Look at /etc/login.defs for the system_max_[ug]id values

It makes little sense to make the boundary between systemd and user guids
configurable. Nevertheless, a completely fixed compile-time define is not
enough in two scenarios:
- the systemd_uid_max boundary has moved over time. The default used to be
  500 for a long time. Systems which are upgraded over time might have users
  in the wrong range, but changing existing systems is complicated and
  expensive (offline disks, backups, remote systems, read-only media, etc.)
- systems are used in a heterogenous enviornment, where some vendors pick
  one value and others another.
So let's make this boundary overridable using /etc/login.defs.

Fixes #3855, #10184.

docs/UIDS-GIDS.md

@@ -171,10 +171,11 @@ pick — given that 64K UIDs are assigned to each container according to this
 allocation logic, the maximum UID used for this range is hence
 1878982656+65535=1879048191.)
 
-Note that systemd does not make any of these values runtime-configurable. All
-these boundaries are chosen during build time. That said, the system UID/GID
-boundary is traditionally configured in /etc/login.defs, though systemd won't
-look there during runtime.
+Systemd has compile-time default for these boundaries. Using those defaults is
+recommended. It will nevertheless query `/etc/login.defs` at runtime, when
+compiled with `-Dcompat-mutable-uid-boundaries=true` and that file is present.
+Support for this is considered only a compatibility feature and should not be
+used except when upgrading systems which were creating with different defaults.
 
 ## Considerations for container managers
 

meson.build

@@ -1443,6 +1443,7 @@ foreach term : ['analyze',
                 'idn',
                 'ima',
                 'initrd',
+                'compat-mutable-uid-boundaries',
                 'ldconfig',
                 'localed',
                 'logind',
@@ -3627,6 +3628,7 @@ foreach tuple : [
         ['libcurl'],
         ['idn'],
         ['initrd'],
+        ['compat-mutable-uid-boundaries'],
         ['libidn2'],
         ['libidn'],
         ['libiptc'],

meson_options.txt

@@ -28,9 +28,9 @@ option('static-libsystemd', type : 'combo',
        description : '''install a static library for libsystemd''')
 option('static-libudev', type : 'combo',
        choices : ['false', 'true', 'pic', 'no-pic'],
-       description : '''install a static library for libudev''')
+       description : 'install a static library for libudev')
 option('standalone-binaries', type : 'boolean', value : 'false',
-       description : '''also build standalone versions of supported binaries''')
+       description : 'also build standalone versions of supported binaries')
 
 option('sysvinit-path', type : 'string', value : '/etc/init.d',
        description : 'the directory where the SysV init scripts are located')
@@ -40,8 +40,10 @@ option('telinit-path', type : 'string', value : '/lib/sysvinit/telinit',
        description : 'path to telinit')
 option('rc-local', type : 'string',
        value : '/etc/rc.local')
-option('initrd', type: 'boolean',
+option('initrd', type : 'boolean',
        description : 'install services for use when running systemd in initrd')
+option('compat-mutable-uid-boundaries', type : 'boolean', value : 'false',
+       description : 'look at uid boundaries in /etc/login.defs for compatibility')
 
 option('quotaon-path', type : 'string', description : 'path to quotaon')
 option('quotacheck-path', type : 'string', description : 'path to quotacheck')

src/shared/user-record.c

@@ -5,6 +5,8 @@
 #include "cgroup-util.h"
 #include "dns-domain.h"
 #include "env-util.h"
+#include "fd-util.h"
+#include "fileio.h"
 #include "fs-util.h"
 #include "hexdecoct.h"
 #include "hostname-util.h"
@@ -21,6 +23,100 @@
 #define DEFAULT_RATELIMIT_BURST 30
 #define DEFAULT_RATELIMIT_INTERVAL_USEC (1*USEC_PER_MINUTE)
 
+#if ENABLE_COMPAT_MUTABLE_UID_BOUNDARIES
+static int parse_alloc_uid(const char *path, const char *name, const char *t, uid_t *ret_uid) {
+        uid_t uid;
+        int r;
+
+        r = parse_uid(t, &uid);
+        if (r < 0)
+                return log_debug_errno(r, "%s: failed to parse %s %s, ignoring: %m", path, name, t);
+        if (uid == 0)
+                uid = 1;
+
+        *ret_uid = uid;
+        return 0;
+}
+
+static int read_login_defs(UGIDAllocationRange *ret_defs, const char *path) {
+        _cleanup_fclose_ FILE *f = NULL;
+        UGIDAllocationRange defs = {
+                .system_uid_max = SYSTEM_UID_MAX,
+                .system_gid_max = SYSTEM_GID_MAX,
+        };
+        int r;
+
+        if (!path)
+                path = "/etc/login.defs";
+
+        r = fopen_unlocked(path, "re", &f);
+        if (r == -ENOENT)
+                goto assign;
+        if (r < 0)
+                return log_debug_errno(r, "Failed to open %s: %m", path);
+
+        for (;;) {
+                _cleanup_free_ char *line = NULL;
+                char *t;
+
+                r = read_line(f, LINE_MAX, &line);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to read %s: %m", path);
+                if (r == 0)
+                        break;
+
+                if ((t = first_word(line, "SYS_UID_MAX")))
+                        (void) parse_alloc_uid(path, "SYS_UID_MAX", t, &defs.system_uid_max);
+                else if ((t = first_word(line, "SYS_GID_MAX")))
+                        (void) parse_alloc_uid(path, "SYS_GID_MAX", t, &defs.system_gid_max);
+        }
+
+ assign:
+        *ret_defs = defs;
+        return 0;
+}
+#endif
+
+const UGIDAllocationRange *acquire_ugid_allocation_range(void) {
+#if ENABLE_COMPAT_MUTABLE_UID_BOUNDARIES
+        static thread_local UGIDAllocationRange defs = {
+#else
+        static const UGIDAllocationRange defs = {
+#endif
+                .system_uid_max = SYSTEM_UID_MAX,
+                .system_gid_max = SYSTEM_GID_MAX,
+        };
+
+#if ENABLE_COMPAT_MUTABLE_UID_BOUNDARIES
+        /* This function will ignore failure to read the file, so it should only be called from places where
+         * we don't crucially depend on the answer. In other words, it's appropriate for journald, but
+         * probably not for sysusers. */
+
+        static thread_local bool initialized = false;
+
+        if (!initialized) {
+                (void) read_login_defs(&defs, NULL);
+                initialized = true;
+        }
+#endif
+
+        return &defs;
+}
+
+bool uid_is_system(uid_t uid) {
+        const UGIDAllocationRange *defs;
+        assert_se(defs = acquire_ugid_allocation_range());
+
+        return uid <= defs->system_uid_max;
+}
+
+bool gid_is_system(gid_t gid) {
+        const UGIDAllocationRange *defs;
+        assert_se(defs = acquire_ugid_allocation_range());
+
+        return gid <= defs->system_gid_max;
+}
+
 UserRecord* user_record_new(void) {
         UserRecord *h;
 

src/shared/user-record.h

@@ -17,13 +17,8 @@
 /* The default disk size to use when nothing else is specified, relative to free disk space */
 #define USER_DISK_SIZE_DEFAULT_PERCENT 85
 
-static inline bool uid_is_system(uid_t uid) {
-        return uid <= SYSTEM_UID_MAX;
-}
-
-static inline bool gid_is_system(gid_t gid) {
-        return gid <= SYSTEM_GID_MAX;
-}
+bool uid_is_system(uid_t uid);
+bool gid_is_system(gid_t gid);
 
 static inline bool uid_is_dynamic(uid_t uid) {
         return DYNAMIC_UID_MIN <= uid && uid <= DYNAMIC_UID_MAX;
@@ -41,6 +36,13 @@ static inline bool gid_is_container(gid_t gid) {
         return uid_is_container((uid_t) gid);
 }
 
+typedef struct UGIDAllocationRange {
+        uid_t system_uid_max;
+        gid_t system_gid_max;
+} UGIDAllocationRange;
+
+const UGIDAllocationRange *acquire_ugid_allocation_range(void);
+
 typedef enum UserDisposition {
         USER_INTRINSIC,   /* root and nobody */
         USER_SYSTEM,      /* statically allocated users for system services */

src/test/meson.build

@@ -339,6 +339,10 @@ tests += [
          [],
          []],
 
+        [['src/test/test-user-record.c'],
+         [],
+         []],
+
         [['src/test/test-user-util.c'],
          [],
          []],

src/test/test-user-record.c

@@ -0,0 +1,54 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include <unistd.h>
+#include <sys/types.h>
+
+#include "format-util.h"
+#include "tests.h"
+#include "user-record.h"
+
+static void test_acquire_ugid_allocation_range(void) {
+        log_info("/* %s */", __func__);
+
+        const UGIDAllocationRange *defs;
+        assert_se(defs = acquire_ugid_allocation_range());
+
+        log_info("system_uid_max="UID_FMT, defs->system_uid_max);
+        log_info("system_gid_max="GID_FMT, defs->system_gid_max);
+}
+
+static void test_uid_is_system(void) {
+        log_info("/* %s */", __func__);
+
+        uid_t uid = 0;
+        log_info("uid_is_system("UID_FMT") = %s", uid, yes_no(uid_is_system(uid)));
+
+        uid = 999;
+        log_info("uid_is_system("UID_FMT") = %s", uid, yes_no(uid_is_system(uid)));
+
+        uid = getuid();
+        log_info("uid_is_system("UID_FMT") = %s", uid, yes_no(uid_is_system(uid)));
+}
+
+static void test_gid_is_system(void) {
+        log_info("/* %s */", __func__);
+
+        gid_t gid = 0;
+        log_info("gid_is_system("GID_FMT") = %s", gid, yes_no(gid_is_system(gid)));
+
+        gid = 999;
+        log_info("gid_is_system("GID_FMT") = %s", gid, yes_no(gid_is_system(gid)));
+
+        gid = getgid();
+        log_info("gid_is_system("GID_FMT") = %s", gid, yes_no(gid_is_system(gid)));
+}
+
+int main(int argc, char *argv[]) {
+        test_setup_logging(LOG_DEBUG);
+
+        test_acquire_ugid_allocation_range();
+        test_uid_is_system();
+        test_gid_is_system();
+
+        return 0;
+}