run0: Add --empower

--empower gives full privileges to a non-root user. Currently this
includes all capabilities but we leave the option open to add more
privileges via this option in the future.

Why is this useful? When running privileged development or debugging
commands from your home directory (think bpftrace, strace and such),
you want any files written by these tools to be owned by your current
user, and not by the root user. run0 --empower will allow you to run
all privileged operations (assuming the tools check for capabilities
and not UIDs), while any files written by the tools will still be owned
by the current user.

src/run/run.c

@@ -25,6 +25,7 @@
 #include "bus-util.h"
 #include "bus-wait-for-jobs.h"
 #include "calendarspec.h"
+#include "capability-util.h"
 #include "capsule-util.h"
 #include "chase.h"
 #include "env-util.h"
@@ -117,6 +118,7 @@ static char *arg_shell_prompt_prefix = NULL;
 static int arg_lightweight = -1;
 static char *arg_area = NULL;
 static bool arg_via_shell = false;
+static bool arg_empower = false;
 
 STATIC_DESTRUCTOR_REGISTER(arg_description, freep);
 STATIC_DESTRUCTOR_REGISTER(arg_environment, strv_freep);
@@ -244,6 +246,7 @@ static int help_sudo_mode(void) {
                "     --lightweight=BOOLEAN        Control whether to register a session with service manager\n"
                "                                  or without\n"
                "     --area=AREA                  Home area to log into\n"
+               "     --empower                    Give privileges to selected or current user\n"
                "\nSee the %s for details.\n",
                program_invocation_short_name,
                ansi_highlight(),
@@ -253,11 +256,15 @@ static int help_sudo_mode(void) {
         return 0;
 }
 
+static bool become_root(void) {
+        return !arg_exec_user || STR_IN_SET(arg_exec_user, "root", "0");
+}
+
 static bool privileged_execution(void) {
         if (arg_runtime_scope != RUNTIME_SCOPE_SYSTEM)
                 return false;
 
-        return !arg_exec_user || STR_IN_SET(arg_exec_user, "root", "0");
+        return become_root() || arg_empower;
 }
 
 static int add_timer_property(const char *name, const char *val) {
@@ -859,6 +866,7 @@ static int parse_argv_sudo_mode(int argc, char *argv[]) {
                 ARG_LIGHTWEIGHT,
                 ARG_AREA,
                 ARG_VIA_SHELL,
+                ARG_EMPOWER,
         };
 
         /* If invoked as "run0" binary, let's expose a more sudo-like interface. We add various extensions
@@ -888,6 +896,7 @@ static int parse_argv_sudo_mode(int argc, char *argv[]) {
                 { "shell-prompt-prefix", required_argument, NULL, ARG_SHELL_PROMPT_PREFIX },
                 { "lightweight",         required_argument, NULL, ARG_LIGHTWEIGHT         },
                 { "area",                required_argument, NULL, ARG_AREA                },
+                { "empower",             no_argument,       NULL, ARG_EMPOWER             },
                 {},
         };
 
@@ -1027,6 +1036,10 @@ static int parse_argv_sudo_mode(int argc, char *argv[]) {
                         arg_via_shell = true;
                         break;
 
+                case ARG_EMPOWER:
+                        arg_empower = true;
+                        break;
+
                 case '?':
                         return -EINVAL;
 
@@ -1034,9 +1047,13 @@ static int parse_argv_sudo_mode(int argc, char *argv[]) {
                         assert_not_reached();
                 }
 
-        if (!arg_exec_user && arg_area) {
+        if (!arg_exec_user && (arg_area || arg_empower)) {
                 /* If the user specifies --area= but not --user= then consider this an area switch request,
-                 * and default to logging into our own account */
+                 * and default to logging into our own account.
+                 *
+                 * If the user specifies --empower but not --user= then consider this a request to empower
+                 * the current user. */
+
                 arg_exec_user = getusername_malloc();
                 if (!arg_exec_user)
                         return log_oom();
@@ -1211,8 +1228,8 @@ static int parse_argv_sudo_mode(int argc, char *argv[]) {
 
                 if (arg_lightweight >= 0) {
                         const char *class =
-                                arg_lightweight ? (arg_stdio == ARG_STDIO_PTY ? (privileged_execution() ? "user-early-light" : "user-light") : "background-light") :
-                                                  (arg_stdio == ARG_STDIO_PTY ? (privileged_execution() ? "user-early" : "user") : "background");
+                                arg_lightweight ? (arg_stdio == ARG_STDIO_PTY ? (become_root() ? "user-early-light" : "user-light") : "background-light") :
+                                                  (arg_stdio == ARG_STDIO_PTY ? (become_root() ? "user-early" : "user") : "background");
 
                         log_debug("Setting XDG_SESSION_CLASS to '%s'.", class);
 
@@ -1371,6 +1388,12 @@ static int transient_service_set_properties(sd_bus_message *m, const char *pty_p
                         return bus_log_create_error(r);
         }
 
+        if (arg_empower) {
+                r = sd_bus_message_append(m, "(sv)", "AmbientCapabilities", "t", CAP_MASK_ALL);
+                if (r < 0)
+                        return bus_log_create_error(r);
+        }
+
         if (arg_nice_set) {
                 r = sd_bus_message_append(m, "(sv)", "Nice", "i", arg_nice);
                 if (r < 0)