bug#64937: "who" reports funny dates

[Paul Eggert]

Thanks for doing all that work. I looked into it, and found a problem: a 
command like "who /var/log/wtmp" stops working because the systemd 
emulation of read_utmp only supports plain "who" (rougnly equivalent to 
"who /var/run/utmp" on Fedora). So I installed it into coreutils, but 
the default is systemd is disabled. This should give people time to 
experiment with it.


Thorsten, is there some way to get the equivalent of /var/log/wtmp with 
systemd?


Also, I simplified the use of the new readutmp interface a bit, by going 
back to the old way where you simply call 'free' once to free the 
storage. Although I toyed with the idea of simplifying readutmp.h 
further, by moving most of it into readutmp.c and having just struct 
gl_utmp public (this would simplify coreutils quite a bit), I ran out of 
time and patience and decided to ship what I had. So I nstalled the 
first ten attached patches into Gnulib, and the last patch into coreutils.


I haven't tested this with the latest systemd so there could well be 
typos in that part of the code.From 39a4cb0afdf4f2a1e6c2f3176b84e5b4cfe8996d Mon Sep 17 00:00:00 2001
From: Paul Eggert 
Date: Thu, 3 Aug 2023 15:31:48 -0700
Subject: [PATCH 1/9] readutmp: simplify extract_trimmed_name via ximemdup0

* lib/readutmp.c (extract_trimmed_name): Simplify.
* modules/readutmp (Depends-on):
Add strnlen, which was a missing dependency.

* lib/readutmp.c: Include xmemdup0.
(extract_trimmed_name): Simplify.
* modules/readutmp (Depends-on): Add xmemdup0.
Add strnlen, which was a missing dependency already.
---
 ChangeLog|  7 +++
 lib/readutmp.c   | 28 +++-
 modules/readutmp |  1 +
 3 files changed, 19 insertions(+), 17 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index b84cece6ff..7fa4e7b64a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2023-08-03  Paul Eggert  
+
+	readutmp: simplify extract_trimmed_name via ximemdup0
+	* lib/readutmp.c (extract_trimmed_name): Simplify.
+	* modules/readutmp (Depends-on):
+	Add strnlen, which was a missing dependency.
+
 2023-08-03  Bruno Haible  
 
 	alignasof, stdalign: Avoid some -Wundef warnings from config.h.
diff --git a/lib/readutmp.c b/lib/readutmp.c
index 11dd1655c5..9057a36494 100644
--- a/lib/readutmp.c
+++ b/lib/readutmp.c
@@ -47,29 +47,23 @@
 # pragma GCC diagnostic ignored "-Wsizeof-pointer-memaccess"
 #endif
 
+/* Work around .  */
+#if 11 <= __GNUC__
+# pragma GCC diagnostic ignored "-Wstringop-overread"
+#endif
+
 /* Copy UT_USER (UT) into storage obtained from malloc.  Then remove any
trailing spaces from the copy, NUL terminate it, and return the copy.  */
 
 char *
 extract_trimmed_name (const STRUCT_UTMP *ut)
 {
-  char *p, *trimmed_name;
-
-#if READUTMP_USE_SYSTEMD
-  trimmed_name = xstrdup (UT_USER (ut));
-#else
-  trimmed_name = xmalloc (UT_USER_SIZE + 1);
-  strncpy (trimmed_name, UT_USER (ut), UT_USER_SIZE);
-  /* Append a trailing NUL.  Some systems pad names shorter than the
- maximum with spaces, others pad with NULs.  */
-  trimmed_name[UT_USER_SIZE] = '\0';
-#endif
-  /* Remove any trailing spaces.  */
-  for (p = trimmed_name + strlen (trimmed_name);
-   trimmed_name < p && p[-1] == ' ';
-   *--p = '\0')
-;
-  return trimmed_name;
+  char const *name = UT_USER (ut);
+  idx_t len = strnlen (name, UT_USER_SIZE);
+  char const *p;
+  for (p = name + len; name < p && p[-1] == ' '; p--)
+continue;
+  return ximemdup0 (name, p - name);
 }
 
 #if READ_UTMP_SUPPORTED
diff --git a/modules/readutmp b/modules/readutmp
index 04893a4487..484edd1842 100644
--- a/modules/readutmp
+++ b/modules/readutmp
@@ -12,6 +12,7 @@ extensions
 xalloc
 stdbool
 stdint
+strnlen
 sys_time
 fopen-gnu
 unlocked-io-internal
-- 
2.39.2

From 1ccde926759e8638d4b86de3dabbd948ad921edc Mon Sep 17 00:00:00 2001
From: Paul Eggert 
Date: Thu, 3 Aug 2023 15:53:27 -0700
Subject: [PATCH 2/9] =?UTF-8?q?readutmp:=20go=20back=20to=20simple=20?=
 =?UTF-8?q?=E2=80=98free=E2=80=99?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Omit the new free_utmp function.  Instead, allocate storage
in one block, so that using code can still just call ‘free’.
* lib/readutmp.c (struct utmp_alloc) [READUTMP_USE_SYSTEMD]: New type.
(add_utmp) [READUTMP_USE_SYSTEMD]: New function.
(read_utmp) [READUTMP_USE_SYSTEMD]: Use it.
Also, use malloc a bit less heavily.
(free_utmp): Remove.
* tests/test-readutmp.c (main): Call free, not free_utmp.
---
 ChangeLog |  10 ++
 NEWS  |   4 +-
 lib/readutmp.c| 285 +++---
 lib/readutmp.h|   4 -
 tests/test-readutmp.c |   4 +-
 5 files changed, 167 insertions(+), 140 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 7fa4e7b64a..56b27d129f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,15 @@
 2023-08-03  Paul Eggert  
 
+	readutmp: go back to simple ‘free’
+	Omit the new free_utmp function.

bug#64937: "who" reports funny dates

[Thorsten Kukuk via GNU coreutils Bug Reports]

Hi,

On Fri, Aug 04, Paul Eggert wrote:

> Thanks for doing all that work. I looked into it, and found a problem: a 
> command like "who /var/log/wtmp" stops working because the systemd 
> emulation of read_utmp only supports plain "who" (rougnly equivalent to 
> "who /var/run/utmp" on Fedora). So I installed it into coreutils, but 
> the default is systemd is disabled. This should give people time to 
> experiment with it.
> 
> Thorsten, is there some way to get the equivalent of /var/log/wtmp with 
> systemd?

systemd does not collect wtmp data, because the applications don't 
report this data to systemd. And I don't think it makes sense to 
implement that or that such patches would be accepted by systemd
developers.

wtmp uses the same struct as utmp and thus faces the same problems,
including the Y2038 issue.
For this reason we (openSUSE/SUSE) switched to wtmpdb:
https://github.com/thkukuk/wtmpdb and don't support /var/log/wtmp
anymore.

And yes, "who /var/log/wtmp" will not work with this, too. But is this
really a required or usefull use case?
/usr/bin/last can give you the same output.

  Thorsten

-- 
Thorsten Kukuk, Distinguished Engineer, Senior Architect, Future Technologies
SUSE Software Solutions Germany GmbH, Frankenstraße 146, 90461 Nuernberg, 
Germany
Managing Director: Ivo Totev, Andrew McDonald, Werner Knoblich
(HRB 36809, AG Nürnberg)