The issue is within libselinux:

-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
 12 int security_check_context_raw(const char * con)
 13 {
 :
 22         if (!selinux_mnt) {
 23                 errno = ENOENT;
 24                 return -1;
 25         }
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

we have selinux_mnt == NULL

Does is work if you add "s0" to the context?

# chcon unconfined_u:object_r:usr_t:s0 <file>

I believe that chcon needs a complete context if all options are omitted.

Nope, doesn't work either.

Honestly I don't know if it's a libselinux issue or coreutils issue.

I see a difference in chcon.c RHEL7/RHEL8:

RHEL7:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
557     {
558       context_t context;
559       specified_context = argv[optind++];
560       context = context_new (specified_context);
561       if (!context)
562         error (EXIT_FAILURE, 0, _("invalid context: %s"),
563                quotearg_colon (specified_context));
564       context_free (context);
565     }
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Line 561 always succeeds, even with SELinux disabled.

RHEL8:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
558     {
559       specified_context = argv[optind++];
560       if (security_check_context (se_const (specified_context)) < 0)
561         die (EXIT_FAILURE, errno, _("invalid context: %s"),
562              quote (specified_context));
563     }
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Line 560 fails with SELinux disabled.

I would tend to see back to coreutils ...

Note anyway that this is some oddity: with SELinux disabled, you can set whatever label you want, even if it doesn't exist (assuming the file already exists of course)

Enabled:

# chcon -t boulet_t /tmp/foo
chcon: failed to change context of ‘/tmp/foo’ to ‘unconfined_u:object_r:boulet_t:s0’: Invalid argument


Disabled:

# chcon -t boulet_t /tmp/foo

--> no error

The code in coreutils was changed here:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
commit e4081e44e0dcc4bfe11d0da0ae47518df087349d
Author: Namhyung Kim <namhyung>
Date:   Tue Jul 1 00:12:41 2014 +0900

    chcon: use security_check_context() for context validation
    
    context_new() and _free() are used for checking validity of a
    specified context.  libselinux provides security_check_context
    for this purpose so use it.
    
    Note that context_new() can fail for a valid context - e.g. ENOMEM.
    
    * src/chcon.c (main): Use security_check_context().

---
 src/chcon.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/src/chcon.c b/src/chcon.c
index 32d4b0f..8c18167 100644
--- a/src/chcon.c
+++ b/src/chcon.c
@@ -555,13 +555,10 @@ main (int argc, char **argv)
     }
   else
     {
-      context_t context;
       specified_context = argv[optind++];
-      context = context_new (specified_context);
-      if (!context)
-        error (EXIT_FAILURE, 0, _("invalid context: %s"),
+      if (security_check_context (specified_context) < 0)
+        error (EXIT_FAILURE, errno, _("invalid context: %s"),
                quotearg_colon (specified_context));
-      context_free (context);
     }
 
   if (reference_file && component_specified)
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------


I'm tempted to say that it's not possible to chcon when SELinux is disabled, since it doesn't know which contexts are available (may depend on the policy used ...), hence I would consider "context_new()" not failing when SELinux is disabled is a bug.

context_new() and other context_* routines are used for manipulating security context so that caller doesn't need to know internal implementation. It doesn't do any check. E.g. on SELinux enabled system, you can call context_new() on invalid context it will pass:

^&^ python3
Python 3.7.5 (default, Oct 17 2019, 12:16:48) 
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import selinux
>>> c = selinux.context_new("a:b:c:d")
>>> selinux.context_type_get(c)
'c'
>>> selinux.context_user_get(c)
'a'
>>> selinux.context_role_get(c)
'b'

So it's not a bug in context_new() that it's not failing when SELinux is disabled.

But security_check_context() can't work with disabled SELinux.

In this case, I'd say that chcon should not try to valide the context if SELinux is disabled.
It should be enough add another check before context validation:

    if (is_selinux_enabled && security_check_context (specified_context) < 0)

if (is_selinux_enabled() && security_check_context (specified_context) < 0)

And it looks like the same check is not applied when -u/-r/-t used. It just computes a new context based on given values and applies it.

I believe that the original behavior when you can set any context in SELinux disabled mode is correct.

Thank you for finding the cause and proposing a solution!  I have submitted it upstream for review:

https://lists.gnu.org/archive/html/coreutils/2019-12/msg00002.html

Credits to Petr :-)

upstream commit:

https://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=v8.31-64-g5118a2e39

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (coreutils bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2020:4454