core,logind,systemctl,journald: replace calls to strerror() with setting errno + %m

strerror() is not thread safe and calling it just isn't worth the effort
required to justify why it would be safe in those cases. It's easier to just
use %m which is thread-safe out of the box. I don't think that any of the
changes in the patch cause any functional difference. This is just about
getting rid of calls to strerror() in general.

When we print an error message and fail to format the string, using something
like "(null)" is good enough. This is very very unlikely to happen anyway.

src/core/cgroup.c

@@ -398,17 +398,13 @@ static char *format_cgroup_memory_limit_comparison(char *buf, size_t l, Unit *u,
          * In the absence of reliably being able to detect whether memcg swap support is available or not,
          * only complain if the error is not ENOENT. */
         if (r > 0 || IN_SET(r, -ENODATA, -EOWNERDEAD) ||
-            (r == -ENOENT && streq(property_name, "MemorySwapMax"))) {
+            (r == -ENOENT && streq(property_name, "MemorySwapMax")))
                 buf[0] = 0;
-                return buf;
-        }
-
-        if (r < 0) {
-                (void) snprintf(buf, l, " (error getting kernel value: %s)", strerror_safe(r));
-                return buf;
-        }
-
-        (void) snprintf(buf, l, " (different value in kernel: %" PRIu64 ")", kval);
+        else if (r < 0) {
+                errno = -r;
+                (void) snprintf(buf, l, " (error getting kernel value: %m)");
+        } else
+                (void) snprintf(buf, l, " (different value in kernel: %" PRIu64 ")", kval);
 
         return buf;
 }

src/core/execute.c

@@ -6428,9 +6428,10 @@ static void exec_command_dump(ExecCommand *c, FILE *f, const char *prefix) {
         prefix2 = strjoina(prefix, "\t");
 
         cmd = quote_command_line(c->argv, SHELL_ESCAPE_EMPTY);
+
         fprintf(f,
                 "%sCommand Line: %s\n",
-                prefix, cmd ?: strerror_safe(ENOMEM));
+                prefix, strnull(cmd));
 
         exec_status_dump(&c->exec_status, f, prefix2);
 }

src/core/selinux-access.c

@@ -137,6 +137,7 @@ _printf_(2, 3) static int log_callback(int type, const char *fmt, ...) {
 }
 
 static int access_init(sd_bus_error *error) {
+        int r;
 
         if (!mac_selinux_use())
                 return 0;
@@ -145,23 +146,20 @@ static int access_init(sd_bus_error *error) {
                 return 1;
 
         if (avc_open(NULL, 0) != 0) {
-                int saved_errno = errno;
-                bool enforce;
+                r = -errno;  /* Save original errno for later */
 
-                enforce = security_getenforce() != 0;
-                log_full_errno(enforce ? LOG_ERR : LOG_WARNING, saved_errno, "Failed to open the SELinux AVC: %m");
+                bool enforce = security_getenforce() != 0;
+                log_full_errno(enforce ? LOG_ERR : LOG_WARNING, r, "Failed to open the SELinux AVC: %m");
 
-                /* If enforcement isn't on, then let's suppress this
-                 * error, and just don't do any AVC checks. The
-                 * warning we printed is hence all the admin will
-                 * see. */
+                /* If enforcement isn't on, then let's suppress this error, and just don't do any AVC checks.
+                 * The warning we printed is hence all the admin will see. */
                 if (!enforce)
                         return 0;
 
-                /* Return an access denied error, if we couldn't load
-                 * the AVC but enforcing mode was on, or we couldn't
-                 * determine whether it is one. */
-                return sd_bus_error_setf(error, SD_BUS_ERROR_ACCESS_DENIED, "Failed to open the SELinux AVC: %s", strerror_safe(saved_errno));
+                /* Return an access denied error based on the original errno, if we couldn't load the AVC but
+                 * enforcing mode was on, or we couldn't determine whether it is one. */
+                errno = -r;
+                return sd_bus_error_setf(error, SD_BUS_ERROR_ACCESS_DENIED, "Failed to open the SELinux AVC: %m");
         }
 
         selinux_set_callback(SELINUX_CB_AUDIT, (union selinux_callback) { .func_audit = audit_callback });

src/core/socket.c

@@ -739,16 +739,14 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
                 switch (p->type) {
                 case SOCKET_SOCKET: {
                         _cleanup_free_ char *k = NULL;
-                        const char *t;
                         int r;
 
                         r = socket_address_print(&p->address, &k);
-                        if (r < 0)
-                                t = strerror_safe(r);
-                        else
-                                t = k;
-
-                        fprintf(f, "%s%s: %s\n", prefix, listen_lookup(socket_address_family(&p->address), p->address.type), t);
+                        if (r < 0) {
+                                errno = -r;
+                                fprintf(f, "%s%s: %m\n", prefix, listen_lookup(socket_address_family(&p->address), p->address.type));
+                        } else
+                                fprintf(f, "%s%s: %s\n", prefix, listen_lookup(socket_address_family(&p->address), p->address.type), k);
                         break;
                 }
                 case SOCKET_SPECIAL:

src/core/unit-serialize.c

@@ -849,8 +849,10 @@ void unit_dump(Unit *u, FILE *f, const char *prefix) {
                 fprintf(f,
                         "%s\tMerged into: %s\n",
                         prefix, u->merged_into->id);
-        else if (u->load_state == UNIT_ERROR)
-                fprintf(f, "%s\tLoad Error Code: %s\n", prefix, strerror_safe(u->load_error));
+        else if (u->load_state == UNIT_ERROR) {
+                errno = abs(u->load_error);
+                fprintf(f, "%s\tLoad Error Code: %m\n", prefix);
+        }
 
         for (const char *n = sd_bus_track_first(u->bus_track); n; n = sd_bus_track_next(u->bus_track))
                 fprintf(f, "%s\tBus Ref: %s\n", prefix, n);

src/journal/journald-server.c

@@ -1094,7 +1094,8 @@ void server_driver_message(Server *s, pid_t object_pid, const char *message_id,
                 /* We failed to format the message. Emit a warning instead. */
                 char buf[LINE_MAX];
 
-                xsprintf(buf, "MESSAGE=Entry printing failed: %s", strerror_safe(r));
+                errno = -r;
+                xsprintf(buf, "MESSAGE=Entry printing failed: %m");
 
                 n = 3;
                 iovec[n++] = IOVEC_MAKE_STRING("PRIORITY=4");

src/systemctl/systemctl-show.c

@@ -670,8 +670,10 @@ static void print_status_info(
 
         if (i->status_text)
                 printf("     Status: \"%s\"\n", i->status_text);
-        if (i->status_errno > 0)
-                printf("      Error: %i (%s)\n", i->status_errno, strerror_safe(i->status_errno));
+        if (i->status_errno > 0) {
+                errno = i->status_errno;
+                printf("      Error: %i (%m)\n", i->status_errno);
+        }
 
         if (i->ip_ingress_bytes != UINT64_MAX && i->ip_egress_bytes != UINT64_MAX)
                 printf("         IP: %s in, %s out\n",