[Bug 8306] New: libutil's logout doesn't clear utmp entries

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=8306

           Summary: libutil's logout doesn't clear utmp entries
           Product: uClibc
           Version: 0.9.33.2
          Platform: All
        OS/Version: Other
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Standard Compliance
        AssignedTo: unassigned at uclibc.org
        ReportedBy: john.ata at baesystems.com
                CC: uclibc-cvs at uclibc.org
   Estimated Hours: 0.0


Hi,

The module logout() in libutil does not actually remove the utmp entry that it
attempts to do.  This is because it uses getutline() to read the utmp entry
which is returned in utmp’s internal static buffer.  It then modifies the
static entry directly and attempts to write it out with pututline().  However,
pututline() reads the original entry before writing it into the internal static
buffer (only one internal utmp buffer) so it then always writes the original
unchanged record back out.  One fix is to copy the static buffer into the
temporary tmp stack variable (no longer needed) after reading but before
writing.  This has been tested and appears to work well.

+--- uclibc/libutil/logout.c    2012-05-15 03:20:09.000000000 -0400
++++ uclibc/libutil/logout.c    2015-08-14 16:59:06.625944541 -0400
+@@ -45,6 +45,10 @@
+   /* Read the record.  */
+   if ((ut = getutline(&tmp)) != NULL)
+     {
++      /* We can't use the utmp static buffer on the rewrite so copy over */
++      memcpy(&tmp, ut, sizeof tmp);
++      ut = &tmp;
++
+       /* Clear information about who & from where.  */
+       memset (ut->ut_name, 0, sizeof ut->ut_name);
+ #if _HAVE_UT_HOST - 0

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