From: Evgeny Grin (Karlson2k) <k2k@drgrin.dev>
Date: Wed, 16 Jul 2025 15:50:53 +0000 (+0200)
Subject: lib/utmp.c: Align generated "ut_id" with modern software
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=59c5acb5dee310c3909a348a557224119f7d345f;p=thirdparty%2Fshadow.git

lib/utmp.c: Align generated "ut_id" with modern software

Modern software (systemd, utemper) usually use full "ut_line" as "ut_id"
if string is up to four chars or last four chars if "ut_line" is longer.
For reference:
* libutemper (used by many graphical terminal emulator applications
  (konsole, xterm, others) to deal with utmp file):
  https://github.com/altlinux/libutempter/blob/4caa8ab94ff8b207228fa723a89214bf5e929321/libutempter/utempter.c#L99-L103
* systemd:
  uses full name of the device:
  https://github.com/systemd/systemd/blob/8013beb4a2221680b2c741bac6b3a33fe66406e1/units/getty%40.service.in#L41-L44
  or **last** four characters:
  https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#UtmpIdentifier=

The code in the commit is optimised for visual C code size.

Signed-off-by: Evgeny Grin (Karlson2k) <k2k@drgrin.dev>
Reviewed-by: Alejandro Colomar <alx@kernel.org>
---

diff --git a/lib/utmp.c b/lib/utmp.c
index 11e6b190d..fade895f2 100644
--- a/lib/utmp.c
+++ b/lib/utmp.c
@@ -26,6 +26,7 @@
 #include "alloc/x/xcalloc.h"
 #include "alloc/x/xmalloc.h"
 #include "sizeof.h"
+#include "string/strchr/strnul.h"
 #include "string/strcmp/streq.h"
 #include "string/strcmp/strprefix.h"
 #include "string/strcpy/strncpy.h"
@@ -274,7 +275,7 @@ prepare_utmp(const char *name, const char *line, const char *host,
 	if (NULL != ut) {
 		STRNCPY(utent->ut_id, ut->ut_id);
 	} else {
-		STRNCPY(utent->ut_id, strprefix(line, "tty") ?: line);
+		STRNCPY(utent->ut_id, strnul(line) - MIN(strlen(line), countof(utent->ut_id)));
 	}
 #if defined(HAVE_STRUCT_UTMPX_UT_NAME)
 	STRNCPY(utent->ut_name, name);