linux-kernel - [PATCH v2 2/2] spi: Avoid undefined behaviour when counting unused native CSs

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Message-Id: <20210420164425.40287-2-andriy.shevchenko@linux.intel.com>
Date:   Tue, 20 Apr 2021 19:44:25 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: [PATCH v2 2/2] spi: Avoid undefined behaviour when counting unused native CSs

ffz(), that has been used to count unused native CSs,
might cause undefined behaviour when called against ~0U.
To fix that, open code it with ffs(~value) - 1.

Fixes: 7d93aecdb58d ("spi: Add generic support for unused native cs with cs-gpios")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
v2: decoded UB abbreviation (Mark)
 drivers/spi/spi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 9c3730a9f7d5..01f95bee2ac8 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2609,7 +2609,7 @@ static int spi_get_gpio_descs(struct spi_controller *ctlr)
 		native_cs_mask |= BIT(i);
 	}
 
-	ctlr->unused_native_cs = ffz(native_cs_mask);
+	ctlr->unused_native_cs = ffs(~native_cs_mask) - 1;
 
 	if ((ctlr->flags & SPI_MASTER_GPIO_SS) && num_cs_gpios &&
 	    ctlr->max_native_cs && ctlr->unused_native_cs >= ctlr->max_native_cs) {
-- 
2.30.2