spi: Skip zero-length transfers in spi_transfer_one_message()

With the introduction of 26751de25d25 ("spi: bcm2835: Micro-optimise
FIFO loops") it has become apparent that some users might initiate
zero-length SPI transfers. A fact the micro-optimization omitted, and
which turned out to cause crashes[1].

Instead of changing the micro-optimization itself, use a bigger hammer
and skip zero-length transfers altogether for drivers using the default
transfer_one_message() implementation.

Reported-by: Phil Elwell <phil@raspberrypi.com>
Fixes: 26751de25d25 ("spi: bcm2835: Micro-optimise FIFO loops")
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
[1] https://github.com/raspberrypi/linux/issues/4100
Link: https://lore.kernel.org/r/20210211180820.25757-1-nsaenzjulienne@suse.de
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 720ab34784c1d7c7bc430f0212706902dca62359..ccca3a7409facd2088e9a2e4d906da897d82060a 100644 (file)
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1267,7 +1267,7 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
                        ptp_read_system_prets(xfer->ptp_sts);
                }
 
-               if (xfer->tx_buf || xfer->rx_buf) {
+               if ((xfer->tx_buf || xfer->rx_buf) && xfer->len) {
                        reinit_completion(&ctlr->xfer_completion);
 
 fallback_pio: