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login: fix ut_line comparison logic 

ut_line[] is not a string, it's a fixed-width character field,
and may not be NUL terminated.  Thus, the use of strcmp is incorrect.
strncmp is more appropriate as it stops at the field size.

Note that differences beyond the field size do not count here,
as (1) this test doesn't do that, and (2) such differences are
traditionally ignored (i.e. logins that are silently truncated to
8 characters, etc)

While this is "only a test", we should still demonstrate the
correct way of doing things.  Also, using strncmp avoids a
"not a string" warning from gcc if you use -O1 or lower,
where it can't deduce that overflow won't happen.

Reviewed-by: Sam James <sam@gentoo.org>
Reviewed-by: Collin Funk <collin.funk1@gmail.com>
This commit is contained in:
DJ Delorie 2025-08-07 17:07:53 -04:00
parent 320cf1e1b5
commit 6dbaed693a
1 changed files with 5 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
login/tst-utmp.c
View File

@ -33,6 +33,7 @@
# define getutline getutxline
# define getutid getutxid
# define pututline pututxline
# define UT_LINESIZE __UT_LINESIZE
#else
# include <utmp.h>
#endif
@ -153,7 +154,7 @@ simulate_login (const char *line, const char *user)
for (n = 0; n < num_entries; n++)
{
if (strcmp (line, entry[n].ut_line) == 0
if (strncmp (line, entry[n].ut_line, UT_LINESIZE) == 0
|| entry[n].ut_type == DEAD_PROCESS)
{
if (entry[n].ut_pid == DEAD_PROCESS)
@ -186,7 +187,7 @@ simulate_logout (const char *line)
for (n = 0; n < num_entries; n++)
{
if (strcmp (line, entry[n].ut_line) == 0)
if (strncmp (line, entry[n].ut_line, UT_LINESIZE) == 0)
{
entry[n].ut_type = DEAD_PROCESS;
strncpy (entry[n].ut_user, "", sizeof (entry[n].ut_user));
@ -230,7 +231,7 @@ check_login (const char *line)
for (n = 0; n < num_entries; n++)
{
if (strcmp (line, entry[n].ut_line) == 0)
if (strncmp (line, entry[n].ut_line, UT_LINESIZE) == 0)
{
if (memcmp (up, &entry[n], sizeof (struct utmp)))
{
@ -287,7 +288,7 @@ check_id (const char *id)
for (n = 0; n < num_entries; n++)
{
if (strcmp (id, entry[n].ut_id) == 0)
if (strncmp (id, entry[n].ut_id, sizeof (entry[n].ut_id)) == 0)
{
if (memcmp (up, &entry[n], sizeof (struct utmp)))
{