[PATCH v3] net: mdio-gpio: replace deprecated strncpy with strscpy

[PATCH v3] net: mdio-gpio: replace deprecated strncpy with strscpy

[Justin Stitt]

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);

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.

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

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 v3:
- swap another instance of MII_BUS_ID_SIZE to sizeof() (thanks Russell)
- rebase onto mainline bee0e7762ad2c602
- Link to v2: https://lore.kernel.org/r/20231207-strncpy-drivers-net-mdio-mdio-gpio-c-v2-1-c28d52dd3dfe@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  "superseded" label being improperly applied.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 | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/mdio/mdio-gpio.c b/drivers/net/mdio/mdio-gpio.c
index 897b88c50bbb..778db310a28d 100644
--- a/drivers/net/mdio/mdio-gpio.c
+++ b/drivers/net/mdio/mdio-gpio.c
@@ -123,9 +123,9 @@ static struct mii_bus *mdio_gpio_bus_init(struct device *dev,
 	new_bus->parent = dev;
 
 	if (bus_id != -1)
-		snprintf(new_bus->id, MII_BUS_ID_SIZE, "gpio-%x", bus_id);
+		snprintf(new_bus->id, sizeof(new_bus->id), "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: bee0e7762ad2c6025b9f5245c040fcc36ef2bde8
change-id: 20231012-strncpy-drivers-net-mdio-mdio-gpio-c-bddd9ed0c630

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

Re: [PATCH v3] net: mdio-gpio: replace deprecated strncpy with strscpy

[Russell King (Oracle)]

On Mon, Dec 11, 2023 at 07:10:00PM +0000, Justin Stitt wrote:
> 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);
> 
> 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.
> 
> We can also use sizeof() instead of a length macro as this more closely
> ties the maximum buffer size to the destination buffer. Do this for two
> instances.
> 
> 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>

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Thanks!

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Re: [PATCH v3] net: mdio-gpio: replace deprecated strncpy with strscpy

[Kees Cook]

On Mon, Dec 11, 2023 at 07:10:00PM +0000, Justin Stitt wrote:
> 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);
> 
> 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.
> 
> We can also use sizeof() instead of a length macro as this more closely
> ties the maximum buffer size to the destination buffer. Do this for two
> instances.
> 
> 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>

Looks like a clean replacement. (And a nice sizeof cleanup.)

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

Re: [PATCH v3] net: mdio-gpio: replace deprecated strncpy with strscpy

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 11 Dec 2023 19:10:00 +0000 you wrote:
> 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);
> 
> [...]

Here is the summary with links:
  - [v3] net: mdio-gpio: replace deprecated strncpy with strscpy
    https://git.kernel.org/netdev/net-next/c/79ac11393328

You are awesome, thank you!
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