sysext: Check for /etc/initrd-release in given --root= tree

Both sysext and confext used the host's /etc/initrd-release file even
when --root=/somewhere was specified. A workaround was the
SYSTEMD_IN_INITRD= env var but without knowing this it was quite
confusing. Aside from users validating their extensions, the primary
use case for this to matter is when the extensions are set up from the
initrd where the initrd-release file is present when running but we want
to prepare the extensions for the final system and thus should match
for the right scope.
Make systemd-sysext check for /etc/initrd-release inside the given
--root= tree. An alternative would be to always ignore the
initrd-release check when --root= is passed but this way it is more
consistent. The image policy logic for EFI-loader-passed extensions
won't take effect when --root= is used, though.

src/shared/discover-image.c

@@ -619,6 +619,7 @@ static int image_make(
 static int pick_image_search_path(
                 RuntimeScope scope,
                 ImageClass class,
+                const char *root,
                 char ***ret) {
 
         int r;
@@ -635,11 +636,11 @@ static int pick_image_search_path(
         if (scope < 0) {
                 _cleanup_strv_free_ char **a = NULL, **b = NULL;
 
-                r = pick_image_search_path(RUNTIME_SCOPE_USER, class, &a);
+                r = pick_image_search_path(RUNTIME_SCOPE_USER, class, root, &a);
                 if (r < 0)
                         return r;
 
-                r = pick_image_search_path(RUNTIME_SCOPE_SYSTEM, class, &b);
+                r = pick_image_search_path(RUNTIME_SCOPE_SYSTEM, class, root, &b);
                 if (r < 0)
                         return r;
 
@@ -655,8 +656,15 @@ static int pick_image_search_path(
 
         case RUNTIME_SCOPE_SYSTEM: {
                 const char *ns;
+                bool is_initrd;
+
+                r = chase_and_access("/etc/initrd-release", root, CHASE_PREFIX_ROOT, F_OK, /* ret_path= */ NULL);
+                if (r < 0 && r != -ENOENT)
+                        return r;
+                is_initrd = r >= 0;
+
                 /* Use the initrd search path if there is one, otherwise use the common one */
-                ns = in_initrd() && image_search_path_initrd[class] ?
+                ns = is_initrd && image_search_path_initrd[class] ?
                         image_search_path_initrd[class] :
                         image_search_path[class];
                 if (!ns)
@@ -763,7 +771,7 @@ int image_find(RuntimeScope scope,
                 return -ENOMEM;
 
         _cleanup_strv_free_ char **search = NULL;
-        r = pick_image_search_path(scope, class, &search);
+        r = pick_image_search_path(scope, class, root, &search);
         if (r < 0)
                 return r;
 
@@ -954,7 +962,7 @@ int image_discover(
         assert(images);
 
         _cleanup_strv_free_ char **search = NULL;
-        r = pick_image_search_path(scope, class, &search);
+        r = pick_image_search_path(scope, class, root, &search);
         if (r < 0)
                 return r;
 
@@ -2101,7 +2109,7 @@ bool image_in_search_path(
         assert(image);
 
         _cleanup_strv_free_ char **search = NULL;
-        r = pick_image_search_path(scope, class, &search);
+        r = pick_image_search_path(scope, class, root, &search);
         if (r < 0)
                 return r;