[PATCH] accessibility: speakup: refactor deprecated strncpy

[Justin Stitt <justinstitt@google.com>]

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

Use `strscpy` as it guarantees NUL-termination of its destination buffer [2]
which allows for simpler and less ambiguous code.

Also, change `strlen(buf)` to `strlen(ptr)` to be consistent with
further usage within the scope of the function. Note that these are
equivalent:
|419 	const char *ptr = buf;

Link: 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/accessibility/speakup/kobjects.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/accessibility/speakup/kobjects.c b/drivers/accessibility/speakup/kobjects.c
index a7522d409802..8aa416c5f3fc 100644
--- a/drivers/accessibility/speakup/kobjects.c
+++ b/drivers/accessibility/speakup/kobjects.c
@@ -422,12 +422,11 @@ static ssize_t synth_direct_store(struct kobject *kobj,
 	if (!synth)
 		return -EPERM;
 
-	len = strlen(buf);
+	len = strlen(ptr);
 	spin_lock_irqsave(&speakup_info.spinlock, flags);
 	while (len > 0) {
 		bytes = min_t(size_t, len, 250);
-		strncpy(tmp, ptr, bytes);
-		tmp[bytes] = '\0';
+		strscpy(tmp, ptr, bytes);
 		string_unescape_any_inplace(tmp);
 		synth_printf("%s", tmp);
 		ptr += bytes;

---
base-commit: f9604036a3fb6149badf346994b46b03f9292db7
change-id: 20230824-strncpy-drivers-accessibility-speakup-kobjects-c-4009e7df0936

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