nss-systemd: make dynamic users enumerable by `getent`

This adds `setpwent()`, `getpwent_r()`, `endpwent()`, `setgrent()`,
`getgrent_r()`, and `endgrent()` interfaces to nss-systemd library.
Thus, dynamic users can be enumerated by e.g. `getent passwd` command.

2 files changed, 338 insertions, 0 deletions

diff --git a/src/nss-systemd/nss-systemd.c b/src/nss-systemd/nss-systemd.c
index a8705e6677..d5a061a551 100644
--- a/src/nss-systemd/nss-systemd.c
+++ b/src/nss-systemd/nss-systemd.c
@@ -19,13 +19,17 @@
 ***/
 
 #include <nss.h>
+#include <pthread.h>
 
 #include "sd-bus.h"
 
 #include "alloc-util.h"
 #include "bus-common-errors.h"
+#include "dirent-util.h"
 #include "env-util.h"
+#include "fd-util.h"
 #include "fs-util.h"
+#include "list.h"
 #include "macro.h"
 #include "nss-util.h"
 #include "signal-util.h"
@@ -73,8 +77,40 @@ static const struct group nobody_group = {
         .gr_mem = (char*[]) { NULL },
 };
 
+typedef struct UserEntry UserEntry;
+typedef struct GetentData GetentData;
+
+struct UserEntry {
+        uid_t id;
+        char *name;
+
+        GetentData *data;
+        LIST_FIELDS(UserEntry, entries);
+};
+
+struct GetentData {
+        /* As explained in NOTES section of getpwent_r(3) as 'getpwent_r() is not really
+         * reentrant since it shares the reading position in the stream with all other threads',
+         * we need to protect the data in UserEntry from multithreaded programs which may call
+         * setpwent(), getpwent_r(), or endpwent() simultaneously. So, each function locks the
+         * data by using the mutex below. */
+        pthread_mutex_t mutex;
+
+        UserEntry *position;
+        LIST_HEAD(UserEntry, entries);
+};
+
+static GetentData getpwent_data = { PTHREAD_MUTEX_INITIALIZER, NULL, NULL };
+static GetentData getgrent_data = { PTHREAD_MUTEX_INITIALIZER, NULL, NULL };
+
 NSS_GETPW_PROTOTYPES(systemd);
 NSS_GETGR_PROTOTYPES(systemd);
+enum nss_status _nss_systemd_endpwent(void) _public_;
+enum nss_status _nss_systemd_setpwent(int stayopen) _public_;
+enum nss_status _nss_systemd_getpwent_r(struct passwd *result, char *buffer, size_t buflen, int *errnop) _public_;
+enum nss_status _nss_systemd_endgrent(void) _public_;
+enum nss_status _nss_systemd_setgrent(int stayopen) _public_;
+enum nss_status _nss_systemd_getgrent_r(struct group *result, char *buffer, size_t buflen, int *errnop) _public_;
 
 static int direct_lookup_name(const char *name, uid_t *ret) {
         _cleanup_free_ char *s = NULL;
@@ -530,3 +566,299 @@ fail:
         *errnop = -r;
         return NSS_STATUS_UNAVAIL;
 }
+
+static void user_entry_free(UserEntry *p) {
+        if (!p)
+                return;
+
+        if (p->data)
+                LIST_REMOVE(entries, p->data->entries, p);
+
+        free(p->name);
+        free(p);
+}
+
+static int user_entry_add(GetentData *data, const char *name, uid_t id) {
+        UserEntry *p;
+
+        assert(data);
+
+        /* This happens when User= or Group= already exists statically. */
+        if (!uid_is_dynamic(id))
+                return -EINVAL;
+
+        p = new0(UserEntry, 1);
+        if (!p)
+                return -ENOMEM;
+
+        p->name = strdup(name);
+        if (!p->name) {
+                free(p);
+                return -ENOMEM;
+        }
+        p->id = id;
+        p->data = data;
+
+        LIST_PREPEND(entries, data->entries, p);
+
+        return 0;
+}
+
+static void systemd_endent(GetentData *data) {
+        UserEntry *p;
+
+        assert(data);
+
+        while ((p = data->entries))
+                user_entry_free(p);
+
+        data->position = NULL;
+}
+
+static enum nss_status nss_systemd_endent(GetentData *p) {
+        BLOCK_SIGNALS(NSS_SIGNALS_BLOCK);
+
+        assert_se(pthread_mutex_lock(&p->mutex) == 0);
+        systemd_endent(p);
+        assert_se(pthread_mutex_unlock(&p->mutex) == 0);
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_endpwent(void) {
+        return nss_systemd_endent(&getpwent_data);
+}
+
+enum nss_status _nss_systemd_endgrent(void) {
+        return nss_systemd_endent(&getgrent_data);
+}
+
+static int direct_enumeration(GetentData *p) {
+        _cleanup_closedir_ DIR *d = NULL;
+        struct dirent *de;
+        int r;
+
+        assert(p);
+
+        d = opendir("/run/systemd/dynamic-uid/");
+        if (!d)
+                return -errno;
+
+        FOREACH_DIRENT(de, d, return -errno) {
+                _cleanup_free_ char *name = NULL;
+                uid_t uid, verified;
+
+                if (!dirent_is_file(de))
+                        continue;
+
+                r = parse_uid(de->d_name, &uid);
+                if (r < 0)
+                        continue;
+
+                r = direct_lookup_uid(uid, &name);
+                if (r == -ENOMEM)
+                        return r;
+                if (r < 0)
+                        continue;
+
+                r = direct_lookup_name(name, &verified);
+                if (r < 0)
+                        continue;
+
+                if (uid != verified)
+                        continue;
+
+                r = user_entry_add(p, name, uid);
+                if (r == -ENOMEM)
+                        return r;
+                if (r < 0)
+                        continue;
+        }
+
+        return 0;
+}
+
+static enum nss_status systemd_setent(GetentData *p) {
+        _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
+        _cleanup_(sd_bus_message_unrefp) sd_bus_message* reply = NULL;
+        _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
+        const char *name;
+        uid_t id;
+        int bypass, r;
+
+        BLOCK_SIGNALS(NSS_SIGNALS_BLOCK);
+
+        assert(p);
+
+        assert_se(pthread_mutex_lock(&p->mutex) == 0);
+
+        systemd_endent(p);
+
+        if (getenv_bool_secure("SYSTEMD_NSS_DYNAMIC_BYPASS") > 0)
+                goto finish;
+
+        bypass = getenv_bool_secure("SYSTEMD_NSS_BYPASS_BUS");
+
+        if (bypass <= 0) {
+                r = sd_bus_open_system(&bus);
+                if (r < 0)
+                        bypass = 1;
+        }
+
+        if (bypass > 0) {
+                r = direct_enumeration(p);
+                if (r < 0)
+                        goto fail;
+
+                goto finish;
+        }
+
+        r = sd_bus_call_method(bus,
+                               "org.freedesktop.systemd1",
+                               "/org/freedesktop/systemd1",
+                               "org.freedesktop.systemd1.Manager",
+                               "GetDynamicUsers",
+                               &error,
+                               &reply,
+                               NULL);
+        if (r < 0)
+                goto fail;
+
+        r = sd_bus_message_enter_container(reply, 'a', "(us)");
+        if (r < 0)
+                goto fail;
+
+        while ((r = sd_bus_message_read(reply, "(us)", &id, &name)) > 0) {
+                r = user_entry_add(p, name, id);
+                if (r == -ENOMEM)
+                        goto fail;
+                if (r < 0)
+                        continue;
+        }
+        if (r < 0)
+                goto fail;
+
+        r = sd_bus_message_exit_container(reply);
+        if (r < 0)
+                goto fail;
+
+finish:
+        p->position = p->entries;
+        assert_se(pthread_mutex_unlock(&p->mutex) == 0);
+
+        return NSS_STATUS_SUCCESS;
+
+fail:
+        systemd_endent(p);
+        assert_se(pthread_mutex_unlock(&p->mutex) == 0);
+
+        return NSS_STATUS_UNAVAIL;
+}
+
+enum nss_status _nss_systemd_setpwent(int stayopen) {
+        return systemd_setent(&getpwent_data);
+}
+
+enum nss_status _nss_systemd_setgrent(int stayopen) {
+        return systemd_setent(&getgrent_data);
+}
+
+enum nss_status _nss_systemd_getpwent_r(struct passwd *result, char *buffer, size_t buflen, int *errnop) {
+        enum nss_status ret;
+        UserEntry *p;
+        size_t len;
+
+        BLOCK_SIGNALS(NSS_SIGNALS_BLOCK);
+
+        assert(result);
+        assert(buffer);
+        assert(errnop);
+
+        assert_se(pthread_mutex_lock(&getpwent_data.mutex) == 0);
+
+        LIST_FOREACH(entries, p, getpwent_data.position) {
+                len = strlen(p->name) + 1;
+                if (buflen < len) {
+                        *errnop = ERANGE;
+                        ret = NSS_STATUS_TRYAGAIN;
+                        goto finalize;
+                }
+
+                memcpy(buffer, p->name, len);
+
+                result->pw_name = buffer;
+                result->pw_uid = p->id;
+                result->pw_gid = p->id;
+                result->pw_gecos = (char*) DYNAMIC_USER_GECOS;
+                result->pw_passwd = (char*) DYNAMIC_USER_PASSWD;
+                result->pw_dir = (char*) DYNAMIC_USER_DIR;
+                result->pw_shell = (char*) DYNAMIC_USER_SHELL;
+                break;
+        }
+        if (!p) {
+                *errnop = ENOENT;
+                ret = NSS_STATUS_NOTFOUND;
+                goto finalize;
+        }
+
+        /* On success, step to the next entry. */
+        p = p->entries_next;
+        ret = NSS_STATUS_SUCCESS;
+
+finalize:
+        /* Save position for the next call. */
+        getpwent_data.position = p;
+
+        assert_se(pthread_mutex_unlock(&getpwent_data.mutex) == 0);
+
+        return ret;
+}
+
+enum nss_status _nss_systemd_getgrent_r(struct group *result, char *buffer, size_t buflen, int *errnop) {
+        enum nss_status ret;
+        UserEntry *p;
+        size_t len;
+
+        BLOCK_SIGNALS(NSS_SIGNALS_BLOCK);
+
+        assert(result);
+        assert(buffer);
+        assert(errnop);
+
+        assert_se(pthread_mutex_lock(&getgrent_data.mutex) == 0);
+
+        LIST_FOREACH(entries, p, getgrent_data.position) {
+                len = sizeof(char*) + strlen(p->name) + 1;
+                if (buflen < len) {
+                        *errnop = ERANGE;
+                        ret = NSS_STATUS_TRYAGAIN;
+                        goto finalize;
+                }
+
+                memzero(buffer, sizeof(char*));
+                strcpy(buffer + sizeof(char*), p->name);
+
+                result->gr_name = buffer + sizeof(char*);
+                result->gr_gid = p->id;
+                result->gr_passwd = (char*) DYNAMIC_USER_PASSWD;
+                result->gr_mem = (char**) buffer;
+                break;
+        }
+        if (!p) {
+                *errnop = ENOENT;
+                ret = NSS_STATUS_NOTFOUND;
+                goto finalize;
+        }
+
+        /* On success, step to the next entry. */
+        p = p->entries_next;
+        ret = NSS_STATUS_SUCCESS;
+
+finalize:
+        /* Save position for the next call. */
+        getgrent_data.position = p;
+
+        assert_se(pthread_mutex_unlock(&getgrent_data.mutex) == 0);
+
+        return ret;
+}
diff --git a/src/nss-systemd/nss-systemd.sym b/src/nss-systemd/nss-systemd.sym
index e3fe56f684..95038c833b 100644
--- a/src/nss-systemd/nss-systemd.sym
+++ b/src/nss-systemd/nss-systemd.sym
@@ -13,7 +13,13 @@
 global:
         _nss_systemd_getpwnam_r;
         _nss_systemd_getpwuid_r;
+        _nss_systemd_endpwent;
+        _nss_systemd_setpwent;
+        _nss_systemd_getpwent_r;
         _nss_systemd_getgrnam_r;
         _nss_systemd_getgrgid_r;
+        _nss_systemd_endgrent;
+        _nss_systemd_setgrent;
+        _nss_systemd_getgrent_r;
 local: *;
 };