[PATCH 4.9 19/83] include/linux/bitops.h: sanitize rotate primitives

[Greg Kroah-Hartman]

From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>

commit ef4d6f6b275c498f8e5626c99dbeefdc5027f843 upstream.

The ror32 implementation (word >> shift) | (word << (32 - shift) has
undefined behaviour if shift is outside the [1, 31] range.  Similarly
for the 64 bit variants.  Most callers pass a compile-time constant
(naturally in that range), but there's an UBSAN report that these may
actually be called with a shift count of 0.

Instead of special-casing that, we can make them DTRT for all values of
shift while also avoiding UB.  For some reason, this was already partly
done for rol32 (which was well-defined for [0, 31]).  gcc 8 recognizes
these patterns as rotates, so for example

  __u32 rol32(__u32 word, unsigned int shift)
  {
	return (word << (shift & 31)) | (word >> ((-shift) & 31));
  }

compiles to

0000000000000020 <rol32>:
  20:   89 f8                   mov    %edi,%eax
  22:   89 f1                   mov    %esi,%ecx
  24:   d3 c0                   rol    %cl,%eax
  26:   c3                      retq

Older compilers unfortunately do not do as well, but this only affects
the small minority of users that don't pass constants.

Due to integer promotions, ro[lr]8 were already well-defined for shifts
in [0, 8], and ro[lr]16 were mostly well-defined for shifts in [0, 16]
(only mostly - u16 gets promoted to _signed_ int, so if bit 15 is set,
word << 16 is undefined).  For consistency, update those as well.

Link: http://lkml.kernel.org/r/20190410211906.2190-1-linux@xxxxxxxxxxxxxxxxxx
Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Reported-by: Ido Schimmel <idosch@xxxxxxxxxxxx>
Tested-by: Ido Schimmel <idosch@xxxxxxxxxxxx>
Reviewed-by: Will Deacon <will.deacon@xxxxxxx>
Cc: Vadim Pasternak <vadimp@xxxxxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx>
Cc: Pavel Machek <pavel@xxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/bitops.h |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -58,7 +58,7 @@ static __always_inline unsigned long hwe
  */
 static inline __u64 rol64(__u64 word, unsigned int shift)
 {
-	return (word << shift) | (word >> (64 - shift));
+	return (word << (shift & 63)) | (word >> ((-shift) & 63));
 }
 
 /**
@@ -68,7 +68,7 @@ static inline __u64 rol64(__u64 word, un
  */
 static inline __u64 ror64(__u64 word, unsigned int shift)
 {
-	return (word >> shift) | (word << (64 - shift));
+	return (word >> (shift & 63)) | (word << ((-shift) & 63));
 }
 
 /**
@@ -78,7 +78,7 @@ static inline __u64 ror64(__u64 word, un
  */
 static inline __u32 rol32(__u32 word, unsigned int shift)
 {
-	return (word << shift) | (word >> ((-shift) & 31));
+	return (word << (shift & 31)) | (word >> ((-shift) & 31));
 }
 
 /**
@@ -88,7 +88,7 @@ static inline __u32 rol32(__u32 word, un
  */
 static inline __u32 ror32(__u32 word, unsigned int shift)
 {
-	return (word >> shift) | (word << (32 - shift));
+	return (word >> (shift & 31)) | (word << ((-shift) & 31));
 }
 
 /**
@@ -98,7 +98,7 @@ static inline __u32 ror32(__u32 word, un
  */
 static inline __u16 rol16(__u16 word, unsigned int shift)
 {
-	return (word << shift) | (word >> (16 - shift));
+	return (word << (shift & 15)) | (word >> ((-shift) & 15));
 }
 
 /**
@@ -108,7 +108,7 @@ static inline __u16 rol16(__u16 word, un
  */
 static inline __u16 ror16(__u16 word, unsigned int shift)
 {
-	return (word >> shift) | (word << (16 - shift));
+	return (word >> (shift & 15)) | (word << ((-shift) & 15));
 }
 
 /**
@@ -118,7 +118,7 @@ static inline __u16 ror16(__u16 word, un
  */
 static inline __u8 rol8(__u8 word, unsigned int shift)
 {
-	return (word << shift) | (word >> (8 - shift));
+	return (word << (shift & 7)) | (word >> ((-shift) & 7));
 }
 
 /**
@@ -128,7 +128,7 @@ static inline __u8 rol8(__u8 word, unsig
  */
 static inline __u8 ror8(__u8 word, unsigned int shift)
 {
-	return (word >> shift) | (word << (8 - shift));
+	return (word >> (shift & 7)) | (word << ((-shift) & 7));
 }
 
 /**