[PATCH v2] x86/tdx: replace deprecated strncpy with strtomem_pad

[Justin Stitt]

strncpy works perfectly here in all cases, however, it is deprecated and
as such we should prefer more robust and less ambiguous string apis.

Let's use `strtomem_pad` as this matches the functionality of `strncpy`
and is _not_ deprecated.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@xxxxxxxxxxxxxxx
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Changes in v2:
- update subject (thanks Kees)
- update commit message (thanks Dave)
- rebase onto mainline cbf3a2cb156a2c91
- Link to v1: https://lore.kernel.org/r/20230911-strncpy-arch-x86-coco-tdx-tdx-c-v1-1-4b38155727f3@xxxxxxxxxx
---
Note: build-tested

Note: Ingo Molnar has some concerns about the comment being out of sync
[1] but I believe the comment still has a place as we can still
theoretically copy 64 bytes into our destination buffer without a
NUL-byte. The extra information about the 65th byte being NUL may serve
helpful to future travelers of this code. What do we think? I can drop
the comment in a v3 if needed.

[1]: https://lore.kernel.org/all/ZRc+JqO7XvyHg%2FnB@xxxxxxxxx/
---
 arch/x86/coco/tdx/tdx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 1d6b863c42b0..2e1be592c220 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -119,7 +119,7 @@ static void __noreturn tdx_panic(const char *msg)
 	} message;
 
 	/* VMM assumes '\0' in byte 65, if the message took all 64 bytes */
-	strncpy(message.str, msg, 64);
+	strtomem_pad(message.str, msg, '\0');
 
 	args.r8  = message.r8;
 	args.r9  = message.r9;

---
base-commit: cbf3a2cb156a2c911d8f38d8247814b4c07f49a2
change-id: 20230911-strncpy-arch-x86-coco-tdx-tdx-c-98b0b966bb8d

Best regards,
--
Justin Stitt <justinstitt@xxxxxxxxxx>