Adding upstream version 252.22.upstream/252.22upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

5 files changed, 1638 insertions, 0 deletions

diff --git a/src/nss-systemd/nss-systemd.c b/src/nss-systemd/nss-systemd.c
new file mode 100644
index 0000000..1d6e253
--- /dev/null
+++ b/src/nss-systemd/nss-systemd.c
@@ -0,0 +1,1084 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <nss.h>
+#include <pthread.h>
+#include <string.h>
+
+#include "env-util.h"
+#include "errno-util.h"
+#include "fd-util.h"
+#include "log.h"
+#include "macro.h"
+#include "missing_threads.h"
+#include "nss-systemd.h"
+#include "nss-util.h"
+#include "pthread-util.h"
+#include "signal-util.h"
+#include "strv.h"
+#include "user-record-nss.h"
+#include "user-util.h"
+#include "userdb-glue.h"
+#include "userdb.h"
+
+static const struct passwd root_passwd = {
+        .pw_name = (char*) "root",
+        .pw_passwd = (char*) PASSWORD_SEE_SHADOW,
+        .pw_uid = 0,
+        .pw_gid = 0,
+        .pw_gecos = (char*) "Super User",
+        .pw_dir = (char*) "/root",
+        .pw_shell = NULL,
+};
+
+static const struct spwd root_spwd = {
+        .sp_namp = (char*) "root",
+        .sp_pwdp = (char*) PASSWORD_LOCKED_AND_INVALID,
+        .sp_lstchg = -1,
+        .sp_min = -1,
+        .sp_max = -1,
+        .sp_warn = -1,
+        .sp_inact = -1,
+        .sp_expire = -1,
+        .sp_flag = ULONG_MAX, /* this appears to be what everybody does ... */
+};
+
+static const struct passwd nobody_passwd = {
+        .pw_name = (char*) NOBODY_USER_NAME,
+        .pw_passwd = (char*) PASSWORD_LOCKED_AND_INVALID,
+        .pw_uid = UID_NOBODY,
+        .pw_gid = GID_NOBODY,
+        .pw_gecos = (char*) "Kernel Overflow User",
+        .pw_dir = (char*) "/",
+        .pw_shell = (char*) NOLOGIN,
+};
+
+static const struct spwd nobody_spwd = {
+        .sp_namp = (char*) NOBODY_USER_NAME,
+        .sp_pwdp = (char*) PASSWORD_LOCKED_AND_INVALID,
+        .sp_lstchg = -1,
+        .sp_min = -1,
+        .sp_max = -1,
+        .sp_warn = -1,
+        .sp_inact = -1,
+        .sp_expire = -1,
+        .sp_flag = ULONG_MAX, /* this appears to be what everybody does ... */
+};
+
+static const struct group root_group = {
+        .gr_name = (char*) "root",
+        .gr_gid = 0,
+        .gr_passwd = (char*) PASSWORD_SEE_SHADOW,
+        .gr_mem = (char*[]) { NULL },
+};
+
+static const struct sgrp root_sgrp = {
+        .sg_namp = (char*) "root",
+        .sg_passwd = (char*) PASSWORD_LOCKED_AND_INVALID,
+};
+
+static const struct group nobody_group = {
+        .gr_name = (char*) NOBODY_GROUP_NAME,
+        .gr_gid = GID_NOBODY,
+        .gr_passwd = (char*) PASSWORD_LOCKED_AND_INVALID,
+        .gr_mem = (char*[]) { NULL },
+};
+
+static const struct sgrp nobody_sgrp = {
+        .sg_namp = (char*) NOBODY_GROUP_NAME,
+        .sg_passwd = (char*) PASSWORD_LOCKED_AND_INVALID,
+};
+
+typedef 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
+         * UserDBIterator 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;
+        UserDBIterator *iterator;
+
+        /* Applies to group iterations only: true while we iterate over groups defined through NSS, false
+         * otherwise. */
+        bool by_membership;
+} GetentData;
+
+static GetentData getpwent_data = {
+        .mutex = PTHREAD_MUTEX_INITIALIZER,
+};
+
+static GetentData getgrent_data = {
+        .mutex = PTHREAD_MUTEX_INITIALIZER,
+};
+
+static GetentData getspent_data = {
+        .mutex = PTHREAD_MUTEX_INITIALIZER,
+};
+
+static GetentData getsgent_data = {
+        .mutex = PTHREAD_MUTEX_INITIALIZER,
+};
+
+static void setup_logging_once(void) {
+        static pthread_once_t once = PTHREAD_ONCE_INIT;
+        assert_se(pthread_once(&once, log_parse_environment_variables) == 0);
+}
+
+#define NSS_ENTRYPOINT_BEGIN                    \
+        BLOCK_SIGNALS(NSS_SIGNALS_BLOCK);       \
+        setup_logging_once()
+
+NSS_GETPW_PROTOTYPES(systemd);
+NSS_GETSP_PROTOTYPES(systemd);
+NSS_GETGR_PROTOTYPES(systemd);
+NSS_GETSG_PROTOTYPES(systemd);
+NSS_PWENT_PROTOTYPES(systemd);
+NSS_SPENT_PROTOTYPES(systemd);
+NSS_GRENT_PROTOTYPES(systemd);
+NSS_SGENT_PROTOTYPES(systemd);
+NSS_INITGROUPS_PROTOTYPE(systemd);
+
+/* Since our NSS functions implement reentrant glibc APIs, we have to guarantee
+ * all the string pointers we return point into the buffer provided by the
+ * caller, not into our own static memory. */
+
+static enum nss_status copy_synthesized_passwd(
+                struct passwd *dest,
+                const struct passwd *src,
+                const char *fallback_shell,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        assert(dest);
+        assert(src);
+        assert(src->pw_name);
+        assert(src->pw_passwd);
+        assert(src->pw_gecos);
+        assert(src->pw_dir);
+
+        const char *shell = ASSERT_PTR(src->pw_shell ?: fallback_shell);
+
+        size_t required =
+                strlen(src->pw_name) + 1 +
+                strlen(src->pw_passwd) + 1 +
+                strlen(src->pw_gecos) + 1 +
+                strlen(src->pw_dir) + 1 +
+                strlen(shell) + 1;
+
+        if (buflen < required) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->pw_name = buffer;
+        dest->pw_passwd = stpcpy(dest->pw_name, src->pw_name) + 1;
+        dest->pw_gecos = stpcpy(dest->pw_passwd, src->pw_passwd) + 1;
+        dest->pw_dir = stpcpy(dest->pw_gecos, src->pw_gecos) + 1;
+        dest->pw_shell = stpcpy(dest->pw_dir, src->pw_dir) + 1;
+        strcpy(dest->pw_shell, shell);
+
+        return NSS_STATUS_SUCCESS;
+}
+
+static enum nss_status copy_synthesized_spwd(
+                struct spwd *dest,
+                const struct spwd *src,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        assert(dest);
+        assert(src);
+        assert(src->sp_namp);
+        assert(src->sp_pwdp);
+
+        size_t required =
+                strlen(src->sp_namp) + 1 +
+                strlen(src->sp_pwdp) + 1;
+
+        if (buflen < required) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->sp_namp = buffer;
+        dest->sp_pwdp = stpcpy(dest->sp_namp, src->sp_namp) + 1;
+        strcpy(dest->sp_pwdp, src->sp_pwdp);
+
+        return NSS_STATUS_SUCCESS;
+}
+
+static enum nss_status copy_synthesized_group(
+                struct group *dest,
+                const struct group *src,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        assert(dest);
+        assert(src);
+        assert(src->gr_name);
+        assert(src->gr_passwd);
+        assert(src->gr_mem);
+        assert(!*src->gr_mem); /* Our synthesized records' gr_mem is always just NULL... */
+
+        size_t required =
+                strlen(src->gr_name) + 1 +
+                strlen(src->gr_passwd) + 1 +
+                sizeof(char*); /* ...but that NULL still needs to be stored into the buffer! */
+
+        if (buflen < ALIGN(required)) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->gr_name = buffer;
+        dest->gr_passwd = stpcpy(dest->gr_name, src->gr_name) + 1;
+        dest->gr_mem = ALIGN_PTR(stpcpy(dest->gr_passwd, src->gr_passwd) + 1);
+        *dest->gr_mem = NULL;
+
+        return NSS_STATUS_SUCCESS;
+}
+
+static enum nss_status copy_synthesized_sgrp(
+                struct sgrp *dest,
+                const struct sgrp *src,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        assert(dest);
+        assert(src);
+        assert(src->sg_namp);
+        assert(src->sg_passwd);
+
+        size_t required =
+                strlen(src->sg_namp) + 1 +
+                strlen(src->sg_passwd) + 1;
+
+        if (buflen < required) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->sg_namp = buffer;
+        dest->sg_passwd = stpcpy(dest->sg_namp, src->sg_namp) + 1;
+        strcpy(dest->sg_passwd, src->sg_passwd);
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_getpwnam_r(
+                const char *name,
+                struct passwd *pwd,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        enum nss_status status;
+        int e;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(name);
+        assert(pwd);
+        assert(errnop);
+
+        /* If the username is not valid, then we don't know it. Ideally libc would filter these for us
+         * anyway. We don't generate EINVAL here, because it isn't really out business to complain about
+         * invalid user names. */
+        if (!valid_user_group_name(name, VALID_USER_RELAX))
+                return NSS_STATUS_NOTFOUND;
+
+        /* Synthesize entries for the root and nobody users, in case they are missing in /etc/passwd */
+        if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
+
+                if (streq(name, root_passwd.pw_name))
+                        return copy_synthesized_passwd(pwd, &root_passwd,
+                                                       default_root_shell(NULL),
+                                                       buffer, buflen, errnop);
+
+                if (streq(name, nobody_passwd.pw_name)) {
+                        if (!synthesize_nobody())
+                                return NSS_STATUS_NOTFOUND;
+
+                        return copy_synthesized_passwd(pwd, &nobody_passwd,
+                                                       NULL,
+                                                       buffer, buflen, errnop);
+                }
+
+        } else if (STR_IN_SET(name, root_passwd.pw_name, nobody_passwd.pw_name))
+                return NSS_STATUS_NOTFOUND;
+
+        status = userdb_getpwnam(name, pwd, buffer, buflen, &e);
+        if (IN_SET(status, NSS_STATUS_UNAVAIL, NSS_STATUS_TRYAGAIN)) {
+                UNPROTECT_ERRNO;
+                *errnop = e;
+                return status;
+        }
+
+        return status;
+}
+
+enum nss_status _nss_systemd_getpwuid_r(
+                uid_t uid,
+                struct passwd *pwd,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        enum nss_status status;
+        int e;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(pwd);
+        assert(errnop);
+
+        if (!uid_is_valid(uid))
+                return NSS_STATUS_NOTFOUND;
+
+        /* Synthesize data for the root user and for nobody in case they are missing from /etc/passwd */
+        if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
+
+                if (uid == root_passwd.pw_uid)
+                        return copy_synthesized_passwd(pwd, &root_passwd,
+                                                       default_root_shell(NULL),
+                                                       buffer, buflen, errnop);
+
+                if (uid == nobody_passwd.pw_uid) {
+                        if (!synthesize_nobody())
+                                return NSS_STATUS_NOTFOUND;
+
+                        return copy_synthesized_passwd(pwd, &nobody_passwd,
+                                                       NULL,
+                                                       buffer, buflen, errnop);
+                }
+
+        } else if (uid == root_passwd.pw_uid || uid == nobody_passwd.pw_uid)
+                return NSS_STATUS_NOTFOUND;
+
+        status = userdb_getpwuid(uid, pwd, buffer, buflen, &e);
+        if (IN_SET(status, NSS_STATUS_UNAVAIL, NSS_STATUS_TRYAGAIN)) {
+                UNPROTECT_ERRNO;
+                *errnop = e;
+                return status;
+        }
+
+        return status;
+}
+
+enum nss_status _nss_systemd_getspnam_r(
+                const char *name,
+                struct spwd *spwd,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        enum nss_status status;
+        int e;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(name);
+        assert(spwd);
+        assert(errnop);
+
+        if (!valid_user_group_name(name, VALID_USER_RELAX))
+                return NSS_STATUS_NOTFOUND;
+
+        /* Synthesize entries for the root and nobody users, in case they are missing in /etc/passwd */
+        if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
+
+                if (streq(name, root_spwd.sp_namp))
+                        return copy_synthesized_spwd(spwd, &root_spwd, buffer, buflen, errnop);
+
+                if (streq(name, nobody_spwd.sp_namp)) {
+                        if (!synthesize_nobody())
+                                return NSS_STATUS_NOTFOUND;
+
+                        return copy_synthesized_spwd(spwd, &nobody_spwd, buffer, buflen, errnop);
+                }
+
+        } else if (STR_IN_SET(name, root_spwd.sp_namp, nobody_spwd.sp_namp))
+                return NSS_STATUS_NOTFOUND;
+
+        status = userdb_getspnam(name, spwd, buffer, buflen, &e);
+        if (IN_SET(status, NSS_STATUS_UNAVAIL, NSS_STATUS_TRYAGAIN)) {
+                UNPROTECT_ERRNO;
+                *errnop = e;
+                return status;
+        }
+
+        return status;
+}
+
+#pragma GCC diagnostic ignored "-Wsizeof-pointer-memaccess"
+
+enum nss_status _nss_systemd_getgrnam_r(
+                const char *name,
+                struct group *gr,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        enum nss_status status;
+        int e;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(name);
+        assert(gr);
+        assert(errnop);
+
+        if (!valid_user_group_name(name, VALID_USER_RELAX))
+                return NSS_STATUS_NOTFOUND;
+
+        /* Synthesize records for root and nobody, in case they are missing from /etc/group */
+        if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
+
+                if (streq(name, root_group.gr_name))
+                        return copy_synthesized_group(gr, &root_group, buffer, buflen, errnop);
+
+                if (streq(name, nobody_group.gr_name)) {
+                        if (!synthesize_nobody())
+                                return NSS_STATUS_NOTFOUND;
+
+                        return copy_synthesized_group(gr, &nobody_group, buffer, buflen, errnop);
+                }
+
+        } else if (STR_IN_SET(name, root_group.gr_name, nobody_group.gr_name))
+                return NSS_STATUS_NOTFOUND;
+
+        status = userdb_getgrnam(name, gr, buffer, buflen, &e);
+        if (IN_SET(status, NSS_STATUS_UNAVAIL, NSS_STATUS_TRYAGAIN)) {
+                UNPROTECT_ERRNO;
+                *errnop = e;
+                return status;
+        }
+
+        return status;
+}
+
+enum nss_status _nss_systemd_getgrgid_r(
+                gid_t gid,
+                struct group *gr,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        enum nss_status status;
+        int e;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(gr);
+        assert(errnop);
+
+        if (!gid_is_valid(gid))
+                return NSS_STATUS_NOTFOUND;
+
+        /* Synthesize records for root and nobody, in case they are missing from /etc/group */
+        if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
+
+                if (gid == root_group.gr_gid)
+                        return copy_synthesized_group(gr, &root_group, buffer, buflen, errnop);
+
+                if (gid == nobody_group.gr_gid) {
+                        if (!synthesize_nobody())
+                                return NSS_STATUS_NOTFOUND;
+
+                        return copy_synthesized_group(gr, &nobody_group, buffer, buflen, errnop);
+                }
+
+        } else if (gid == root_group.gr_gid || gid == nobody_group.gr_gid)
+                return NSS_STATUS_NOTFOUND;
+
+        status = userdb_getgrgid(gid, gr, buffer, buflen, &e);
+        if (IN_SET(status, NSS_STATUS_UNAVAIL, NSS_STATUS_TRYAGAIN)) {
+                UNPROTECT_ERRNO;
+                *errnop = e;
+                return status;
+        }
+
+        return status;
+}
+
+enum nss_status _nss_systemd_getsgnam_r(
+                const char *name,
+                struct sgrp *sgrp,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        enum nss_status status;
+        int e;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(name);
+        assert(sgrp);
+        assert(errnop);
+
+        if (!valid_user_group_name(name, VALID_USER_RELAX))
+                return NSS_STATUS_NOTFOUND;
+
+        /* Synthesize records for root and nobody, in case they are missing from /etc/group */
+        if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
+
+                if (streq(name, root_sgrp.sg_namp))
+                        return copy_synthesized_sgrp(sgrp, &root_sgrp, buffer, buflen, errnop);
+
+                if (streq(name, nobody_sgrp.sg_namp)) {
+                        if (!synthesize_nobody())
+                                return NSS_STATUS_NOTFOUND;
+
+                        return copy_synthesized_sgrp(sgrp, &nobody_sgrp, buffer, buflen, errnop);
+                }
+
+        } else if (STR_IN_SET(name, root_sgrp.sg_namp, nobody_sgrp.sg_namp))
+                return NSS_STATUS_NOTFOUND;
+
+        status = userdb_getsgnam(name, sgrp, buffer, buflen, &e);
+        if (IN_SET(status, NSS_STATUS_UNAVAIL, NSS_STATUS_TRYAGAIN)) {
+                UNPROTECT_ERRNO;
+                *errnop = e;
+                return status;
+        }
+
+        return status;
+}
+
+static enum nss_status nss_systemd_endent(GetentData *p) {
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(p);
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&p->mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        p->iterator = userdb_iterator_free(p->iterator);
+        p->by_membership = false;
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_endpwent(void) {
+        return nss_systemd_endent(&getpwent_data);
+}
+
+enum nss_status _nss_systemd_endspent(void) {
+        return nss_systemd_endent(&getspent_data);
+}
+
+enum nss_status _nss_systemd_endgrent(void) {
+        return nss_systemd_endent(&getgrent_data);
+}
+
+enum nss_status _nss_systemd_endsgent(void) {
+        return nss_systemd_endent(&getsgent_data);
+}
+
+enum nss_status _nss_systemd_setpwent(int stayopen) {
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getpwent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        getpwent_data.iterator = userdb_iterator_free(getpwent_data.iterator);
+        getpwent_data.by_membership = false;
+
+        /* Don't synthesize root/nobody when iterating. Let nss-files take care of that. If the two records
+         * are missing there, then that's fine, after all getpwent() is known to be possibly incomplete
+         * (think: LDAP/NIS type situations), and our synthesizing of root/nobody is a robustness fallback
+         * only, which matters for getpwnam()/getpwuid() primarily, which are the main NSS entrypoints to the
+         * user database. */
+        r = userdb_all(nss_glue_userdb_flags() | USERDB_DONT_SYNTHESIZE, &getpwent_data.iterator);
+        return r < 0 ? NSS_STATUS_UNAVAIL : NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_setgrent(int stayopen) {
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getgrent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        getgrent_data.iterator = userdb_iterator_free(getgrent_data.iterator);
+        getgrent_data.by_membership = false;
+
+        /* See _nss_systemd_setpwent() for an explanation why we use USERDB_DONT_SYNTHESIZE here */
+        r = groupdb_all(nss_glue_userdb_flags() | USERDB_DONT_SYNTHESIZE, &getgrent_data.iterator);
+        return r < 0 ? NSS_STATUS_UNAVAIL : NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_setspent(int stayopen) {
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getspent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        getspent_data.iterator = userdb_iterator_free(getspent_data.iterator);
+        getspent_data.by_membership = false;
+
+        /* See _nss_systemd_setpwent() for an explanation why we use USERDB_DONT_SYNTHESIZE here */
+        r = userdb_all(nss_glue_userdb_flags() | USERDB_DONT_SYNTHESIZE, &getspent_data.iterator);
+        return r < 0 ? NSS_STATUS_UNAVAIL : NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_setsgent(int stayopen) {
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getsgent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        getsgent_data.iterator = userdb_iterator_free(getsgent_data.iterator);
+        getsgent_data.by_membership = false;
+
+        /* See _nss_systemd_setpwent() for an explanation why we use USERDB_DONT_SYNTHESIZE here */
+        r = groupdb_all(nss_glue_userdb_flags() | USERDB_DONT_SYNTHESIZE, &getsgent_data.iterator);
+        return r < 0 ? NSS_STATUS_UNAVAIL : NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_getpwent_r(
+                struct passwd *result,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        _cleanup_(user_record_unrefp) UserRecord *ur = NULL;
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(result);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getpwent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        if (!getpwent_data.iterator) {
+                UNPROTECT_ERRNO;
+                *errnop = EHOSTDOWN;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        r = userdb_iterator_get(getpwent_data.iterator, &ur);
+        if (r == -ESRCH)
+                return NSS_STATUS_NOTFOUND;
+        if (r < 0) {
+                UNPROTECT_ERRNO;
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        r = nss_pack_user_record(ur, result, buffer, buflen);
+        if (r < 0) {
+                UNPROTECT_ERRNO;
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_getgrent_r(
+                struct group *result,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        _cleanup_(group_record_unrefp) GroupRecord *gr = NULL;
+        _cleanup_free_ char **members = NULL;
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(result);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getgrent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        if (!getgrent_data.iterator) {
+                UNPROTECT_ERRNO;
+                *errnop = EHOSTDOWN;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        if (!getgrent_data.by_membership) {
+                r = groupdb_iterator_get(getgrent_data.iterator, &gr);
+                if (r == -ESRCH) {
+                        /* So we finished iterating native groups now. Let's now continue with iterating
+                         * native memberships, and generate additional group entries for any groups
+                         * referenced there that are defined in NSS only. This means for those groups there
+                         * will be two or more entries generated during iteration, but this is apparently how
+                         * this is supposed to work, and what other implementations do too. Clients are
+                         * supposed to merge the group records found during iteration automatically. */
+                        getgrent_data.iterator = userdb_iterator_free(getgrent_data.iterator);
+
+                        r = membershipdb_all(nss_glue_userdb_flags(), &getgrent_data.iterator);
+                        if (r < 0 && r != -ESRCH) {
+                                UNPROTECT_ERRNO;
+                                *errnop = -r;
+                                return NSS_STATUS_UNAVAIL;
+                        }
+
+                        getgrent_data.by_membership = true;
+                } else if (r < 0) {
+                        UNPROTECT_ERRNO;
+                        *errnop = -r;
+                        return NSS_STATUS_UNAVAIL;
+                } else if (!STR_IN_SET(gr->group_name, root_group.gr_name, nobody_group.gr_name)) {
+                        r = membershipdb_by_group_strv(gr->group_name, nss_glue_userdb_flags(), &members);
+                        if (r < 0 && r != -ESRCH) {
+                                UNPROTECT_ERRNO;
+                                *errnop = -r;
+                                return NSS_STATUS_UNAVAIL;
+                        }
+                }
+        }
+
+        if (getgrent_data.by_membership) {
+                _cleanup_(_nss_systemd_unblockp) bool blocked = false;
+
+                if (!getgrent_data.iterator)
+                        return NSS_STATUS_NOTFOUND;
+
+                for (;;) {
+                        _cleanup_free_ char *user_name = NULL, *group_name = NULL;
+
+                        r = membershipdb_iterator_get(getgrent_data.iterator, &user_name, &group_name);
+                        if (r == -ESRCH)
+                                return NSS_STATUS_NOTFOUND;
+                        if (r < 0) {
+                                UNPROTECT_ERRNO;
+                                *errnop = -r;
+                                return NSS_STATUS_UNAVAIL;
+                        }
+
+                        if (STR_IN_SET(user_name, root_passwd.pw_name, nobody_passwd.pw_name))
+                                continue;
+                        if (STR_IN_SET(group_name, root_group.gr_name, nobody_group.gr_name))
+                                continue;
+
+                        /* We are about to recursively call into NSS, let's make sure we disable recursion into our own code. */
+                        if (!blocked) {
+                                r = _nss_systemd_block(true);
+                                if (r < 0) {
+                                        UNPROTECT_ERRNO;
+                                        *errnop = -r;
+                                        return NSS_STATUS_UNAVAIL;
+                                }
+
+                                blocked = true;
+                        }
+
+                        r = nss_group_record_by_name(group_name, false, &gr);
+                        if (r == -ESRCH)
+                                continue;
+                        if (r < 0) {
+                                log_debug_errno(r, "Failed to do NSS check for group '%s', ignoring: %m", group_name);
+                                continue;
+                        }
+
+                        members = strv_new(user_name);
+                        if (!members) {
+                                UNPROTECT_ERRNO;
+                                *errnop = ENOMEM;
+                                return NSS_STATUS_TRYAGAIN;
+                        }
+
+                        /* Note that we currently generate one group entry per user that is part of a
+                         * group. It's a bit ugly, but equivalent to generating a single entry with a set of
+                         * members in them. */
+                        break;
+                }
+        }
+
+        r = nss_pack_group_record(gr, members, result, buffer, buflen);
+        if (r < 0) {
+                UNPROTECT_ERRNO;
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_getspent_r(
+                struct spwd *result,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        _cleanup_(user_record_unrefp) UserRecord *ur = NULL;
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(result);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getspent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        if (!getspent_data.iterator) {
+                UNPROTECT_ERRNO;
+                *errnop = EHOSTDOWN;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        for (;;) {
+                r = userdb_iterator_get(getspent_data.iterator, &ur);
+                if (r == -ESRCH)
+                        return NSS_STATUS_NOTFOUND;
+                if (r < 0) {
+                        UNPROTECT_ERRNO;
+                        *errnop = -r;
+                        return NSS_STATUS_UNAVAIL;
+                }
+
+                if (!ur->incomplete) /* don't synthesize shadow records for records where we couldn't read shadow data */
+                        break;
+
+                ur = user_record_unref(ur);
+        }
+
+        r = nss_pack_user_record_shadow(ur, result, buffer, buflen);
+        if (r < 0) {
+                UNPROTECT_ERRNO;
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_getsgent_r(
+                struct sgrp *result,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        _cleanup_(group_record_unrefp) GroupRecord *gr = NULL;
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(result);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        _cleanup_(pthread_mutex_unlock_assertp) pthread_mutex_t *_l = pthread_mutex_lock_assert(&getsgent_data.mutex);
+        (void) _l; /* make llvm shut up about _l not being used. */
+
+        if (!getsgent_data.iterator) {
+                UNPROTECT_ERRNO;
+                *errnop = EHOSTDOWN;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        for (;;) {
+                r = groupdb_iterator_get(getsgent_data.iterator, &gr);
+                if (r == -ESRCH)
+                        return NSS_STATUS_NOTFOUND;
+                if (r < 0) {
+                        UNPROTECT_ERRNO;
+                        *errnop = -r;
+                        return NSS_STATUS_UNAVAIL;
+                }
+
+                if (!gr->incomplete) /* don't synthesize shadow records for records where we couldn't read shadow data */
+                        break;
+
+                gr = group_record_unref(gr);
+        }
+
+        r = nss_pack_group_record_shadow(gr, result, buffer, buflen);
+        if (r < 0) {
+                UNPROTECT_ERRNO;
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status _nss_systemd_initgroups_dyn(
+                const char *user_name,
+                gid_t gid,
+                long *start,
+                long *size,
+                gid_t **groupsp,
+                long int limit,
+                int *errnop) {
+
+        _cleanup_(userdb_iterator_freep) UserDBIterator *iterator = NULL;
+        bool any = false;
+        int r;
+
+        PROTECT_ERRNO;
+        NSS_ENTRYPOINT_BEGIN;
+
+        assert(user_name);
+        assert(start);
+        assert(size);
+        assert(groupsp);
+        assert(errnop);
+
+        if (!valid_user_group_name(user_name, VALID_USER_RELAX))
+                return NSS_STATUS_NOTFOUND;
+
+        /* Don't allow extending these two special users, the same as we won't resolve them via getpwnam() */
+        if (STR_IN_SET(user_name, root_passwd.pw_name, nobody_passwd.pw_name))
+                return NSS_STATUS_NOTFOUND;
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        r = membershipdb_by_user(user_name, nss_glue_userdb_flags(), &iterator);
+        if (r < 0) {
+                UNPROTECT_ERRNO;
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        for (;;) {
+                _cleanup_(group_record_unrefp) GroupRecord *g = NULL;
+                _cleanup_free_ char *group_name = NULL;
+
+                r = membershipdb_iterator_get(iterator, NULL, &group_name);
+                if (r == -ESRCH)
+                        break;
+                if (r < 0) {
+                        UNPROTECT_ERRNO;
+                        *errnop = -r;
+                        return NSS_STATUS_UNAVAIL;
+                }
+
+                /* The group might be defined via traditional NSS only, hence let's do a full look-up without
+                 * disabling NSS. This means we are operating recursively here. */
+
+                r = groupdb_by_name(group_name, (nss_glue_userdb_flags() & ~USERDB_EXCLUDE_NSS) | USERDB_SUPPRESS_SHADOW, &g);
+                if (r == -ESRCH)
+                        continue;
+                if (r < 0) {
+                        log_debug_errno(r, "Failed to resolve group '%s', ignoring: %m", group_name);
+                        continue;
+                }
+
+                if (g->gid == gid)
+                        continue;
+
+                if (*start >= *size) {
+                        gid_t *new_groups;
+                        long new_size;
+
+                        if (limit > 0 && *size >= limit) /* Reached the limit.? */
+                                break;
+
+                        if (*size > LONG_MAX/2) { /* Check for overflow */
+                                UNPROTECT_ERRNO;
+                                *errnop = ENOMEM;
+                                return NSS_STATUS_TRYAGAIN;
+                        }
+
+                        new_size = *start * 2;
+                        if (limit > 0 && new_size > limit)
+                                new_size = limit;
+
+                        /* Enlarge buffer */
+                        new_groups = reallocarray(*groupsp, new_size, sizeof(**groupsp));
+                        if (!new_groups) {
+                                UNPROTECT_ERRNO;
+                                *errnop = ENOMEM;
+                                return NSS_STATUS_TRYAGAIN;
+                        }
+
+                        *groupsp = new_groups;
+                        *size = new_size;
+                }
+
+                (*groupsp)[(*start)++] = g->gid;
+                any = true;
+        }
+
+        return any ? NSS_STATUS_SUCCESS : NSS_STATUS_NOTFOUND;
+}
+
+static thread_local unsigned _blocked = 0;
+
+_public_ int _nss_systemd_block(bool b) {
+
+        /* This blocks recursively: it's blocked for as many times this function is called with `true` until
+         * it is called an equal time with `false`. */
+
+        if (b) {
+                if (_blocked >= UINT_MAX)
+                        return -EOVERFLOW;
+
+                _blocked++;
+        } else {
+                if (_blocked <= 0)
+                        return -EOVERFLOW;
+
+                _blocked--;
+        }
+
+        return b; /* Return what is passed in, i.e. the new state from the PoV of the caller */
+}
+
+_public_ bool _nss_systemd_is_blocked(void) {
+        return _blocked > 0;
+}
diff --git a/src/nss-systemd/nss-systemd.h b/src/nss-systemd/nss-systemd.h
new file mode 100644
index 0000000..e97b801
--- /dev/null
+++ b/src/nss-systemd/nss-systemd.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdbool.h>
+
+int _nss_systemd_block(bool b);
+bool _nss_systemd_is_blocked(void);
+
+/* For use with the _cleanup_() macro */
+static inline void _nss_systemd_unblockp(bool *b) {
+        if (*b)
+                assert_se(_nss_systemd_block(false) >= 0);
+}
diff --git a/src/nss-systemd/nss-systemd.sym b/src/nss-systemd/nss-systemd.sym
new file mode 100644
index 0000000..5602f00
--- /dev/null
+++ b/src/nss-systemd/nss-systemd.sym
@@ -0,0 +1,36 @@
+/***
+  SPDX-License-Identifier: LGPL-2.1-or-later
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+***/
+
+{
+global:
+        _nss_systemd_getpwnam_r;
+        _nss_systemd_getspnam_r;
+        _nss_systemd_getpwuid_r;
+        _nss_systemd_endpwent;
+        _nss_systemd_setpwent;
+        _nss_systemd_getpwent_r;
+        _nss_systemd_endspent;
+        _nss_systemd_setspent;
+        _nss_systemd_getspent_r;
+        _nss_systemd_getgrnam_r;
+        _nss_systemd_getsgnam_r;
+        _nss_systemd_getgrgid_r;
+        _nss_systemd_endgrent;
+        _nss_systemd_setgrent;
+        _nss_systemd_getgrent_r;
+        _nss_systemd_endsgent;
+        _nss_systemd_setsgent;
+        _nss_systemd_getsgent_r;
+        _nss_systemd_initgroups_dyn;
+
+        /* These two are not used by glibc, but can be used by apps to explicitly disable nss-systemd for the calling thread. */
+        _nss_systemd_block;
+        _nss_systemd_is_blocked;
+local: *;
+};
diff --git a/src/nss-systemd/userdb-glue.c b/src/nss-systemd/userdb-glue.c
new file mode 100644
index 0000000..c69667d
--- /dev/null
+++ b/src/nss-systemd/userdb-glue.c
@@ -0,0 +1,478 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "env-util.h"
+#include "fd-util.h"
+#include "nss-systemd.h"
+#include "strv.h"
+#include "user-record-nss.h"
+#include "user-record.h"
+#include "user-util.h"
+#include "userdb-glue.h"
+#include "userdb.h"
+
+UserDBFlags nss_glue_userdb_flags(void) {
+        UserDBFlags flags = USERDB_EXCLUDE_NSS;
+
+        /* Make sure that we don't go in circles when allocating a dynamic UID by checking our own database */
+        if (getenv_bool_secure("SYSTEMD_NSS_DYNAMIC_BYPASS") > 0)
+                flags |= USERDB_EXCLUDE_DYNAMIC_USER;
+
+        return flags;
+}
+
+int nss_pack_user_record(
+                UserRecord *hr,
+                struct passwd *pwd,
+                char *buffer,
+                size_t buflen) {
+
+        const char *rn, *hd, *shell;
+        size_t required;
+
+        assert(hr);
+        assert(pwd);
+
+        assert(hr->user_name);
+        required = strlen(hr->user_name) + 1;
+
+        required += 2; /* strlen(PASSWORD_SEE_SHADOW) + 1 */
+
+        assert_se(rn = user_record_real_name(hr));
+        required += strlen(rn) + 1;
+
+        assert_se(hd = user_record_home_directory(hr));
+        required += strlen(hd) + 1;
+
+        assert_se(shell = user_record_shell(hr));
+        required += strlen(shell) + 1;
+
+        if (buflen < required)
+                return -ERANGE;
+
+        *pwd = (struct passwd) {
+                .pw_name = buffer,
+                .pw_uid = hr->uid,
+                .pw_gid = user_record_gid(hr),
+        };
+
+        assert(buffer);
+
+        pwd->pw_passwd = stpcpy(pwd->pw_name, hr->user_name) + 1;
+        pwd->pw_gecos = stpcpy(pwd->pw_passwd, PASSWORD_SEE_SHADOW) + 1;
+        pwd->pw_dir = stpcpy(pwd->pw_gecos, rn) + 1;
+        pwd->pw_shell = stpcpy(pwd->pw_dir, hd) + 1;
+        strcpy(pwd->pw_shell, shell);
+
+        return 0;
+}
+
+enum nss_status userdb_getpwnam(
+                const char *name,
+                struct passwd *pwd,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        _cleanup_(user_record_unrefp) UserRecord *hr = NULL;
+        int r;
+
+        assert(pwd);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        r = userdb_by_name(name, nss_glue_userdb_flags()|USERDB_SUPPRESS_SHADOW, &hr);
+        if (r == -ESRCH)
+                return NSS_STATUS_NOTFOUND;
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        r = nss_pack_user_record(hr, pwd, buffer, buflen);
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status userdb_getpwuid(
+                uid_t uid,
+                struct passwd *pwd,
+                char *buffer,
+                size_t buflen,
+                int *errnop) {
+
+        _cleanup_(user_record_unrefp) UserRecord *hr = NULL;
+        int r;
+
+        assert(pwd);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        r = userdb_by_uid(uid, nss_glue_userdb_flags()|USERDB_SUPPRESS_SHADOW, &hr);
+        if (r == -ESRCH)
+                return NSS_STATUS_NOTFOUND;
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        r = nss_pack_user_record(hr, pwd, buffer, buflen);
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+int nss_pack_user_record_shadow(
+                UserRecord *hr,
+                struct spwd *spwd,
+                char *buffer,
+                size_t buflen) {
+
+        const char *hashed;
+        size_t required;
+
+        assert(hr);
+        assert(spwd);
+
+        assert(hr->user_name);
+        required = strlen(hr->user_name) + 1;
+
+        assert_se(hashed = strv_isempty(hr->hashed_password) ? PASSWORD_LOCKED_AND_INVALID : hr->hashed_password[0]);
+        required += strlen(hashed) + 1;
+
+        if (buflen < required)
+                return -ERANGE;
+
+        *spwd = (struct spwd) {
+                .sp_namp = buffer,
+                .sp_lstchg = hr->last_password_change_usec == 0 ? 1 :               /* map 0 to 1, since 0 means please change password on next login */
+                             hr->last_password_change_usec == UINT64_MAX ? -1 :
+                             (long int) (hr->last_password_change_usec / USEC_PER_DAY),
+                .sp_min = hr->password_change_min_usec != UINT64_MAX ? (long int) (hr->password_change_min_usec / USEC_PER_DAY) : -1,
+                .sp_max = hr->password_change_max_usec != UINT64_MAX ? (long int) (hr->password_change_max_usec / USEC_PER_DAY) : -1,
+                .sp_warn = hr->password_change_warn_usec != UINT64_MAX ? (long int) (hr->password_change_warn_usec / USEC_PER_DAY) : -1,
+                .sp_inact = hr->password_change_inactive_usec != UINT64_MAX ? (long int) (hr->password_change_inactive_usec / USEC_PER_DAY) : -1,
+                .sp_expire = hr->locked > 0 || hr->not_after_usec == 0 ? 1 : /* already expired/locked */
+                             hr->not_after_usec == UINT64_MAX ? -1 :
+                             (long int) (hr->not_after_usec / USEC_PER_DAY),
+                .sp_flag = ULONG_MAX,
+        };
+
+        assert(buffer);
+
+        spwd->sp_pwdp = stpcpy(spwd->sp_namp, hr->user_name) + 1;
+        strcpy(spwd->sp_pwdp, hashed);
+
+        return 0;
+}
+
+enum nss_status userdb_getspnam(
+                const char *name,
+                struct spwd *spwd,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        _cleanup_(user_record_unrefp) UserRecord *hr = NULL;
+        int r;
+
+        assert(spwd);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        r = userdb_by_name(name, nss_glue_userdb_flags(), &hr);
+        if (r == -ESRCH)
+                return NSS_STATUS_NOTFOUND;
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        if (hr->incomplete) /* protected records missing? */
+                return NSS_STATUS_NOTFOUND;
+
+        r = nss_pack_user_record_shadow(hr, spwd, buffer, buflen);
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+int nss_pack_group_record(
+                GroupRecord *g,
+                char **extra_members,
+                struct group *gr,
+                char *buffer,
+                size_t buflen) {
+
+        char **array = NULL, *p;
+        size_t required, n = 0, i = 0;
+
+        assert(g);
+        assert(gr);
+
+        assert(g->group_name);
+        required = strlen(g->group_name) + 1;
+
+        STRV_FOREACH(m, g->members) {
+                required += sizeof(char*);  /* space for ptr array entry */
+                required += strlen(*m) + 1;
+                n++;
+        }
+        STRV_FOREACH(m, extra_members) {
+                if (strv_contains(g->members, *m))
+                        continue;
+
+                required += sizeof(char*);
+                required += strlen(*m) + 1;
+                n++;
+        }
+
+        required += sizeof(char*); /* trailing NULL in ptr array entry */
+
+        if (buflen < required)
+                return -ERANGE;
+
+        array = (char**) buffer; /* place ptr array at beginning of buffer, under assumption buffer is aligned */
+        p = buffer + sizeof(void*) * (n + 1); /* place member strings right after the ptr array */
+
+        STRV_FOREACH(m, g->members) {
+                array[i++] = p;
+                p = stpcpy(p, *m) + 1;
+        }
+        STRV_FOREACH(m, extra_members) {
+                if (strv_contains(g->members, *m))
+                        continue;
+
+                array[i++] = p;
+                p = stpcpy(p, *m) + 1;
+        }
+
+        assert_se(i == n);
+        array[n] = NULL;
+
+        *gr = (struct group) {
+                .gr_name = strcpy(p, g->group_name),
+                .gr_gid = g->gid,
+                .gr_passwd = (char*) PASSWORD_SEE_SHADOW,
+                .gr_mem = array,
+        };
+
+        return 0;
+}
+
+enum nss_status userdb_getgrnam(
+                const char *name,
+                struct group *gr,
+                char *buffer,
+                size_t buflen,
+                int *errnop) {
+
+        _cleanup_(group_record_unrefp) GroupRecord *g = NULL;
+        _cleanup_strv_free_ char **members = NULL;
+        int r;
+
+        assert(gr);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        r = groupdb_by_name(name, nss_glue_userdb_flags()|USERDB_SUPPRESS_SHADOW, &g);
+        if (r < 0 && r != -ESRCH) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        r = membershipdb_by_group_strv(name, nss_glue_userdb_flags()|USERDB_SUPPRESS_SHADOW, &members);
+        if (r < 0 && r != -ESRCH) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        if (!g) {
+                _unused_ _cleanup_(_nss_systemd_unblockp) bool blocked = false;
+
+                if (strv_isempty(members))
+                        return NSS_STATUS_NOTFOUND;
+
+                /* Grmbl, so we are supposed to extend a group entry, but the group entry itself is not
+                 * accessible via non-NSS. Hence let's do what we have to do, and query NSS after all to
+                 * acquire it, so that we can extend it (that's because glibc's group merging feature will
+                 * merge groups only if both GID and name match and thus we need to have both first). It
+                 * sucks behaving recursively likely this, but it's apparently what everybody does. We break
+                 * the recursion for ourselves via the _nss_systemd_block_nss() lock. */
+
+                r = _nss_systemd_block(true);
+                if (r < 0)
+                        return r;
+
+                blocked = true;
+
+                r = nss_group_record_by_name(name, false, &g);
+                if (r == -ESRCH)
+                        return NSS_STATUS_NOTFOUND;
+                if (r < 0) {
+                        *errnop = -r;
+                        return NSS_STATUS_UNAVAIL;
+                }
+        }
+
+        r = nss_pack_group_record(g, members, gr, buffer, buflen);
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+enum nss_status userdb_getgrgid(
+                gid_t gid,
+                struct group *gr,
+                char *buffer,
+                size_t buflen,
+                int *errnop) {
+
+
+        _cleanup_(group_record_unrefp) GroupRecord *g = NULL;
+        _cleanup_strv_free_ char **members = NULL;
+        bool from_nss;
+        int r;
+
+        assert(gr);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        r = groupdb_by_gid(gid, nss_glue_userdb_flags()|USERDB_SUPPRESS_SHADOW, &g);
+        if (r < 0 && r != -ESRCH) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        if (!g) {
+                _unused_ _cleanup_(_nss_systemd_unblockp) bool blocked = false;
+
+                /* So, quite possibly we have to extend an existing group record with additional members. But
+                 * to do this we need to know the group name first. The group didn't exist via non-NSS
+                 * queries though, hence let's try to acquire it here recursively via NSS. */
+
+                r = _nss_systemd_block(true);
+                if (r < 0)
+                        return r;
+
+                blocked = true;
+
+                r = nss_group_record_by_gid(gid, false, &g);
+                if (r == -ESRCH)
+                        return NSS_STATUS_NOTFOUND;
+                if (r < 0) {
+                        *errnop = -r;
+                        return NSS_STATUS_UNAVAIL;
+                }
+
+                from_nss = true;
+        } else
+                from_nss = false;
+
+        r = membershipdb_by_group_strv(g->group_name, nss_glue_userdb_flags()|USERDB_SUPPRESS_SHADOW, &members);
+        if (r < 0 && r != -ESRCH) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        /* If we acquired the record via NSS then there's no reason to respond unless we have to augment the
+         * list of members of the group */
+        if (from_nss && strv_isempty(members))
+                return NSS_STATUS_NOTFOUND;
+
+        r = nss_pack_group_record(g, members, gr, buffer, buflen);
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
+
+int nss_pack_group_record_shadow(
+                GroupRecord *hr,
+                struct sgrp *sgrp,
+                char *buffer,
+                size_t buflen) {
+
+        const char *hashed;
+        size_t required;
+
+        assert(hr);
+        assert(sgrp);
+
+        assert(hr->group_name);
+        required = strlen(hr->group_name) + 1;
+
+        assert_se(hashed = strv_isempty(hr->hashed_password) ? PASSWORD_LOCKED_AND_INVALID : hr->hashed_password[0]);
+        required += strlen(hashed) + 1;
+
+        if (buflen < required)
+                return -ERANGE;
+
+        *sgrp = (struct sgrp) {
+                .sg_namp = buffer,
+        };
+
+        assert(buffer);
+
+        sgrp->sg_passwd = stpcpy(sgrp->sg_namp, hr->group_name) + 1;
+        strcpy(sgrp->sg_passwd, hashed);
+
+        return 0;
+}
+
+enum nss_status userdb_getsgnam(
+                const char *name,
+                struct sgrp *sgrp,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        _cleanup_(group_record_unrefp) GroupRecord *hr = NULL;
+        int r;
+
+        assert(sgrp);
+        assert(errnop);
+
+        if (_nss_systemd_is_blocked())
+                return NSS_STATUS_NOTFOUND;
+
+        r = groupdb_by_name(name, nss_glue_userdb_flags(), &hr);
+        if (r == -ESRCH)
+                return NSS_STATUS_NOTFOUND;
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_UNAVAIL;
+        }
+
+        if (hr->incomplete) /* protected records missing? */
+                return NSS_STATUS_NOTFOUND;
+
+        r = nss_pack_group_record_shadow(hr, sgrp, buffer, buflen);
+        if (r < 0) {
+                *errnop = -r;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        return NSS_STATUS_SUCCESS;
+}
diff --git a/src/nss-systemd/userdb-glue.h b/src/nss-systemd/userdb-glue.h
new file mode 100644
index 0000000..386cc88
--- /dev/null
+++ b/src/nss-systemd/userdb-glue.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <nss.h>
+#include <pwd.h>
+#include <grp.h>
+#include <sys/types.h>
+
+#include "userdb.h"
+
+UserDBFlags nss_glue_userdb_flags(void);
+
+int nss_pack_user_record(UserRecord *hr, struct passwd *pwd, char *buffer, size_t buflen);
+int nss_pack_group_record(GroupRecord *g, char **extra_members, struct group *gr, char *buffer, size_t buflen);
+
+int nss_pack_user_record_shadow(UserRecord *hr, struct spwd *spwd, char *buffer, size_t buflen);
+int nss_pack_group_record_shadow(GroupRecord *hr, struct sgrp *sgrp, char *buffer,size_t buflen);
+
+enum nss_status userdb_getpwnam(const char *name, struct passwd *pwd, char *buffer, size_t buflen, int *errnop);
+enum nss_status userdb_getpwuid(uid_t uid, struct passwd *pwd, char *buffer, size_t buflen, int *errnop);
+
+enum nss_status userdb_getspnam(const char *name, struct spwd *spwd, char *buffer, size_t buflen, int *errnop);
+
+enum nss_status userdb_getgrnam(const char *name, struct group *gr, char *buffer, size_t buflen, int *errnop);
+enum nss_status userdb_getgrgid(gid_t gid, struct group *gr, char *buffer, size_t buflen, int *errnop);
+
+enum nss_status userdb_getsgnam(const char *name, struct sgrp *sgrp, char *buffer, size_t buflen, int *errnop);