various: try to use DEFAULT_USER_SHELL for root too

/bin/sh as a shell is punishing. There is no good reason to make
the occasional root login unpleasant.

Since /bin/sh is usually /bin/bash in compat mode, i.e. if one is
available, the other will be too, /bin/bash is almost as good as a default.
But to avoid a regression in the situation where /bin/bash (or
DEFAULT_USER_SHELL) is not installed, we check with access() and fall back
to /bin/sh. This should make this change in behaviour less risky.

(FWIW, e.g. Fedora/RHEL use /bin/bash as default for root.)

This is a follow-up of sorts for 53350c7bba,
which added the default-user-shell option, but most likely with the idea
of using /bin/bash less ;)

Fixes #24369.

src/basic/user-util.c

@@ -13,6 +13,7 @@
 #include "sd-messages.h"
 
 #include "alloc-util.h"
+#include "chase-symlinks.h"
 #include "errno-util.h"
 #include "fd-util.h"
 #include "fileio.h"
@@ -136,7 +137,6 @@ char *getusername_malloc(void) {
 }
 
 bool is_nologin_shell(const char *shell) {
-
         return PATH_IN_SET(shell,
                            /* 'nologin' is the friendliest way to disable logins for a user account. It prints a nice
                             * message and exits. Different distributions place the binary at different places though,
@@ -154,6 +154,21 @@ bool is_nologin_shell(const char *shell) {
                            "/usr/bin/true");
 }
 
+const char* default_root_shell(const char *root) {
+        /* We want to use the preferred shell, i.e. DEFAULT_USER_SHELL, which usually
+         * will be /bin/bash. Fall back to /bin/sh if DEFAULT_USER_SHELL is not found,
+         * or any access errors. */
+
+        int r = chase_symlinks(DEFAULT_USER_SHELL, root, CHASE_PREFIX_ROOT, NULL, NULL);
+        if (r < 0 && r != -ENOENT)
+                log_debug_errno(r, "Failed to look up shell '%s%s%s': %m",
+                                strempty(root), root ? "/" : "", DEFAULT_USER_SHELL);
+        if (r > 0)
+                return DEFAULT_USER_SHELL;
+
+        return "/bin/sh";
+}
+
 static int synthesize_user_creds(
                 const char **username,
                 uid_t *uid, gid_t *gid,
@@ -176,7 +191,7 @@ static int synthesize_user_creds(
                         *home = "/root";
 
                 if (shell)
-                        *shell = "/bin/sh";
+                        *shell = default_root_shell(NULL);
 
                 return 0;
         }
@@ -635,7 +650,7 @@ int get_shell(char **_s) {
         /* Hardcode shell for root and nobody to avoid NSS */
         u = getuid();
         if (u == 0) {
-                s = strdup("/bin/sh");
+                s = strdup(default_root_shell(NULL));
                 if (!s)
                         return -ENOMEM;
 

src/basic/user-util.h

@@ -130,6 +130,7 @@ int putsgent_sane(const struct sgrp *sg, FILE *stream);
 #endif
 
 bool is_nologin_shell(const char *shell);
+const char* default_root_shell(const char *root);
 
 int is_this_me(const char *username);