hw/i2c/microbit_i2c: Don't index off end of twi_read_sequence[]

If the guest tries to read more bytes from our fake stub I2C device
than we have provided, we incorrectly read one byte beyond the end of
this array. Avoid this, and instead keep reporting the RXD register
as containing the last byte of the "data transfer".

Cc: qemu-stable@nongnu.org
Fixes: 9d68bf564ec ("arm: Stub out NRF51 TWI magnetometer/accelerometer detection")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3408
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20260501162634.4092394-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

diff --git a/hw/i2c/microbit_i2c.c b/hw/i2c/microbit_i2c.c
index 2291d6370e25e2ffb6da33c551e5002ace9ffb36..d9689b6f1ae0fcffd094467c6bd0a10142e5119f 100644 (file)
--- a/hw/i2c/microbit_i2c.c
+++ b/hw/i2c/microbit_i2c.c
@@ -41,8 +41,13 @@ static uint64_t microbit_i2c_read(void *opaque, hwaddr addr, unsigned int size)
         data = 0x01;
         break;
     case NRF51_TWI_REG_RXD:
+        /*
+         * Return the next byte from our fake data sequence. If
+         * the guest keeps reading the register after that, keep
+         * returning the same last byte value.
+         */
         data = twi_read_sequence[s->read_idx];
-        if (s->read_idx < G_N_ELEMENTS(twi_read_sequence)) {
+        if (s->read_idx + 1 < G_N_ELEMENTS(twi_read_sequence)) {
             s->read_idx++;
         }
         break;