tree-wide: make specifier expansion --root= aware

This fixes repart's, systemctl's, sysusers' and tmpfiles' specifier
expansion to honour the root dir specified with --root=. This is
relevant for specifiers such as %m, %o, … which are directly sourced
from files on disk.

This doesn't try to be overly smart: specifiers referring to runtime
concepts (i.e. boot ID, architecture, hostname) rather than files on the
medium are left as is. There's certainly a point to be made that they
should fail in case --root= is specified, but I am not entirely convinced
about that, and it's certainly something we can look into later if
there's reason to.

I wondered for a while how to hook this up best, but given that quite a
large number of specifiers resolve to data from files on disks, and most
of our tools needs this, I ultimately decided to make the root dir a
first class parameter to specifier_printf().

Replaces: #16187
Fixes: #16183

src/tmpfiles/tmpfiles.c

@@ -200,8 +200,8 @@ STATIC_DESTRUCTOR_REGISTER(arg_exclude_prefixes, freep);
 STATIC_DESTRUCTOR_REGISTER(arg_root, freep);
 STATIC_DESTRUCTOR_REGISTER(arg_image, freep);
 
-static int specifier_machine_id_safe(char specifier, const void *data, const void *userdata, char **ret);
-static int specifier_directory(char specifier, const void *data, const void *userdata, char **ret);
+static int specifier_machine_id_safe(char specifier, const void *data, const char *root, const void *userdata, char **ret);
+static int specifier_directory(char specifier, const void *data, const char *root, const void *userdata, char **ret);
 
 static const Specifier specifier_table[] = {
         { 'a', specifier_architecture,    NULL },
@@ -228,21 +228,20 @@ static const Specifier specifier_table[] = {
         {}
 };
 
-static int specifier_machine_id_safe(char specifier, const void *data, const void *userdata, char **ret) {
+static int specifier_machine_id_safe(char specifier, const void *data, const char *root, const void *userdata, char **ret) {
         int r;
 
-        /* If /etc/machine_id is missing or empty (e.g. in a chroot environment)
-         * return a recognizable error so that the caller can skip the rule
-         * gracefully. */
+        /* If /etc/machine_id is missing or empty (e.g. in a chroot environment) return a recognizable error
+         * so that the caller can skip the rule gracefully. */
 
-        r = specifier_machine_id(specifier, data, userdata, ret);
+        r = specifier_machine_id(specifier, data, root, userdata, ret);
         if (IN_SET(r, -ENOENT, -ENOMEDIUM))
                 return -ENXIO;
 
         return r;
 }
 
-static int specifier_directory(char specifier, const void *data, const void *userdata, char **ret) {
+static int specifier_directory(char specifier, const void *data, const char *root, const void *userdata, char **ret) {
         struct table_entry {
                 uint64_t type;
                 const char *suffix;
@@ -262,8 +261,10 @@ static int specifier_directory(char specifier, const void *data, const void *use
                 [DIRECTORY_LOGS] =    { SD_PATH_USER_CONFIGURATION, "log" },
         };
 
-        unsigned i;
         const struct table_entry *paths;
+        _cleanup_free_ char *p = NULL;
+        unsigned i;
+        int r;
 
         assert_cc(ELEMENTSOF(paths_system) == ELEMENTSOF(paths_user));
         paths = arg_user ? paths_user : paths_system;
@@ -271,7 +272,22 @@ static int specifier_directory(char specifier, const void *data, const void *use
         i = PTR_TO_UINT(data);
         assert(i < ELEMENTSOF(paths_system));
 
-        return sd_path_lookup(paths[i].type, paths[i].suffix, ret);
+        r = sd_path_lookup(paths[i].type, paths[i].suffix, &p);
+        if (r < 0)
+                return r;
+
+        if (arg_root) {
+                _cleanup_free_ char *j = NULL;
+
+                j = path_join(arg_root, p);
+                if (!j)
+                        return -ENOMEM;
+
+                *ret = TAKE_PTR(j);
+        } else
+                *ret = TAKE_PTR(p);
+
+        return 0;
 }
 
 static int log_unresolvable_specifier(const char *filename, unsigned line) {
@@ -2778,7 +2794,7 @@ static int specifier_expansion_from_arg(Item *i) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to unescape parameter to write: %s", i->argument);
 
-                r = specifier_printf(unescaped, PATH_MAX-1, specifier_table, NULL, &resolved);
+                r = specifier_printf(unescaped, PATH_MAX-1, specifier_table, arg_root, NULL, &resolved);
                 if (r < 0)
                         return r;
 
@@ -2788,7 +2804,7 @@ static int specifier_expansion_from_arg(Item *i) {
         case SET_XATTR:
         case RECURSIVE_SET_XATTR:
                 STRV_FOREACH(xattr, i->xattrs) {
-                        r = specifier_printf(*xattr, SIZE_MAX, specifier_table, NULL, &resolved);
+                        r = specifier_printf(*xattr, SIZE_MAX, specifier_table, arg_root, NULL, &resolved);
                         if (r < 0)
                                 return r;
 
@@ -3021,7 +3037,7 @@ static int parse_line(
         i.allow_failure = allow_failure;
         i.try_replace = try_replace;
 
-        r = specifier_printf(path, PATH_MAX-1, specifier_table, NULL, &i.path);
+        r = specifier_printf(path, PATH_MAX-1, specifier_table, arg_root, NULL, &i.path);
         if (r == -ENXIO)
                 return log_unresolvable_specifier(fname, line);
         if (r < 0) {