[PATCH v2] um: refactor deprecated strncpy to strtomem

[Justin Stitt <justinstitt@google.com>]

Favor a more robust and less ambiguous interface over `strncpy`.

Use `strtomem` here since `console_buf` is not expected to be
NUL-terminated.

Also mark char buffer as `__nonstring` as per Kees' suggestion here [1]

Link: https://github.com/KSPP/linux/issues/90 [1]
Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Changes in v2:
- Keep usage of ARRAY_SIZE (thanks Bill)
- Remove unnecessary import (thanks Kees and Bill)
- Link to v1: https://lore.kernel.org/r/20230807-arch-um-v1-1-86dbbfb59709@google.com
---
Notes:
I only build tested this patch.

I am also making an assumption that `console_buf` need not be
NUL-terminated. Am I right in believing this?
---
 arch/um/drivers/mconsole_kern.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index 5026e7b9adfe..d5fa9c8dcd18 100644
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -554,7 +554,7 @@ struct mconsole_output {
 
 static DEFINE_SPINLOCK(client_lock);
 static LIST_HEAD(clients);
-static char console_buf[MCONSOLE_MAX_DATA];
+static char console_buf[MCONSOLE_MAX_DATA] __nonstring;
 
 static void console_write(struct console *console, const char *string,
 			  unsigned int len)
@@ -567,7 +567,7 @@ static void console_write(struct console *console, const char *string,
 
 	while (len > 0) {
 		n = min((size_t) len, ARRAY_SIZE(console_buf));
-		strncpy(console_buf, string, n);
+		strtomem(console_buf, string);
 		string += n;
 		len -= n;
 

---
base-commit: c1a515d3c0270628df8ae5f5118ba859b85464a2
change-id: 20230807-arch-um-3ef24413427e

Best regards,
--
Justin Stitt <justinstitt@google.com>