[PATCH v2] kdb: replace deprecated strncpy

[Justin Stitt]

We should move away from using strncpy because it is deprecated [1].

Since these buffers want to be NUL-terminated, let's use strscpy() which
guarantees this behavior.

The code in question enables the visual autocomplete when using kdb tab
completion. After pressing tab a couple times when sitting on a partial
symbol it will attempt to fill it in. In my testing, strscpy() provides
the exact same autocomplete behavior that strncpy() provides here (i.e:
it fills in the same number of characters for the user).

You can confirm this by enabling kdb [3] and booting up the kernel. I
performed my tests with qemu with this incantation (wow these get long):

$ /usr/bin/qemu-system-x86_64 -display none -nodefaults -cpu Nehalem
-append 'console=ttyS0,115200 earlycon=uart8250,io,0x3f8 rdinit=/bin/sh
kgdboc=ttyS0,115200 nokaslr' -kernel $BUILD_DIR/arch/x86/boot/bzImage
-initrd $REPOS/boot-utils/images/x86_64/rootfs.cpio -m 512m -serial
mon:stdio

.. then you can type some symbols and see that autocomplete still kicks
in and performs exactly the same.

For example:
tes <tab><tab> gives you "test",
then "test_ap" <tab><tab> gives you "test_aperfmperf"

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 [2]
Link: https://www.kernel.org/doc/html/v5.0/dev-tools/kgdb.html#using-kdb [3]
Cc: linux-hardening@xxxxxxxxxxxxxxx
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Changes in v2:
- use strscpy over memcpy (thanks Daniel T.)
- Link to v1: https://lore.kernel.org/r/20240403-strncpy-kernel-debug-kdb-kdb_io-c-v1-1-7f78a08e9ff4@xxxxxxxxxx
---
---
 kernel/debug/kdb/kdb_io.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 9443bc63c5a2..60be22132020 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -368,9 +368,9 @@ static char *kdb_read(char *buffer, size_t bufsize)
 			kdb_printf("%s", buffer);
 		} else if (tab != 2 && count > 0) {
 			len_tmp = strlen(p_tmp);
-			strncpy(p_tmp+len_tmp, cp, lastchar-cp+1);
+			strscpy(p_tmp+len_tmp, cp, lastchar-cp+1);
 			len_tmp = strlen(p_tmp);
-			strncpy(cp, p_tmp+len, len_tmp-len + 1);
+			strscpy(cp, p_tmp+len, len_tmp-len + 1);
 			len = len_tmp - len;
 			kdb_printf("%s", cp);
 			cp += len;

---
base-commit: 026e680b0a08a62b1d948e5a8ca78700bfac0e6e
change-id: 20240402-strncpy-kernel-debug-kdb-kdb_io-c-53e5ed26da3d

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