media: rc: ir-spi: avoid overflow in multiplication

Switch to u64 arithmetic and use DIV_ROUND_CLOSEST_ULL() to avoid
the overflow.

buffer[i] is unsigned int and is limited by the lirc core to
IR_MAX_DURATION, which is 500000.

idata->freq is u32, which has a max value of 0xFFFFFFFF.

In the case where buffer[i] is 500000, idata->freq overflows the u32
multiplication for any values >= 8590.

0xFFFFFFFF / 500000 ~= 8589

By casting buffer[i] to u64, idata->freq can be any u32 value without
overflowing the multiplication.

0xFFFFFFFFFFFFFFFF / 500000 ~= 36893488147419 (> 4294967295)

The result of the final operation will fit back into the unsigned int
limits without any issues.

500000 * 0xFFFFFFFF / 1000000 = 0x80000000 (< 0xFFFFFFFF)

Signed-off-by: Cosmin Tanislav <[e-mail]>
Signed-off-by: Sean Young <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

drivers/media/rc/ir-spi.c

···
 
 	/* convert the pulse/space signal to raw binary signal */
 	for (i = 0; i < count; i++) {
-		buffer[i] = DIV_ROUND_CLOSEST(buffer[i] * idata->freq, 1000000);
+		buffer[i] = DIV_ROUND_CLOSEST_ULL((u64)buffer[i] * idata->freq,
+						  1000000);
 		len += buffer[i];
 	}