[PATCH v2] keys: Replace deprecated strncpy in ecryptfs_fill_auth_tok

[Thorsten Blum <thorsten.blum@linux.dev>]

strncpy() is deprecated for NUL-terminated destination buffers; use
strscpy_pad() instead to retain the zero-padding behavior of strncpy().

strscpy_pad() automatically determines the size of the fixed-length
destination buffer via sizeof() when the optional size argument is
omitted, making an explicit size unnecessary.

In encrypted_init(), the source string 'key_desc' is validated by
valid_ecryptfs_desc() before calling ecryptfs_fill_auth_tok(), and is
therefore NUL-terminated and satisfies the __must_be_cstr() requirement
of strscpy_pad().

No functional changes.

Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
Changes in v2:
- Improve commit message as suggested by Jarkko and Kees
- Link to v1: https://lore.kernel.org/lkml/20251009180316.394708-3-thorsten.blum@linux.dev/
---
 security/keys/encrypted-keys/ecryptfs_format.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/security/keys/encrypted-keys/ecryptfs_format.c b/security/keys/encrypted-keys/ecryptfs_format.c
index 8fdd76105ce3..2fc6f3a66135 100644
--- a/security/keys/encrypted-keys/ecryptfs_format.c
+++ b/security/keys/encrypted-keys/ecryptfs_format.c
@@ -54,8 +54,7 @@ int ecryptfs_fill_auth_tok(struct ecryptfs_auth_tok *auth_tok,
 	auth_tok->version = (((uint16_t)(major << 8) & 0xFF00)
 			     | ((uint16_t)minor & 0x00FF));
 	auth_tok->token_type = ECRYPTFS_PASSWORD;
-	strncpy((char *)auth_tok->token.password.signature, key_desc,
-		ECRYPTFS_PASSWORD_SIG_SIZE);
+	strscpy_pad(auth_tok->token.password.signature, key_desc);
 	auth_tok->token.password.session_key_encryption_key_bytes =
 		ECRYPTFS_MAX_KEY_BYTES;
 	/*
-- 
2.51.0