linux-kernel - [PATCH 4.16 265/279] mtd: rawnand: Fix return type of _

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Date:   Mon, 18 Jun 2018 10:14:10 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Geert Uytterhoeven <geert@...ux-m68k.org>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: [PATCH 4.16 265/279] mtd: rawnand: Fix return type of __DIVIDE() when called with 32-bit

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Geert Uytterhoeven <geert@...ux-m68k.org>

[ Upstream commit 9f825e74d761c13b0cfaa5f65344d64ff970e252 ]

The __DIVIDE() macro checks whether it is called with a 32-bit or 64-bit
dividend, to select the appropriate divide-and-round-up routine.
As the check uses the ternary operator, the result will always be
promoted to a type that can hold both results, i.e. unsigned long long.

When using this result in a division on a 32-bit system, this may lead
to link errors like:

    ERROR: "__udivdi3" [drivers/mtd/nand/raw/nand.ko] undefined!

Fix this by casting the result of the division to the type of the
dividend.

Fixes: 8878b126df769831 ("mtd: nand: add ->exec_op() implementation")
Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Signed-off-by: Boris Brezillon <boris.brezillon@...tlin.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 include/linux/mtd/rawnand.h |   16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

--- a/include/linux/mtd/rawnand.h
+++ b/include/linux/mtd/rawnand.h
@@ -824,12 +824,18 @@ struct nand_op_instr {
  * tBERS (during an erase) which all of them are u64 values that cannot be
  * divided by usual kernel macros and must be handled with the special
  * DIV_ROUND_UP_ULL() macro.
+ *
+ * Cast to type of dividend is needed here to guarantee that the result won't
+ * be an unsigned long long when the dividend is an unsigned long (or smaller),
+ * which is what the compiler does when it sees ternary operator with 2
+ * different return types (picks the largest type to make sure there's no
+ * loss).
  */
-#define __DIVIDE(dividend, divisor) ({					\
-	sizeof(dividend) == sizeof(u32) ?				\
-		DIV_ROUND_UP(dividend, divisor) :			\
-		DIV_ROUND_UP_ULL(dividend, divisor);			\
-		})
+#define __DIVIDE(dividend, divisor) ({						\
+	(__typeof__(dividend))(sizeof(dividend) <= sizeof(unsigned long) ?	\
+			       DIV_ROUND_UP(dividend, divisor) :		\
+			       DIV_ROUND_UP_ULL(dividend, divisor)); 		\
+	})
 #define PSEC_TO_NSEC(x) __DIVIDE(x, 1000)
 #define PSEC_TO_MSEC(x) __DIVIDE(x, 1000000000)