systemctl-whoami: use pidfd to refer to processes

While at it, rephrase the output a bit. Before this commit, if
the pid doesn't exist, we output something hard to interpret -
"Failed to get unit for ourselves".

1 files changed, 134 insertions, 30 deletions

diff --git a/src/systemctl/systemctl-whoami.c b/src/systemctl/systemctl-whoami.c
index 4ee6592525..bac72c8973 100644
--- a/src/systemctl/systemctl-whoami.c
+++ b/src/systemctl/systemctl-whoami.c
@@ -1,24 +1,39 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include "bus-common-errors.h"
 #include "bus-error.h"
 #include "bus-locator.h"
+#include "format-util.h"
+#include "parse-util.h"
+#include "pidref.h"
+#include "process-util.h"
 #include "systemctl.h"
 #include "systemctl-util.h"
 #include "systemctl-whoami.h"
-#include "parse-util.h"
 
-static int lookup_pid(sd_bus *bus, pid_t pid) {
+static int get_unit_by_pid(sd_bus *bus, pid_t pid, char **ret) {
         _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
         _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
         _cleanup_free_ char *unit = NULL;
         const char *path;
         int r;
 
+        assert(bus);
+        assert(pid >= 0); /* 0 is accepted by GetUnitByPID for querying our own process. */
+        assert(ret);
+
         r = bus_call_method(bus, bus_systemd_mgr, "GetUnitByPID", &error, &reply, "u", (uint32_t) pid);
-        if (r < 0)
-                return log_error_errno(r, "Failed to get unit for ourselves: %s", bus_error_message(&error, r));
+        if (r < 0) {
+                if (sd_bus_error_has_name(&error, BUS_ERROR_NO_UNIT_FOR_PID))
+                        return log_error_errno(r, "%s", bus_error_message(&error, r));
+
+                return log_error_errno(r,
+                                       "Failed to get unit that PID " PID_FMT " belongs to: %s",
+                                       pid > 0 ? pid : getpid_cached(),
+                                       bus_error_message(&error, r));
+        }
 
-        r = sd_bus_message_read(reply, "o", &path);
+        r = sd_bus_message_read_basic(reply, 'o', &path);
         if (r < 0)
                 return bus_log_parse_error(r);
 
@@ -26,7 +41,103 @@ static int lookup_pid(sd_bus *bus, pid_t pid) {
         if (r < 0)
                 return log_error_errno(r, "Failed to extract unit name from D-Bus object path '%s': %m", path);
 
-        printf("%s\n", unit);
+        *ret = TAKE_PTR(unit);
+        return 0;
+}
+
+static int lookup_pidfd(sd_bus *bus, const PidRef *pid, char **ret) {
+        _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
+        _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
+        const char *unit;
+        int r;
+
+        assert(bus);
+        assert(pidref_is_set(pid));
+        assert(ret);
+
+        if (pid->fd < 0)
+                return -EOPNOTSUPP;
+
+        r = bus_call_method(bus, bus_systemd_mgr, "GetUnitByPIDFD", &error, &reply, "h", pid->fd);
+        if (r < 0) {
+                if (sd_bus_error_has_name(&error, SD_BUS_ERROR_UNKNOWN_METHOD))
+                        return -EOPNOTSUPP;
+
+                if (sd_bus_error_has_names(&error, BUS_ERROR_NO_UNIT_FOR_PID, BUS_ERROR_NO_SUCH_PROCESS))
+                        return log_error_errno(r, "%s", bus_error_message(&error, r));
+
+                return log_error_errno(r,
+                                       "Failed to get unit that PID " PID_FMT " belongs to: %s",
+                                       pid->pid, bus_error_message(&error, r));
+        }
+
+        r = sd_bus_message_read(reply, "os", NULL, &unit);
+        if (r < 0)
+                return bus_log_parse_error(r);
+
+        char *u = strdup(unit);
+        if (!u)
+                return log_oom();
+
+        *ret = TAKE_PTR(u);
+
+        return 0;
+}
+
+static int lookup_pid(sd_bus *bus, const char *pidstr) {
+        _cleanup_free_ char *unit = NULL;
+        int r;
+
+        assert(bus);
+        assert(pidstr);
+
+        if (arg_transport == BUS_TRANSPORT_LOCAL) {
+                static bool use_pidfd = true;
+                _cleanup_(pidref_done) PidRef pid = PIDREF_NULL;
+
+                r = pidref_set_pidstr(&pid, pidstr);
+                if (r < 0)
+                        return log_error_errno(r,
+                                               r == -ESRCH ?
+                                               "PID %s doesn't exist or is already gone." :
+                                               "Failed to create reference to PID %s: %m",
+                                               pidstr);
+
+                if (use_pidfd) {
+                        r = lookup_pidfd(bus, &pid, &unit);
+                        if (r == -EOPNOTSUPP) {
+                                use_pidfd = false;
+                                log_debug_errno(r, "Unable to look up process using pidfd, ignoring.");
+                        } else if (r < 0)
+                                return r;
+                }
+
+                if (!use_pidfd) {
+                        assert(!unit);
+
+                        r = get_unit_by_pid(bus, pid.pid, &unit);
+                        if (r < 0)
+                                return r;
+
+                        r = pidref_verify(&pid);
+                        if (r < 0)
+                                return log_error_errno(r,
+                                                       "Failed to verify our reference to PID " PID_FMT ": %m",
+                                                       pid.pid);
+                }
+        } else {
+                pid_t pid;
+
+                r = parse_pid(pidstr, &pid);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse PID %s: %m", pidstr);
+
+                r = get_unit_by_pid(bus, pid, &unit);
+                if (r < 0)
+                        return r;
+        }
+
+        puts(unit);
         return 0;
 }
 
@@ -38,33 +149,26 @@ int verb_whoami(int argc, char *argv[], void *userdata) {
         if (r < 0)
                 return r;
 
-        char **pids = strv_skip(argv, 1);
-
-        if (strv_isempty(pids)) {
+        if (argc <= 1) {
+                _cleanup_free_ char *unit = NULL;
 
                 if (arg_transport != BUS_TRANSPORT_LOCAL)
-                        return log_error_errno(SYNTHETIC_ERRNO(EREMOTE), "Refusing to look up local PID on remote host.");
+                        return log_error_errno(SYNTHETIC_ERRNO(EREMOTE), "Refusing to look up our local PID on remote host.");
 
-                return lookup_pid(bus, 0);
-        } else {
-                int ret = 0;
-
-                STRV_FOREACH(p, pids) {
-                        pid_t pid;
-
-                        r = parse_pid(*p, &pid);
-                        if (r < 0) {
-                                log_error_errno(r, "Failed to parse PID: %s", *p);
-                                if (ret >= 0)
-                                        ret = r;
-                                continue;
-                        }
-
-                        r = lookup_pid(bus, pid);
-                        if (r < 0 && ret >= 0)
-                                ret = r;
-                }
+                /* Our own process can never go away while querying, hence no need to open pidfd. */
+
+                r = get_unit_by_pid(bus, 0, &unit);
+                if (r < 0)
+                        return r;
 
-                return ret;
+                puts(unit);
+                return 0;
         }
+
+        r = 0;
+
+        STRV_FOREACH(pid, strv_skip(argv, 1))
+                RET_GATHER(r, lookup_pid(bus, *pid));
+
+        return r;
 }