[PATCH v2] nvdimm/btt: replace deprecated strncpy with strscpy

[Justin Stitt]

Found with grep.

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 super->signature to be NUL-terminated based on its usage with
memcmp against a NUL-term'd buffer:
btt_devs.c:
253 | if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
btt.h:
13  | #define BTT_SIG "BTT_ARENA_INFO\0"

NUL-padding is not required as `super` is already zero-allocated:
btt.c:
985 | super = kzalloc(sizeof(struct btt_sb), GFP_NOIO);
... rendering any additional NUL-padding superfluous.

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

Let's also use the more idiomatic strscpy usage of (dest, src,
sizeof(dest)) instead of (dest, src, XYZ_LEN) for buffers that the
compiler can determine the size of. This more tightly correlates the
destination buffer to the amount of bytes copied.

Side note, this pattern of memcmp() on two NUL-terminated strings should
really be changed to just a strncmp(), if i'm not mistaken? I see
multiple instances of this pattern in this system:

|       if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
|               return false;

where BIT_SIG is defined (weirdly) as a double NUL-terminated string:

|       #define BTT_SIG "BTT_ARENA_INFO\0"

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@xxxxxxxxxxxxxxx
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Changes in v2:
- provide more details about memcmp()
- fix typos in commit msg (thanks Kees)
- fix subject line (thanks Alison)
- Link to v1: https://lore.kernel.org/r/20231018-strncpy-drivers-nvdimm-btt-c-v1-1-58070f7dc5c9@xxxxxxxxxx
---
Note: build-tested only.
---
 drivers/nvdimm/btt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/nvdimm/btt.c b/drivers/nvdimm/btt.c
index d5593b0dc700..9372c36e8f76 100644
--- a/drivers/nvdimm/btt.c
+++ b/drivers/nvdimm/btt.c
@@ -986,7 +986,7 @@ static int btt_arena_write_layout(struct arena_info *arena)
 	if (!super)
 		return -ENOMEM;
 
-	strncpy(super->signature, BTT_SIG, BTT_SIG_LEN);
+	strscpy(super->signature, BTT_SIG, sizeof(super->signature));
 	export_uuid(super->uuid, nd_btt->uuid);
 	export_uuid(super->parent_uuid, parent_uuid);
 	super->flags = cpu_to_le32(arena->flags);

---
base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
change-id: 20231018-strncpy-drivers-nvdimm-btt-c-15f93879989e

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