Use paths specified from environment variables for /etc configuration files

Some configuration files that need updates are directly under in /etc. To
update them atomically, we need write access to /etc. For Ubuntu Core this is
an issue as /etc is not writable. Only a selection of subdirectories can be
writable. The general solution is symlinks or bind mounts to writable places.
But for atomic writes in /etc, that does not work. So Ubuntu has had a patch
for that that did not age well.

Instead we would like to introduce some environment variables for alternate
paths.

 * SYSTEMD_ETC_HOSTNAME: /etc/hostname
 * SYSTEMD_ETC_MACHINE_INFO: /etc/machine-info
 * SYSTEMD_ETC_LOCALTIME: /etc/localtime
 * SYSTEMD_ETC_LOCALE_CONF: /etc/locale.conf
 * SYSTEMD_ETC_VCONSOLE_CONF: /etc/vconsole.conf
 * SYSTEMD_ETC_ADJTIME: /etc/adjtime

While it is for now expected that there is a symlink from the standard, we
still try to read them from that alternate path. This is important for
`/etc/localtime`, which is a symlink, so we cannot have an indirect symlink or
bind mount for it.

Since machine-id is typically written only once and not updated. This commit
does not cover it. An initrd can properly create it and bind mount it.

src/shared/env-file-label.c

@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <sys/stat.h>
+
+#include "env-file.h"
+#include "env-file-label.h"
+#include "locale-setup.h"
+#include "selinux-util.h"
+
+int write_env_file_label(int dir_fd, const char *fname, char **headers, char **l) {
+        int r;
+
+        r = mac_selinux_create_file_prepare(fname, S_IFREG);
+        if (r < 0)
+                return r;
+
+        r = write_env_file(dir_fd, fname, headers, l);
+
+        mac_selinux_create_file_clear();
+
+        return r;
+}
+
+int write_vconsole_conf_label(char **l) {
+        int r;
+
+        r = mac_selinux_create_file_prepare(etc_vconsole_conf(), S_IFREG);
+        if (r < 0)
+                return r;
+
+        r = write_vconsole_conf(AT_FDCWD, etc_vconsole_conf(), l);
+
+        mac_selinux_create_file_clear();
+
+        return r;
+}

src/shared/hostname-setup.c

@@ -139,7 +139,7 @@ int read_etc_hostname(const char *path, bool substitute_wildcards, char **ret) {
         assert(ret);
 
         if (!path)
-                path = "/etc/hostname";
+                path = etc_hostname();
 
         f = fopen(path, "re");
         if (!f)

src/shared/locale-setup.c

@@ -67,7 +67,7 @@ static int locale_context_load_conf(LocaleContext *c, LocaleLoadFlag flag) {
         if (!FLAGS_SET(flag, LOCALE_LOAD_LOCALE_CONF))
                 return 0;
 
-        fd = RET_NERRNO(open("/etc/locale.conf", O_CLOEXEC | O_PATH));
+        fd = RET_NERRNO(open(etc_locale_conf(), O_CLOEXEC | O_PATH));
         if (fd == -ENOENT)
                 return 0;
         if (fd < 0)
@@ -83,7 +83,7 @@ static int locale_context_load_conf(LocaleContext *c, LocaleLoadFlag flag) {
         c->st = st;
         locale_context_clear(c);
 
-        r = parse_env_file_fd(fd, "/etc/locale.conf",
+        r = parse_env_file_fd(fd, etc_locale_conf(),
                               "LANG",              &c->locale[VARIABLE_LANG],
                               "LANGUAGE",          &c->locale[VARIABLE_LANGUAGE],
                               "LC_CTYPE",          &c->locale[VARIABLE_LC_CTYPE],
@@ -199,7 +199,7 @@ int locale_context_save(LocaleContext *c, char ***ret_set, char ***ret_unset) {
                 return r;
 
         if (strv_isempty(set)) {
-                if (unlink("/etc/locale.conf") < 0)
+                if (unlink(etc_locale_conf()) < 0)
                         return errno == ENOENT ? 0 : -errno;
 
                 c->st = (struct stat) {};
@@ -213,14 +213,14 @@ int locale_context_save(LocaleContext *c, char ***ret_set, char ***ret_unset) {
 
         r = write_env_file(
                         AT_FDCWD,
-                        "/etc/locale.conf",
+                        etc_locale_conf(),
                         /* headers= */ NULL,
                         set,
                         WRITE_ENV_FILE_LABEL);
         if (r < 0)
                 return r;
 
-        if (stat("/etc/locale.conf", &c->st) < 0)
+        if (stat(etc_locale_conf(), &c->st) < 0)
                 return -errno;
 
         if (ret_set)
@@ -300,3 +300,21 @@ int locale_setup(char ***environment) {
 
         return 0;
 }
+
+const char* etc_locale_conf(void) {
+        static const char *cached = NULL;
+
+        if (!cached)
+                cached = secure_getenv("SYSTEMD_ETC_LOCALE_CONF") ?: "/etc/locale.conf";
+
+        return cached;
+}
+
+const char* etc_vconsole_conf(void) {
+        static const char *cached = NULL;
+
+        if (!cached)
+                cached = secure_getenv("SYSTEMD_ETC_VCONSOLE_CONF") ?: "/etc/vconsole.conf";
+
+        return cached;
+}

src/shared/locale-setup.h

@@ -28,3 +28,6 @@ void locale_context_take(LocaleContext *c, char *l[_VARIABLE_LC_MAX]);
 bool locale_context_equal(const LocaleContext *c, char *l[_VARIABLE_LC_MAX]);
 
 int locale_setup(char ***environment);
+
+const char* etc_locale_conf(void);
+const char* etc_vconsole_conf(void);