block: Fix overrun in lcm() and move it to lib

lcm() was defined to take integer-sized arguments.  The supplied
arguments are multiplied, however, causing us to overflow given
sufficiently large input.  That in turn led to incorrect optimal I/O
size reporting in some cases (RAID over RAID).

Switch lcm() over to unsigned long similar to gcd() and move the
function from blk-settings.c to lib.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 16 insertions, 1 deletions

diff --git a/lib/Makefile b/lib/Makefile
index 2e152aed7198..0d4015205c64 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -21,7 +21,7 @@ lib-y	+= kobject.o kref.o klist.o
 
 obj-y += bcd.o div64.o sort.o parser.o halfmd4.o debug_locks.o random32.o \
 	 bust_spinlocks.o hexdump.o kasprintf.o bitmap.o scatterlist.o \
-	 string_helpers.o gcd.o list_sort.o
+	 string_helpers.o gcd.o lcm.o list_sort.o
 
 ifeq ($(CONFIG_DEBUG_KOBJECT),y)
 CFLAGS_kobject.o += -DDEBUG
diff --git a/lib/lcm.c b/lib/lcm.c
new file mode 100644
index 000000000000..157cd88a6ffc
--- /dev/null
+++ b/lib/lcm.c
@@ -0,0 +1,15 @@
+#include <linux/kernel.h>
+#include <linux/gcd.h>
+#include <linux/module.h>
+
+/* Lowest common multiple */
+unsigned long lcm(unsigned long a, unsigned long b)
+{
+	if (a && b)
+		return (a * b) / gcd(a, b);
+	else if (b)
+		return b;
+
+	return a;
+}
+EXPORT_SYMBOL_GPL(lcm);