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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Chris Lesiak <chris.lesiak@licor.com>,
        Mark Brown <broonie@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.9 126/128] spi: Fix zero length xfer bug
Date:   Wed, 29 May 2019 20:07:38 -0700
Message-Id: <20190530030456.870544376@linuxfoundation.org>
In-Reply-To: <20190530030432.977908967@linuxfoundation.org>
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[ Upstream commit 5442dcaa0d90fc376bdfc179a018931a8f43dea4 ]

This fixes a bug for messages containing both zero length and
unidirectional xfers.

The function spi_map_msg will allocate dummy tx and/or rx buffers
for use with unidirectional transfers when the hardware can only do
a bidirectional transfer.  That dummy buffer will be used in place
of a NULL buffer even when the xfer length is 0.

Then in the function __spi_map_msg, if he hardware can dma,
the zero length xfer will have spi_map_buf called on the dummy
buffer.

Eventually, __sg_alloc_table is called and returns -EINVAL
because nents == 0.

This fix prevents the error by not using the dummy buffer when
the xfer length is zero.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/spi/spi.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index c2e85e23d538e..d74d341f9890d 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -955,6 +955,8 @@ static int spi_map_msg(struct spi_master *master, struct spi_message *msg)
 		if (max_tx || max_rx) {
 			list_for_each_entry(xfer, &msg->transfers,
 					    transfer_list) {
+				if (!xfer->len)
+					continue;
 				if (!xfer->tx_buf)
 					xfer->tx_buf = master->dummy_tx;
 				if (!xfer->rx_buf)
-- 
2.20.1