))
+ return x ? 32 - __builtin_clz(x) : 0;
+ asm("cntlzw %0,%1" : "=r" (lz) : "r" (x));
+ return 32 - lz;
}

#include <asm-generic/bitops/builtin-__fls.h>

+/*
+ * 64-bit can do this using one cntlzd (count leading zeroes doubleword)
+ * instruction; for 32-bit we use the generic version, which does two
+ * 32-bit fls calls.
+ */
+#ifdef CONFIG_PPC64
static __inline__ int fls64(__u64 x)
{
- return 64 - __builtin_clzll(x);
+ int lz;
+
+ if (__builtin_constant_p(x))
+ return x ? 64 - __builtin_clzll(x) : 0;
+ asm("cntlzd %0,%1" : "=r" (lz) : "r" (x));
+ return 64 - lz;
}
+#else
+#include <asm-generic/bitops/fls64.h>
+#endif

#ifdef CONFIG_PPC64
unsigned int __arch_hweight8(unsigned int w);
--
2.27.0