nspawn: implement configurable syscall whitelisting/blacklisting

Now that we have ported nspawn's seccomp code to the generic code in
seccomp-util, let's extend it to support whitelisting and blacklisting
of specific additional syscalls.

This uses similar syntax as PID1's support for system call filtering,
but in contrast to that always implements a blacklist (and not a
whitelist), as we prepopulate the filter with a blacklist, and the
unit's system call filter logic does not come with anything
prepopulated.

(Later on we might actually want to invert the logic here, and
whitelist rather than blacklist things, but at this point let's not do
that. In case we switch this over later, the syscall add/remove logic of
this commit should be compatible conceptually.)

Fixes: #5163

Replaces: #5944

src/nspawn/nspawn-settings.c

@@ -93,6 +93,8 @@ Settings* settings_free(Settings *s) {
         free(s->pivot_root_new);
         free(s->pivot_root_old);
         free(s->working_directory);
+        strv_free(s->syscall_whitelist);
+        strv_free(s->syscall_blacklist);
 
         strv_free(s->network_interfaces);
         strv_free(s->network_macvlan);
@@ -568,3 +570,51 @@ int config_parse_private_users(
 
         return 0;
 }
+
+int config_parse_syscall_filter(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        Settings *settings = data;
+        bool negative;
+        const char *items;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+
+        negative = rvalue[0] == '~';
+        items = negative ? rvalue + 1 : rvalue;
+
+        for (;;) {
+                _cleanup_free_ char *word = NULL;
+
+                r = extract_first_word(&items, &word, NULL, 0);
+                if (r == 0)
+                        break;
+                if (r == -ENOMEM)
+                        return log_oom();
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse SystemCallFilter= parameter %s, ignoring: %m", rvalue);
+                        return 0;
+                }
+
+                if (negative)
+                        r = strv_extend(&settings->syscall_blacklist, word);
+                else
+                        r = strv_extend(&settings->syscall_whitelist, word);
+                if (r < 0)
+                        return log_oom();
+        }
+
+        return 0;
+}