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Date:   Thu, 07 Dec 2023 21:54:31 +0000
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Message-ID: <20231207-strncpy-drivers-net-mdio-mdio-gpio-c-v2-1-c28d52dd3dfe@google.com>
Subject: [PATCH v2] net: mdio-gpio: replace deprecated strncpy with strscpy
From:   Justin Stitt <justinstitt@google.com>
To:     Andrew Lunn <andrew@lunn.ch>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Russell King <linux@armlinux.org.uk>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org,
        Justin Stitt <justinstitt@google.com>
Content-Type: text/plain; charset="utf-8"
Precedence: bulk

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

We expect new_bus->id to be NUL-terminated but not NUL-padded based on
its prior assignment through snprintf:
|       snprintf(new_bus->id, MII_BUS_ID_SIZE, "gpio-%x", bus_id);

We can also use sizeof() instead of a length macro as this more closely
ties the maximum buffer size to the destination buffer.

Due to this, a suitable replacement is `strscpy` [2] due to the fact
that it guarantees NUL-termination on the destination buffer without
unnecessarily NUL-padding.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Changes in v2:
- change subject line as it was causing problems in patchwork with
  "superseded" label being improperly applied.
- update commit msg with rationale around sizeof() (thanks Kees)
- Link to v1 (lore): https://lore.kernel.org/r/20231012-strncpy-drivers-net-mdio-mdio-gpio-c-v1-1-ab9b06cfcdab@google.com
- Link to v1 (patchwork): https://patchwork.kernel.org/project/netdevbpf/patch/20231012-strncpy-drivers-net-mdio-mdio-gpio-c-v1-1-ab9b06cfcdab@google.com/
- Link to other patch with same subject message: https://patchwork.kernel.org/project/netdevbpf/patch/20231012-strncpy-drivers-net-phy-mdio_bus-c-v1-1-15242e6f9ec4@google.com/
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 drivers/net/mdio/mdio-gpio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/mdio/mdio-gpio.c b/drivers/net/mdio/mdio-gpio.c
index 0fb3c2de0845..a1718d646504 100644
--- a/drivers/net/mdio/mdio-gpio.c
+++ b/drivers/net/mdio/mdio-gpio.c
@@ -125,7 +125,7 @@ static struct mii_bus *mdio_gpio_bus_init(struct device *dev,
 	if (bus_id != -1)
 		snprintf(new_bus->id, MII_BUS_ID_SIZE, "gpio-%x", bus_id);
 	else
-		strncpy(new_bus->id, "gpio", MII_BUS_ID_SIZE);
+		strscpy(new_bus->id, "gpio", sizeof(new_bus->id));
 
 	if (pdata) {
 		new_bus->phy_mask = pdata->phy_mask;

---
base-commit: cbf3a2cb156a2c911d8f38d8247814b4c07f49a2
change-id: 20231012-strncpy-drivers-net-mdio-mdio-gpio-c-bddd9ed0c630

Best regards,
--
Justin Stitt <justinstitt@google.com>