[PATCH] Input: axp20x-pek - refactor deprecated strncpy

[Justin Stitt <justinstitt@google.com>]

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

Ensuring we have a trailing NUL-byte and checking the length of bytes
copied are both intrinsic behavior of strscpy.

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

It should be noted that the original code can silently truncate and so
can this refactoring. However, a check could be added if truncation
is an issue:
| len = strscpy(val_str, buf, sizeof(val_str));
| if (len < 0) { // add this
|   return -E2BIG; // or -EINVAL
| }

Also, now check for `len > 0` instead of just a truthy `len` because
`len` is now a signed type and we could run into problems if strscpy
returned -E2BIG which would pass the truthy test.

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>
---
Note: build-tested only.
---
 drivers/input/misc/axp20x-pek.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/input/misc/axp20x-pek.c b/drivers/input/misc/axp20x-pek.c
index 4581606a28d6..abcf78785b45 100644
--- a/drivers/input/misc/axp20x-pek.c
+++ b/drivers/input/misc/axp20x-pek.c
@@ -134,16 +134,14 @@ static ssize_t axp20x_store_attr(struct device *dev,
 {
 	struct axp20x_pek *axp20x_pek = dev_get_drvdata(dev);
 	char val_str[20];
-	size_t len;
+	ssize_t len;
 	int ret, i;
 	unsigned int val, idx = 0;
 	unsigned int best_err = UINT_MAX;
 
-	val_str[sizeof(val_str) - 1] = '\0';
-	strncpy(val_str, buf, sizeof(val_str) - 1);
-	len = strlen(val_str);
+	len = strscpy(val_str, buf, sizeof(val_str));
 
-	if (len && val_str[len - 1] == '\n')
+	if (len > 0 && val_str[len - 1] == '\n')
 		val_str[len - 1] = '\0';
 
 	ret = kstrtouint(val_str, 10, &val);

---
base-commit: 2cf0f715623872823a72e451243bbf555d10d032
change-id: 20230921-strncpy-drivers-input-misc-axp20x-pek-c-3c4708924bbd

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