On Fri, Mar 22, 2019 at 7:05 AM Arnd Bergmann arnd@arndb.de wrote:
clang points out with hundreds of warnings that the bitrev macros have a problem with constant input:
drivers/hwmon/sht15.c:187:11: error: variable '__x' is uninitialized when used within its own initialization [-Werror,-Wuninitialized] u8 crc = bitrev8(data->val_status & 0x0F); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ include/linux/bitrev.h:102:21: note: expanded from macro 'bitrev8' __constant_bitrev8(__x) : \ ~~~~~~~~~~~~~~~~~~~^~~~ include/linux/bitrev.h:67:11: note: expanded from macro '__constant_bitrev8' u8 __x = x; \ ~~~ ^
Both the bitrev and the __constant_bitrev macros use an internal variable named __x, which goes horribly wrong when passing one to the other.
Oh man, so if you have a macro that expands another macro, you can run into this issue. To see how this expands: https://godbolt.org/z/-khHN3
The obvious fix is to rename one of the variables, so this adds an extra '_'.
It seems we got away with this because
- there are only a few drivers using bitrev macros
- usually there are no constant arguments to those
- when they are constant, they tend to be either 0 or (unsigned)-1 (drivers/isdn/i4l/isdnhdlc.o, drivers/iio/amplifiers/ad8366.c) and give the correct result by pure chance.
In fact, the only driver that I could find that gets different results with this is drivers/net/wan/slic_ds26522.c, which in turn is a driver for fairly rare hardware (adding the maintainer to Cc for testing).
Cc: Zhao Qiang qiang.zhao@nxp.com Cc: Yalin Wang yalin.wang@sonymobile.com Cc: stable@vger.kernel.org Fixes: 556d2f055bf6 ("ARM: 8187/1: add CONFIG_HAVE_ARCH_BITREVERSE to support rbit instruction") Signed-off-by: Arnd Bergmann arnd@arndb.de
I would have preferred to just name it `y` rather than keep tacking on underscores, but it doesn't matter what color the bikeshed is. Thanks for the patch. Reviewed-by: Nick Desaulniers ndesaulniers@google.com
include/linux/bitrev.h | 46 +++++++++++++++++++++--------------------- 1 file changed, 23 insertions(+), 23 deletions(-)
diff --git a/include/linux/bitrev.h b/include/linux/bitrev.h index 50fb0dee23e8..d35b8ec1c485 100644 --- a/include/linux/bitrev.h +++ b/include/linux/bitrev.h @@ -34,41 +34,41 @@ static inline u32 __bitrev32(u32 x)
#define __constant_bitrev32(x) \ ({ \
u32 __x = x; \__x = (__x >> 16) | (__x << 16); \__x = ((__x & (u32)0xFF00FF00UL) >> 8) | ((__x & (u32)0x00FF00FFUL) << 8); \__x = ((__x & (u32)0xF0F0F0F0UL) >> 4) | ((__x & (u32)0x0F0F0F0FUL) << 4); \__x = ((__x & (u32)0xCCCCCCCCUL) >> 2) | ((__x & (u32)0x33333333UL) << 2); \__x = ((__x & (u32)0xAAAAAAAAUL) >> 1) | ((__x & (u32)0x55555555UL) << 1); \__x; \
u32 ___x = x; \___x = (___x >> 16) | (___x << 16); \___x = ((___x & (u32)0xFF00FF00UL) >> 8) | ((___x & (u32)0x00FF00FFUL) << 8); \___x = ((___x & (u32)0xF0F0F0F0UL) >> 4) | ((___x & (u32)0x0F0F0F0FUL) << 4); \___x = ((___x & (u32)0xCCCCCCCCUL) >> 2) | ((___x & (u32)0x33333333UL) << 2); \___x = ((___x & (u32)0xAAAAAAAAUL) >> 1) | ((___x & (u32)0x55555555UL) << 1); \___x; \})
#define __constant_bitrev16(x) \ ({ \
u16 __x = x; \__x = (__x >> 8) | (__x << 8); \__x = ((__x & (u16)0xF0F0U) >> 4) | ((__x & (u16)0x0F0FU) << 4); \__x = ((__x & (u16)0xCCCCU) >> 2) | ((__x & (u16)0x3333U) << 2); \__x = ((__x & (u16)0xAAAAU) >> 1) | ((__x & (u16)0x5555U) << 1); \__x; \
u16 ___x = x; \___x = (___x >> 8) | (___x << 8); \___x = ((___x & (u16)0xF0F0U) >> 4) | ((___x & (u16)0x0F0FU) << 4); \___x = ((___x & (u16)0xCCCCU) >> 2) | ((___x & (u16)0x3333U) << 2); \___x = ((___x & (u16)0xAAAAU) >> 1) | ((___x & (u16)0x5555U) << 1); \___x; \})
#define __constant_bitrev8x4(x) \ ({ \
u32 __x = x; \__x = ((__x & (u32)0xF0F0F0F0UL) >> 4) | ((__x & (u32)0x0F0F0F0FUL) << 4); \__x = ((__x & (u32)0xCCCCCCCCUL) >> 2) | ((__x & (u32)0x33333333UL) << 2); \__x = ((__x & (u32)0xAAAAAAAAUL) >> 1) | ((__x & (u32)0x55555555UL) << 1); \__x; \
u32 ___x = x; \___x = ((___x & (u32)0xF0F0F0F0UL) >> 4) | ((___x & (u32)0x0F0F0F0FUL) << 4); \___x = ((___x & (u32)0xCCCCCCCCUL) >> 2) | ((___x & (u32)0x33333333UL) << 2); \___x = ((___x & (u32)0xAAAAAAAAUL) >> 1) | ((___x & (u32)0x55555555UL) << 1); \___x; \})
#define __constant_bitrev8(x) \ ({ \
u8 __x = x; \__x = (__x >> 4) | (__x << 4); \__x = ((__x & (u8)0xCCU) >> 2) | ((__x & (u8)0x33U) << 2); \__x = ((__x & (u8)0xAAU) >> 1) | ((__x & (u8)0x55U) << 1); \__x; \
u8 ___x = x; \___x = (___x >> 4) | (___x << 4); \___x = ((___x & (u8)0xCCU) >> 2) | ((___x & (u8)0x33U) << 2); \___x = ((___x & (u8)0xAAU) >> 1) | ((___x & (u8)0x55U) << 1); \___x; \})
#define bitrev32(x) \
2.20.0