arm64: csum: Disable KASAN for do_csum()

do_csum() over-reads the source buffer and therefore abuses
READ_ONCE_NOCHECK() to avoid tripping up KASAN. In preparation for
READ_ONCE_NOCHECK() becoming a macro, and therefore losing its
'__no_sanitize_address' annotation, just annotate do_csum() explicitly
and fall back to normal loads.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/lib/csum.c

@@ -14,7 +14,11 @@ static u64 accumulate(u64 sum, u64 data)
 	return tmp + (tmp >> 64);
 }
 
-unsigned int do_csum(const unsigned char *buff, int len)
+/*
+ * We over-read the buffer and this makes KASAN unhappy. Instead, disable
+ * instrumentation and call kasan explicitly.
+ */
+unsigned int __no_sanitize_address do_csum(const unsigned char *buff, int len)
 {
 	unsigned int offset, shift, sum;
 	const u64 *ptr;
@@ -42,7 +46,7 @@ unsigned int do_csum(const unsigned char *buff, int len)
 	 * odd/even alignment, and means we can ignore it until the very end.
 	 */
 	shift = offset * 8;
-	data = READ_ONCE_NOCHECK(*ptr++);
+	data = *ptr++;
 #ifdef __LITTLE_ENDIAN
 	data = (data >> shift) << shift;
 #else
@@ -58,10 +62,10 @@ unsigned int do_csum(const unsigned char *buff, int len)
 	while (unlikely(len > 64)) {
 		__uint128_t tmp1, tmp2, tmp3, tmp4;
 
-		tmp1 = READ_ONCE_NOCHECK(*(__uint128_t *)ptr);
-		tmp2 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 2));
-		tmp3 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 4));
-		tmp4 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 6));
+		tmp1 = *(__uint128_t *)ptr;
+		tmp2 = *(__uint128_t *)(ptr + 2);
+		tmp3 = *(__uint128_t *)(ptr + 4);
+		tmp4 = *(__uint128_t *)(ptr + 6);
 
 		len -= 64;
 		ptr += 8;
@@ -85,7 +89,7 @@ unsigned int do_csum(const unsigned char *buff, int len)
 		__uint128_t tmp;
 
 		sum64 = accumulate(sum64, data);
-		tmp = READ_ONCE_NOCHECK(*(__uint128_t *)ptr);
+		tmp = *(__uint128_t *)ptr;
 
 		len -= 16;
 		ptr += 2;
@@ -100,7 +104,7 @@ unsigned int do_csum(const unsigned char *buff, int len)
 	}
 	if (len > 0) {
 		sum64 = accumulate(sum64, data);
-		data = READ_ONCE_NOCHECK(*ptr);
+		data = *ptr;
 		len -= 8;
 	}
 	/*