summaryrefslogtreecommitdiffstats
path: root/src/core/dynamic-user.c
blob: 7da87fd818bb4ce0c79202e0e29e4c4d5b0da32c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
/* SPDX-License-Identifier: LGPL-2.1-or-later */

#include <sys/file.h>
#include <sys/stat.h>
#include <sys/types.h>

#include "clean-ipc.h"
#include "dynamic-user.h"
#include "fd-util.h"
#include "fileio.h"
#include "format-util.h"
#include "fs-util.h"
#include "io-util.h"
#include "nscd-flush.h"
#include "parse-util.h"
#include "random-util.h"
#include "serialize.h"
#include "socket-util.h"
#include "stdio-util.h"
#include "string-util.h"
#include "strv.h"
#include "user-record.h"
#include "user-util.h"

/* Takes a value generated randomly or by hashing and turns it into a UID in the right range */
#define UID_CLAMP_INTO_RANGE(rnd) (((uid_t) (rnd) % (DYNAMIC_UID_MAX - DYNAMIC_UID_MIN + 1)) + DYNAMIC_UID_MIN)

DEFINE_PRIVATE_TRIVIAL_REF_FUNC(DynamicUser, dynamic_user);

static DynamicUser* dynamic_user_free(DynamicUser *d) {
        if (!d)
                return NULL;

        if (d->manager)
                (void) hashmap_remove(d->manager->dynamic_users, d->name);

        safe_close_pair(d->storage_socket);
        return mfree(d);
}

static int dynamic_user_add(Manager *m, const char *name, int storage_socket[static 2], DynamicUser **ret) {
        DynamicUser *d;
        int r;

        assert(m);
        assert(name);
        assert(storage_socket);

        r = hashmap_ensure_allocated(&m->dynamic_users, &string_hash_ops);
        if (r < 0)
                return r;

        d = malloc0(offsetof(DynamicUser, name) + strlen(name) + 1);
        if (!d)
                return -ENOMEM;

        strcpy(d->name, name);

        d->storage_socket[0] = storage_socket[0];
        d->storage_socket[1] = storage_socket[1];

        r = hashmap_put(m->dynamic_users, d->name, d);
        if (r < 0) {
                free(d);
                return r;
        }

        d->manager = m;

        if (ret)
                *ret = d;

        return 0;
}

static int dynamic_user_acquire(Manager *m, const char *name, DynamicUser** ret) {
        _cleanup_close_pair_ int storage_socket[2] = { -1, -1 };
        DynamicUser *d;
        int r;

        assert(m);
        assert(name);

        /* Return the DynamicUser structure for a specific user name. Note that this won't actually allocate a UID for
         * it, but just prepare the data structure for it. The UID is allocated only on demand, when it's really
         * needed, and in the child process we fork off, since allocation involves NSS checks which are not OK to do
         * from PID 1. To allow the children and PID 1 share information about allocated UIDs we use an anonymous
         * AF_UNIX/SOCK_DGRAM socket (called the "storage socket") that contains at most one datagram with the
         * allocated UID number, plus an fd referencing the lock file for the UID
         * (i.e. /run/systemd/dynamic-uid/$UID). Why involve the socket pair? So that PID 1 and all its children can
         * share the same storage for the UID and lock fd, simply by inheriting the storage socket fds. The socket pair
         * may exist in three different states:
         *
         * a) no datagram stored. This is the initial state. In this case the dynamic user was never realized.
         *
         * b) a datagram containing a UID stored, but no lock fd attached to it. In this case there was already a
         *    statically assigned UID by the same name, which we are reusing.
         *
         * c) a datagram containing a UID stored, and a lock fd is attached to it. In this case we allocated a dynamic
         *    UID and locked it in the file system, using the lock fd.
         *
         * As PID 1 and various children might access the socket pair simultaneously, and pop the datagram or push it
         * back in any time, we also maintain a lock on the socket pair. Note one peculiarity regarding locking here:
         * the UID lock on disk is protected via a BSD file lock (i.e. an fd-bound lock), so that the lock is kept in
         * place as long as there's a reference to the fd open. The lock on the storage socket pair however is a POSIX
         * file lock (i.e. a process-bound lock), as all users share the same fd of this (after all it is anonymous,
         * nobody else could get any access to it except via our own fd) and we want to synchronize access between all
         * processes that have access to it. */

        d = hashmap_get(m->dynamic_users, name);
        if (d) {
                if (ret) {
                        /* We already have a structure for the dynamic user, let's increase the ref count and reuse it */
                        d->n_ref++;
                        *ret = d;
                }
                return 0;
        }

        if (!valid_user_group_name(name, VALID_USER_ALLOW_NUMERIC))
                return -EINVAL;

        if (socketpair(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0, storage_socket) < 0)
                return -errno;

        r = dynamic_user_add(m, name, storage_socket, &d);
        if (r < 0)
                return r;

        storage_socket[0] = storage_socket[1] = -1;

        if (ret) {
                d->n_ref++;
                *ret = d;
        }

        return 1;
}

static int make_uid_symlinks(uid_t uid, const char *name, bool b) {

        char path1[STRLEN("/run/systemd/dynamic-uid/direct:") + DECIMAL_STR_MAX(uid_t) + 1];
        const char *path2;
        int r = 0, k;

        /* Add direct additional symlinks for direct lookups of dynamic UIDs and their names by userspace code. The
         * only reason we have this is because dbus-daemon cannot use D-Bus for resolving users and groups (since it
         * would be its own client then). We hence keep these world-readable symlinks in place, so that the
         * unprivileged dbus user can read the mappings when it needs them via these symlinks instead of having to go
         * via the bus. Ideally, we'd use the lock files we keep for this anyway, but we can't since we use BSD locks
         * on them and as those may be taken by any user with read access we can't make them world-readable. */

        xsprintf(path1, "/run/systemd/dynamic-uid/direct:" UID_FMT, uid);
        if (unlink(path1) < 0 && errno != ENOENT)
                r = -errno;

        if (b && symlink(name, path1) < 0) {
                k = log_warning_errno(errno, "Failed to symlink \"%s\": %m", path1);
                if (r == 0)
                        r = k;
        }

        path2 = strjoina("/run/systemd/dynamic-uid/direct:", name);
        if (unlink(path2) < 0 && errno != ENOENT) {
                k = -errno;
                if (r == 0)
                        r = k;
        }

        if (b && symlink(path1 + STRLEN("/run/systemd/dynamic-uid/direct:"), path2) < 0) {
                k = log_warning_errno(errno,  "Failed to symlink \"%s\": %m", path2);
                if (r == 0)
                        r = k;
        }

        return r;
}

static int pick_uid(char **suggested_paths, const char *name, uid_t *ret_uid) {

        /* Find a suitable free UID. We use the following strategy to find a suitable UID:
         *
         * 1. Initially, we try to read the UID of a number of specified paths. If any of these UIDs works, we use
         *    them. We use in order to increase the chance of UID reuse, if StateDirectory=, CacheDirectory= or
         *    LogsDirectory= are used, as reusing the UID these directories are owned by saves us from having to
         *    recursively chown() them to new users.
         *
         * 2. If that didn't yield a currently unused UID, we hash the user name, and try to use that. This should be
         *    pretty good, as the use ris by default derived from the unit name, and hence the same service and same
         *    user should usually get the same UID as long as our hashing doesn't clash.
         *
         * 3. Finally, if that didn't work, we randomly pick UIDs, until we find one that is empty.
         *
         * Since the dynamic UID space is relatively small we'll stop trying after 100 iterations, giving up. */

        enum {
                PHASE_SUGGESTED,  /* the first phase, reusing directory ownership UIDs */
                PHASE_HASHED,     /* the second phase, deriving a UID from the username by hashing */
                PHASE_RANDOM,     /* the last phase, randomly picking UIDs */
        } phase = PHASE_SUGGESTED;

        static const uint8_t hash_key[] = {
                0x37, 0x53, 0x7e, 0x31, 0xcf, 0xce, 0x48, 0xf5,
                0x8a, 0xbb, 0x39, 0x57, 0x8d, 0xd9, 0xec, 0x59
        };

        unsigned n_tries = 100, current_suggested = 0;
        int r;

        (void) mkdir("/run/systemd/dynamic-uid", 0755);

        for (;;) {
                char lock_path[STRLEN("/run/systemd/dynamic-uid/") + DECIMAL_STR_MAX(uid_t) + 1];
                _cleanup_close_ int lock_fd = -1;
                uid_t candidate;
                ssize_t l;

                if (--n_tries <= 0) /* Give up retrying eventually */
                        return -EBUSY;

                switch (phase) {

                case PHASE_SUGGESTED: {
                        struct stat st;

                        if (!suggested_paths || !suggested_paths[current_suggested]) {
                                /* We reached the end of the suggested paths list, let's try by hashing the name */
                                phase = PHASE_HASHED;
                                continue;
                        }

                        if (stat(suggested_paths[current_suggested++], &st) < 0)
                                continue; /* We can't read the UID of this path, but that doesn't matter, just try the next */

                        candidate = st.st_uid;
                        break;
                }

                case PHASE_HASHED:
                        /* A static user by this name does not exist yet. Let's find a free ID then, and use that. We
                         * start with a UID generated as hash from the user name. */
                        candidate = UID_CLAMP_INTO_RANGE(siphash24(name, strlen(name), hash_key));

                        /* If this one fails, we should proceed with random tries */
                        phase = PHASE_RANDOM;
                        break;

                case PHASE_RANDOM:

                        /* Pick another random UID, and see if that works for us. */
                        random_bytes(&candidate, sizeof(candidate));
                        candidate = UID_CLAMP_INTO_RANGE(candidate);
                        break;

                default:
                        assert_not_reached("unknown phase");
                }

                /* Make sure whatever we picked here actually is in the right range */
                if (!uid_is_dynamic(candidate))
                        continue;

                xsprintf(lock_path, "/run/systemd/dynamic-uid/" UID_FMT, candidate);

                for (;;) {
                        struct stat st;

                        lock_fd = open(lock_path, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
                        if (lock_fd < 0)
                                return -errno;

                        r = flock(lock_fd, LOCK_EX|LOCK_NB); /* Try to get a BSD file lock on the UID lock file */
                        if (r < 0) {
                                if (IN_SET(errno, EBUSY, EAGAIN))
                                        goto next; /* already in use */

                                return -errno;
                        }

                        if (fstat(lock_fd, &st) < 0)
                                return -errno;
                        if (st.st_nlink > 0)
                                break;

                        /* Oh, bummer, we got the lock, but the file was unlinked between the time we opened it and
                         * got the lock. Close it, and try again. */
                        lock_fd = safe_close(lock_fd);
                }

                /* Some superficial check whether this UID/GID might already be taken by some static user */
                if (getpwuid(candidate) ||
                    getgrgid((gid_t) candidate) ||
                    search_ipc(candidate, (gid_t) candidate) != 0) {
                        (void) unlink(lock_path);
                        continue;
                }

                /* Let's store the user name in the lock file, so that we can use it for looking up the username for a UID */
                l = pwritev(lock_fd,
                            (struct iovec[2]) {
                                    IOVEC_INIT_STRING(name),
                                    IOVEC_INIT((char[1]) { '\n' }, 1),
                            }, 2, 0);
                if (l < 0) {
                        r = -errno;
                        (void) unlink(lock_path);
                        return r;
                }

                (void) ftruncate(lock_fd, l);
                (void) make_uid_symlinks(candidate, name, true); /* also add direct lookup symlinks */

                *ret_uid = candidate;
                return TAKE_FD(lock_fd);

        next:
                ;
        }
}

static int dynamic_user_pop(DynamicUser *d, uid_t *ret_uid, int *ret_lock_fd) {
        uid_t uid = UID_INVALID;
        struct iovec iov = IOVEC_INIT(&uid, sizeof(uid));
        int lock_fd;
        ssize_t k;

        assert(d);
        assert(ret_uid);
        assert(ret_lock_fd);

        /* Read the UID and lock fd that is stored in the storage AF_UNIX socket. This should be called with the lock
         * on the socket taken. */

        k = receive_one_fd_iov(d->storage_socket[0], &iov, 1, MSG_DONTWAIT, &lock_fd);
        if (k < 0)
                return (int) k;

        *ret_uid = uid;
        *ret_lock_fd = lock_fd;

        return 0;
}

static int dynamic_user_push(DynamicUser *d, uid_t uid, int lock_fd) {
        struct iovec iov = IOVEC_INIT(&uid, sizeof(uid));

        assert(d);

        /* Store the UID and lock_fd in the storage socket. This should be called with the socket pair lock taken. */
        return send_one_fd_iov(d->storage_socket[1], lock_fd, &iov, 1, MSG_DONTWAIT);
}

static void unlink_uid_lock(int lock_fd, uid_t uid, const char *name) {
        char lock_path[STRLEN("/run/systemd/dynamic-uid/") + DECIMAL_STR_MAX(uid_t) + 1];

        if (lock_fd < 0)
                return;

        xsprintf(lock_path, "/run/systemd/dynamic-uid/" UID_FMT, uid);
        (void) unlink(lock_path);

        (void) make_uid_symlinks(uid, name, false); /* remove direct lookup symlinks */
}

static int lockfp(int fd, int *fd_lock) {
        if (lockf(fd, F_LOCK, 0) < 0)
                return -errno;
        *fd_lock = fd;
        return 0;
}

static void unlockfp(int *fd_lock) {
        if (*fd_lock < 0)
                return;
        lockf(*fd_lock, F_ULOCK, 0);
        *fd_lock = -1;
}

static int dynamic_user_realize(
                DynamicUser *d,
                char **suggested_dirs,
                uid_t *ret_uid, gid_t *ret_gid,
                bool is_user) {

        _cleanup_(unlockfp) int storage_socket0_lock = -1;
        _cleanup_close_ int uid_lock_fd = -1;
        _cleanup_close_ int etc_passwd_lock_fd = -1;
        uid_t num = UID_INVALID; /* a uid if is_user, and a gid otherwise */
        gid_t gid = GID_INVALID; /* a gid if is_user, ignored otherwise */
        bool flush_cache = false;
        int r;

        assert(d);
        assert(is_user == !!ret_uid);
        assert(ret_gid);

        /* Acquire a UID for the user name. This will allocate a UID for the user name if the user doesn't exist
         * yet. If it already exists its existing UID/GID will be reused. */

        r = lockfp(d->storage_socket[0], &storage_socket0_lock);
        if (r < 0)
                return r;

        r = dynamic_user_pop(d, &num, &uid_lock_fd);
        if (r < 0) {
                int new_uid_lock_fd;
                uid_t new_uid;

                if (r != -EAGAIN)
                        return r;

                /* OK, nothing stored yet, let's try to find something useful. While we are working on this release the
                 * lock however, so that nobody else blocks on our NSS lookups. */
                unlockfp(&storage_socket0_lock);

                /* Let's see if a proper, static user or group by this name exists. Try to take the lock on
                 * /etc/passwd, if that fails with EROFS then /etc is read-only. In that case it's fine if we don't
                 * take the lock, given that users can't be added there anyway in this case. */
                etc_passwd_lock_fd = take_etc_passwd_lock(NULL);
                if (etc_passwd_lock_fd < 0 && etc_passwd_lock_fd != -EROFS)
                        return etc_passwd_lock_fd;

                /* First, let's parse this as numeric UID */
                r = parse_uid(d->name, &num);
                if (r < 0) {
                        struct passwd *p;
                        struct group *g;

                        if (is_user) {
                                /* OK, this is not a numeric UID. Let's see if there's a user by this name */
                                p = getpwnam(d->name);
                                if (p) {
                                        num = p->pw_uid;
                                        gid = p->pw_gid;
                                } else {
                                        /* if the user does not exist but the group with the same name exists, refuse operation */
                                        g = getgrnam(d->name);
                                        if (g)
                                                return -EILSEQ;
                                }
                        } else {
                                /* Let's see if there's a group by this name */
                                g = getgrnam(d->name);
                                if (g)
                                        num = (uid_t) g->gr_gid;
                                else {
                                        /* if the group does not exist but the user with the same name exists, refuse operation */
                                        p = getpwnam(d->name);
                                        if (p)
                                                return -EILSEQ;
                                }
                        }
                }

                if (num == UID_INVALID) {
                        /* No static UID assigned yet, excellent. Let's pick a new dynamic one, and lock it. */

                        uid_lock_fd = pick_uid(suggested_dirs, d->name, &num);
                        if (uid_lock_fd < 0)
                                return uid_lock_fd;
                }

                /* So, we found a working UID/lock combination. Let's see if we actually still need it. */
                r = lockfp(d->storage_socket[0], &storage_socket0_lock);
                if (r < 0) {
                        unlink_uid_lock(uid_lock_fd, num, d->name);
                        return r;
                }

                r = dynamic_user_pop(d, &new_uid, &new_uid_lock_fd);
                if (r < 0) {
                        if (r != -EAGAIN) {
                                /* OK, something bad happened, let's get rid of the bits we acquired. */
                                unlink_uid_lock(uid_lock_fd, num, d->name);
                                return r;
                        }

                        /* Great! Nothing is stored here, still. Store our newly acquired data. */
                        flush_cache = true;
                } else {
                        /* Hmm, so as it appears there's now something stored in the storage socket. Throw away what we
                         * acquired, and use what's stored now. */

                        unlink_uid_lock(uid_lock_fd, num, d->name);
                        safe_close(uid_lock_fd);

                        num = new_uid;
                        uid_lock_fd = new_uid_lock_fd;
                }
        } else if (is_user && !uid_is_dynamic(num)) {
                struct passwd *p;

                /* Statically allocated user may have different uid and gid. So, let's obtain the gid. */
                errno = 0;
                p = getpwuid(num);
                if (!p)
                        return errno_or_else(ESRCH);

                gid = p->pw_gid;
        }

        /* If the UID/GID was already allocated dynamically, push the data we popped out back in. If it was already
         * allocated statically, push the UID back too, but do not push the lock fd in. If we allocated the UID
         * dynamically right here, push that in along with the lock fd for it. */
        r = dynamic_user_push(d, num, uid_lock_fd);
        if (r < 0)
                return r;

        if (flush_cache) {
                /* If we allocated a new dynamic UID, refresh nscd, so that it forgets about potentially cached
                 * negative entries. But let's do so after we release the /etc/passwd lock, so that there's no
                 * potential for nscd wanting to lock that for completing the invalidation. */
                etc_passwd_lock_fd = safe_close(etc_passwd_lock_fd);
                (void) nscd_flush_cache(STRV_MAKE("passwd", "group"));
        }

        if (is_user) {
                *ret_uid = num;
                *ret_gid = gid != GID_INVALID ? gid : num;
        } else
                *ret_gid = num;

        return 0;
}

int dynamic_user_current(DynamicUser *d, uid_t *ret) {
        _cleanup_(unlockfp) int storage_socket0_lock = -1;
        _cleanup_close_ int lock_fd = -1;
        uid_t uid;
        int r;

        assert(d);

        /* Get the currently assigned UID for the user, if there's any. This simply pops the data from the storage socket, and pushes it back in right-away. */

        r = lockfp(d->storage_socket[0], &storage_socket0_lock);
        if (r < 0)
                return r;

        r = dynamic_user_pop(d, &uid, &lock_fd);
        if (r < 0)
                return r;

        r = dynamic_user_push(d, uid, lock_fd);
        if (r < 0)
                return r;

        if (ret)
                *ret = uid;

        return 0;
}

static DynamicUser* dynamic_user_unref(DynamicUser *d) {
        if (!d)
                return NULL;

        /* Note that this doesn't actually release any resources itself. If a dynamic user should be fully destroyed
         * and its UID released, use dynamic_user_destroy() instead. NB: the dynamic user table may contain entries
         * with no references, which is commonly the case right before a daemon reload. */

        assert(d->n_ref > 0);
        d->n_ref--;

        return NULL;
}

static int dynamic_user_close(DynamicUser *d) {
        _cleanup_(unlockfp) int storage_socket0_lock = -1;
        _cleanup_close_ int lock_fd = -1;
        uid_t uid;
        int r;

        /* Release the user ID, by releasing the lock on it, and emptying the storage socket. After this the user is
         * unrealized again, much like it was after it the DynamicUser object was first allocated. */

        r = lockfp(d->storage_socket[0], &storage_socket0_lock);
        if (r < 0)
                return r;

        r = dynamic_user_pop(d, &uid, &lock_fd);
        if (r == -EAGAIN)
                /* User wasn't realized yet, nothing to do. */
                return 0;
        if (r < 0)
                return r;

        /* This dynamic user was realized and dynamically allocated. In this case, let's remove the lock file. */
        unlink_uid_lock(lock_fd, uid, d->name);

        (void) nscd_flush_cache(STRV_MAKE("passwd", "group"));
        return 1;
}

static DynamicUser* dynamic_user_destroy(DynamicUser *d) {
        if (!d)
                return NULL;

        /* Drop a reference to a DynamicUser object, and destroy the user completely if this was the last
         * reference. This is called whenever a service is shut down and wants its dynamic UID gone. Note that
         * dynamic_user_unref() is what is called whenever a service is simply freed, for example during a reload
         * cycle, where the dynamic users should not be destroyed, but our datastructures should. */

        dynamic_user_unref(d);

        if (d->n_ref > 0)
                return NULL;

        (void) dynamic_user_close(d);
        return dynamic_user_free(d);
}

int dynamic_user_serialize(Manager *m, FILE *f, FDSet *fds) {
        DynamicUser *d;

        assert(m);
        assert(f);
        assert(fds);

        /* Dump the dynamic user database into the manager serialization, to deal with daemon reloads. */

        HASHMAP_FOREACH(d, m->dynamic_users) {
                int copy0, copy1;

                copy0 = fdset_put_dup(fds, d->storage_socket[0]);
                if (copy0 < 0)
                        return log_error_errno(copy0, "Failed to add dynamic user storage fd to serialization: %m");

                copy1 = fdset_put_dup(fds, d->storage_socket[1]);
                if (copy1 < 0)
                        return log_error_errno(copy1, "Failed to add dynamic user storage fd to serialization: %m");

                (void) serialize_item_format(f, "dynamic-user", "%s %i %i", d->name, copy0, copy1);
        }

        return 0;
}

void dynamic_user_deserialize_one(Manager *m, const char *value, FDSet *fds) {
        _cleanup_free_ char *name = NULL, *s0 = NULL, *s1 = NULL;
        int r, fd0, fd1;

        assert(m);
        assert(value);
        assert(fds);

        /* Parse the serialization again, after a daemon reload */

        r = extract_many_words(&value, NULL, 0, &name, &s0, &s1, NULL);
        if (r != 3 || !isempty(value)) {
                log_debug("Unable to parse dynamic user line.");
                return;
        }

        if (safe_atoi(s0, &fd0) < 0 || !fdset_contains(fds, fd0)) {
                log_debug("Unable to process dynamic user fd specification.");
                return;
        }

        if (safe_atoi(s1, &fd1) < 0 || !fdset_contains(fds, fd1)) {
                log_debug("Unable to process dynamic user fd specification.");
                return;
        }

        r = dynamic_user_add(m, name, (int[]) { fd0, fd1 }, NULL);
        if (r < 0) {
                log_debug_errno(r, "Failed to add dynamic user: %m");
                return;
        }

        (void) fdset_remove(fds, fd0);
        (void) fdset_remove(fds, fd1);
}

void dynamic_user_vacuum(Manager *m, bool close_user) {
        DynamicUser *d;

        assert(m);

        /* Empty the dynamic user database, optionally cleaning up orphaned dynamic users, i.e. destroy and free users
         * to which no reference exist. This is called after a daemon reload finished, in order to destroy users which
         * might not be referenced anymore. */

        HASHMAP_FOREACH(d, m->dynamic_users) {
                if (d->n_ref > 0)
                        continue;

                if (close_user) {
                        log_debug("Removing orphaned dynamic user %s", d->name);
                        (void) dynamic_user_close(d);
                }

                dynamic_user_free(d);
        }
}

int dynamic_user_lookup_uid(Manager *m, uid_t uid, char **ret) {
        char lock_path[STRLEN("/run/systemd/dynamic-uid/") + DECIMAL_STR_MAX(uid_t) + 1];
        _cleanup_free_ char *user = NULL;
        uid_t check_uid;
        int r;

        assert(m);
        assert(ret);

        /* A friendly way to translate a dynamic user's UID into a name. */
        if (!uid_is_dynamic(uid))
                return -ESRCH;

        xsprintf(lock_path, "/run/systemd/dynamic-uid/" UID_FMT, uid);
        r = read_one_line_file(lock_path, &user);
        if (IN_SET(r, -ENOENT, 0))
                return -ESRCH;
        if (r < 0)
                return r;

        /* The lock file might be stale, hence let's verify the data before we return it */
        r = dynamic_user_lookup_name(m, user, &check_uid);
        if (r < 0)
                return r;
        if (check_uid != uid) /* lock file doesn't match our own idea */
                return -ESRCH;

        *ret = TAKE_PTR(user);

        return 0;
}

int dynamic_user_lookup_name(Manager *m, const char *name, uid_t *ret) {
        DynamicUser *d;
        int r;

        assert(m);
        assert(name);

        /* A friendly call for translating a dynamic user's name into its UID */

        d = hashmap_get(m->dynamic_users, name);
        if (!d)
                return -ESRCH;

        r = dynamic_user_current(d, ret);
        if (r == -EAGAIN) /* not realized yet? */
                return -ESRCH;

        return r;
}

int dynamic_creds_acquire(DynamicCreds *creds, Manager *m, const char *user, const char *group) {
        bool acquired = false;
        int r;

        assert(creds);
        assert(m);

        /* A DynamicUser object encapsulates an allocation of both a UID and a GID for a specific name. However, some
         * services use different user and groups. For cases like that there's DynamicCreds containing a pair of user
         * and group. This call allocates a pair. */

        if (!creds->user && user) {
                r = dynamic_user_acquire(m, user, &creds->user);
                if (r < 0)
                        return r;

                acquired = true;
        }

        if (!creds->group) {

                if (creds->user && (!group || streq_ptr(user, group)))
                        creds->group = dynamic_user_ref(creds->user);
                else if (group) {
                        r = dynamic_user_acquire(m, group, &creds->group);
                        if (r < 0) {
                                if (acquired)
                                        creds->user = dynamic_user_unref(creds->user);
                                return r;
                        }
                }
        }

        return 0;
}

int dynamic_creds_realize(DynamicCreds *creds, char **suggested_paths, uid_t *uid, gid_t *gid) {
        uid_t u = UID_INVALID;
        gid_t g = GID_INVALID;
        int r;

        assert(creds);
        assert(uid);
        assert(gid);

        /* Realize both the referenced user and group */

        if (creds->user) {
                r = dynamic_user_realize(creds->user, suggested_paths, &u, &g, true);
                if (r < 0)
                        return r;
        }

        if (creds->group && creds->group != creds->user) {
                r = dynamic_user_realize(creds->group, suggested_paths, NULL, &g, false);
                if (r < 0)
                        return r;
        }

        *uid = u;
        *gid = g;
        return 0;
}

void dynamic_creds_unref(DynamicCreds *creds) {
        assert(creds);

        creds->user = dynamic_user_unref(creds->user);
        creds->group = dynamic_user_unref(creds->group);
}

void dynamic_creds_destroy(DynamicCreds *creds) {
        assert(creds);

        creds->user = dynamic_user_destroy(creds->user);
        creds->group = dynamic_user_destroy(creds->group);
}