[PATCH 4.9 26/78] mm/fadvise.c: fix signed overflow UBSAN complaint

[Greg Kroah-Hartman]

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

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

From: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>

[ Upstream commit a718e28f538441a3b6612da9ff226973376cdf0f ]

Signed integer overflow is undefined according to the C standard.  The
overflow in ksys_fadvise64_64() is deliberate, but since it is signed
overflow, UBSAN complains:

	UBSAN: Undefined behaviour in mm/fadvise.c:76:10
	signed integer overflow:
	4 + 9223372036854775805 cannot be represented in type 'long long int'

Use unsigned types to do math.  Unsigned overflow is defined so UBSAN
will not complain about it.  This patch doesn't change generated code.

[akpm@xxxxxxxxxxxxxxxxxxxx: add comment explaining the casts]
Link: http://lkml.kernel.org/r/20180629184453.7614-1-aryabinin@xxxxxxxxxxxxx
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Reported-by: <icytxw@xxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/fadvise.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -68,8 +68,12 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, l
 		goto out;
 	}
 
-	/* Careful about overflows. Len == 0 means "as much as possible" */
-	endbyte = offset + len;
+	/*
+	 * Careful about overflows. Len == 0 means "as much as possible".  Use
+	 * unsigned math because signed overflows are undefined and UBSan
+	 * complains.
+	 */
+	endbyte = (u64)offset + (u64)len;
 	if (!len || endbyte < len)
 		endbyte = -1;
 	else