package/chrony: run as unprivileged user

There are three ways to run chronyd:
  - start as root, and continue running as root;
  - start as root, then setuid() to a non-root user via either a command
    line option or a configuration directive;
  - start as root, and setuid() to a build-time specified non-root user.

Currently, the first situation is used by Buildroot, which does not
follow security best practices of dropping elevated privileges for
daemon at runtime when that is possible.

We switch to the third situation, where a compile-time default non-root
user is then used at runtime to drop privileges, with libcap used to
keep the capabilities required to call the appropriate syscalls to
adjust the system time (typically, CAP_SYS_TIME to call adjtimex() or
clock_settime() et al.).

This means that libcap is now a mandatory dependency.

To be noted: users who previously had configured their systems to run
chronyd as non root, would have done so with either the command-line
option (`-u`), or the configuration directive (`user`). Those take
precedence over the compile-time default, so this should not break their
systems (presumably, they also run as the `chrony` user). They would
also have taken care to run chronyc as the appropriate user to
manipulate chronyd at runtime via the UNIX socket.

For those who were running chronyd as root, this does not change either:
the functionality is unchanged, and they were running chronyc as root,
which should still be capable of manipulating chronyd via its UNIX
socket.

Take that opportunity to brine chrony's Config.in to current coding
style: enclose sub-option in an if-endif block.

Signed-off-by: James Kent <james.kent@orchestrated-technology.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/chrony/Config.in

@@ -1,16 +1,20 @@
 config BR2_PACKAGE_CHRONY
 	bool "chrony"
 	depends on BR2_USE_MMU # fork()
+	select BR2_PACKAGE_LIBCAP
 	help
 	  Chrony is a pair of programs which are used to maintain the
 	  accuracy of the system clock.
 
 	  http://chrony.tuxfamily.org/
 
+if BR2_PACKAGE_CHRONY
+
 config BR2_PACKAGE_CHRONY_DEBUG_LOGGING
 	bool "enable debug logging support"
-	depends on BR2_PACKAGE_CHRONY
 	help
 	  Enable support for debug logging output from Chrony when
 	  enabled at runtime. If disabled, code for debug logging will
 	  not be compiled in.
+
+endif

package/chrony/chrony.mk

@@ -10,20 +10,19 @@ CHRONY_LICENSE = GPL-2.0
 CHRONY_LICENSE_FILES = COPYING
 CHRONY_CPE_ID_VENDOR = tuxfamily
 CHRONY_SELINUX_MODULES = chronyd
-CHRONY_DEPENDENCIES = host-pkgconf
+CHRONY_DEPENDENCIES = host-pkgconf libcap
 
 CHRONY_CONF_OPTS = \
 	--host-system=Linux \
 	--host-machine=$(BR2_ARCH) \
 	--prefix=/usr \
 	--without-tomcrypt \
+	--with-user=chrony \
 	$(if $(BR2_PACKAGE_CHRONY_DEBUG_LOGGING),--enable-debug,--disable-debug)
 
-ifeq ($(BR2_PACKAGE_LIBCAP),y)
-CHRONY_DEPENDENCIES += libcap
-else
-CHRONY_CONF_OPTS += --without-libcap
-endif
+define CHRONY_USERS
+	chrony -1 chrony -1 * /run/chrony - - Time daemon
+endef
 
 ifeq ($(BR2_PACKAGE_LIBNSS),y)
 CHRONY_DEPENDENCIES += libnss