trun: Deactivate most capabilities before spawning user test

In the previous patch we asked unshare to keep capabilities so that FUSE
mounting works from under regular user. However full set of capabilities
is too much, and in particular if cap_dac_override is present(*), it means
that writes to files that have read-only permission, is not rejected by
kernel.

-> Adjust trun to retain only those capabilities that we actually need
   = CAP_SYS_ADMIN to mount things.

This should fix the following Go build failure:

    --- FAIL: TestReadOnlyWriteFile (0.00s)
        ioutil_test.go:90: Expected an error when writing to read-only file /tmp/TestReadOnlyWriteFile3940340549/blurp.txt
    FAIL
    FAIL	io/ioutil	0.053s

P.S. And if we would unshare to root instead (unshare -Umr) it should be
still a good idea to drop extra capabilities, as we still want to reject
writes to read-only files.

(*) see https://man7.org/linux/man-pages/man7/capabilities.7.html

/helped-by @jerome

nxdtest/nxdtest_test.py

@@ -259,3 +259,18 @@ TestCase('TESTCASE', ['mount', '-t', 'tmpfs', 'none', '/etc'])
 """)
     captured = capsys.readouterr()
     assert "# leaked mount: none /etc tmpfs" in captured.out
+
+
+# verify that inside environment, that nxdtest creates, file permissions are
+# still respected.
+def test_run_writero(run_nxdtest, capsys):
+    twritero = "%s/testprog/twritero" % (dirname(__file__),)
+
+    run_nxdtest(
+        """\
+TestCase('TESTNAME', ['%s'])
+""" % twritero)
+
+    captured = capsys.readouterr()
+    output_lines = captured.out.splitlines()
+    assert re.match(u"# ran 1 test case:  1·ok", output_lines[-1])

nxdtest/testprog/twritero

+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+# Copyright (C) 2021  Nexedi SA and Contributors.
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+"""Program twritero helps to verify that nxdtest runs tests in sane environment.
+
+It verifies that write to a file with readonly permission is rejected.
+"""
+
+from __future__ import print_function, absolute_import
+
+import os, tempfile
+from errno import EACCES
+from golang import func, defer
+
+
+@func
+def main():
+    fd, path = tempfile.mkstemp(prefix='twritero.')
+    def _():
+        os.remove(path)
+    defer(_)
+    os.close(fd)
+
+    os.chmod(path, 0o444)
+
+    try:
+        with open(path, "w+") as f:
+            f.write("zzz")
+    except IOError as e:
+        if e.errno != EACCES:
+            raise
+        print("write to r/o file rejected ok")
+    else:
+        raise AssertionError("write to r/o file not rejected")
+
+
+if __name__ == '__main__':
+    main()

nxdtest/trun.py

@@ -34,6 +34,7 @@ import errno, os, sys, stat, difflib
 from subprocess import check_call as xrun, CalledProcessError
 from os.path import join, devnull
 from golang import func, defer
+import prctl
 
 def main():
     # Try to respawn ourselves in user-namespace where we can mount things, e.g. new /tmp.
@@ -81,6 +82,11 @@ def main():
 # run_in_userns runs f with checks assuming that we are in a user namespace.
 @func
 def run_in_userns(f):
+    # leave only capabilities that are needed for mount/fusermount.
+    # in particular drop cap_dac_override so that file permissions are still
+    # respected (e.g. write to read/only file is rejected).
+    prctl.cap_inheritable.limit('sys_admin')
+
     # mount new /tmp and /dev/shm to isolate this run from other programs and to detect
     # leaked temporary files at the end.
     tmpreg = {

setup.py

@@ -13,7 +13,7 @@ setup(
     keywords    = 'Nexedi testing infrastructure tool tox',
 
     packages    = find_packages(),
-    install_requires = ['erp5.util', 'six', 'pygolang', 'psutil'],
+    install_requires = ['erp5.util', 'six', 'pygolang', 'psutil', 'python-prctl'],
     extras_require = {
                    'test': ['pytest', 'pytest-timeout', 'setproctitle'],
     },