linux-kernel - [PATCH] mm/fadvise: Fix signed overflow UBSAN complaint

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Message-Id: <20180629184453.7614-1-aryabinin@virtuozzo.com>
Date:   Fri, 29 Jun 2018 21:44:53 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     icytxw@...il.com, Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: [PATCH] mm/fadvise: Fix signed overflow UBSAN complaint

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.

Reported-by: <icytxw@...il.com>
Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---
 mm/fadvise.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index afa41491d324..1eaf2002d79a 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -73,7 +73,7 @@ int ksys_fadvise64_64(int fd, loff_t offset, loff_t len, int advice)
 	}
 
 	/* Careful about overflows. Len == 0 means "as much as possible" */
-	endbyte = offset + len;
+	endbyte = (u64)offset + (u64)len;
 	if (!len || endbyte < len)
 		endbyte = -1;
 	else
-- 
2.16.4