[PATCH v3] null_blk: replace strncpy with strscpy

[Justin Stitt]

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

We should favor a more robust and less ambiguous interface.

We expect that both `nullb->disk_name` and `disk->disk_name` be
NUL-terminated:
|     snprintf(nullb->disk_name, sizeof(nullb->disk_name),
|              "%s", config_item_name(&dev->group.cg_item));
...
|       pr_info("disk %s created\n", nullb->disk_name);

It seems like NUL-padding may be required due to __assign_disk_name()
utilizing a memcpy as opposed to a `str*cpy` api.
| static inline void __assign_disk_name(char *name, struct gendisk *disk)
| {
| 	if (disk)
| 		memcpy(name, disk->disk_name, DISK_NAME_LEN);
| 	else
| 		memset(name, 0, DISK_NAME_LEN);
| }

Then we go and print it with `__print_disk_name` which wraps `nullb_trace_disk_name()`.
| #define __print_disk_name(name) nullb_trace_disk_name(p, name)

This function obviously expects a NUL-terminated string.
| const char *nullb_trace_disk_name(struct trace_seq *p, char *name)
| {
| 	const char *ret = trace_seq_buffer_ptr(p);
|
| 	if (name && *name)
| 		trace_seq_printf(p, "disk=%s, ", name);
| 	trace_seq_putc(p, 0);
|
| 	return ret;
| }

>From the above, we need both 1) a NUL-terminated string and 2) a
NUL-padded string. So, let's use strscpy_pad() as per Kees' suggestion
from v1.

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>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Changes in v3:
- reword subject line (thanks Jens)
- rebase onto 3669558bdf354cd352be955ef2764cde6a9bf5ec
- Link to v2: https://lore.kernel.org/r/20230918-strncpy-drivers-block-null_blk-main-c-v2-1-cd9b109edfdf@xxxxxxxxxx

Changes in v2:
- use strscpy_pad (thanks Kees)
- Link to v1: https://lore.kernel.org/r/20230911-strncpy-drivers-block-null_blk-main-c-v1-1-3b3887e7fde4@xxxxxxxxxx
---
Note: build-tested
---
 drivers/block/null_blk/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 968090935eb2..6cd0614d8786 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -1946,7 +1946,7 @@ static int null_gendisk_register(struct nullb *nullb)
 	else
 		disk->fops		= &null_bio_ops;
 	disk->private_data	= nullb;
-	strncpy(disk->disk_name, nullb->disk_name, DISK_NAME_LEN);
+	strscpy_pad(disk->disk_name, nullb->disk_name, DISK_NAME_LEN);
 
 	if (nullb->dev->zoned) {
 		int ret = null_register_zoned_dev(nullb);

---
base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
change-id: 20230911-strncpy-drivers-block-null_blk-main-c-7349153e1c6a

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