cgroup-util,nspawn: Add a special "inherit" cgroup mode for nspawnnotsystemd/v232.2

The "inherit" mode inspects /proc/self/mountinfo to do its best to
replicate the cgroup setup of the outer host.  It is used by default unless
a different specific cgroup setup is to be used; either because the user
requested it (via $UNIFIED_CGROUP_HIERARCHY), or because
pick_cgroup_version() sniffed that the container has a version of systemd
that doesn't support the outer host's setup.

This means that nspawn can now be used when
outer_cgver=CGROUP_UNIFIED_UNKNOWN; AKA when running on a non-systemd host.

Unfortunately, without further rework of cgroup-util, the name=systemd v1
hierarchy and the v2 hierarchy can't be meaningfully differentiated
between, which means that cgroup_setup() can't do anything intelligent with
the v2 hierarchy when outer_cgver=CGROUP_UNIFIED_UNKNOWN.  So in the
meantime there's a test in cgroup_decide_mounts_inherit() that bails if it
sees the v2 hierarchy in a place that would cause
outer_cgver=CGROUP_UNIFIED_UNKNOWN.

4 files changed, 133 insertions, 11 deletions

diff --git a/Makefile.am b/Makefile.am
index 35ec60f736..4f1d729a74 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3113,6 +3113,7 @@ systemd_nspawn_CFLAGS = \
 	$(AM_CFLAGS) \
 	$(ACL_CFLAGS) \
 	$(BLKID_CFLAGS) \
+	$(MOUNT_CFLAGS) \
 	$(SECCOMP_CFLAGS) \
 	$(SELINUX_CFLAGS)
 
@@ -3120,6 +3121,7 @@ systemd_nspawn_LDADD = \
 	libsystemd-shared.la \
 	$(ACL_LIBS) \
 	$(BLKID_LIBS) \
+	$(MOUNT_LIBS) \
 	$(SECCOMP_LIBS) \
 	$(SELINUX_LIBS)
 
diff --git a/src/basic/cgroup-util.h b/src/basic/cgroup-util.h
index 0e5b41103e..3406e0b4fb 100644
--- a/src/basic/cgroup-util.h
+++ b/src/basic/cgroup-util.h
@@ -118,6 +118,7 @@ static inline bool CGROUP_BLKIO_WEIGHT_IS_OK(uint64_t x) {
 #define DEFAULT_USER_TASKS_MAX_PERCENTAGE       33U /* 33% of PIDs, 10813 on default settings */
 
 typedef enum CGroupUnified {
+        CGROUP_UNIFIED_INHERIT = -2,    /* special case only used by nspawn */
         CGROUP_UNIFIED_UNKNOWN = -1,
         CGROUP_UNIFIED_NONE = 0,        /* Both systemd and controllers on legacy */
         CGROUP_UNIFIED_SYSTEMD = 1,     /* Only systemd on unified */
diff --git a/src/nspawn/nspawn-cgroup.c b/src/nspawn/nspawn-cgroup.c
index 3287aff95b..a226866207 100644
--- a/src/nspawn/nspawn-cgroup.c
+++ b/src/nspawn/nspawn-cgroup.c
@@ -18,8 +18,11 @@
 ***/
 
 #include <sys/mount.h>
+#include <libmount.h>
 
 #include "alloc-util.h"
+#include "dirent-util.h"
+#include "escape.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "fs-util.h"
@@ -267,6 +270,13 @@ int cgroup_setup(pid_t pid, CGroupUnified outer_cgver, CGroupUnified inner_cgver
         _cleanup_set_free_ Set *peers = NULL;
         int r;
 
+        /* For purposes of version comparison */
+        if (inner_cgver == CGROUP_UNIFIED_INHERIT)
+                inner_cgver = outer_cgver;
+
+        if (outer_cgver == CGROUP_UNIFIED_UNKNOWN)
+                return 0;
+
         if ((outer_cgver >= CGROUP_UNIFIED_SYSTEMD) != (inner_cgver >= CGROUP_UNIFIED_SYSTEMD)) {
                 /* sync the name=systemd hierarchy with the unified hierarchy */
                 r = sync_cgroup(pid, outer_cgver, inner_cgver, uid_shift);
@@ -316,6 +326,108 @@ int cgroup_setup(pid_t pid, CGroupUnified outer_cgver, CGroupUnified inner_cgver
 
 /********************************************************************/
 
+static int cgroup_decide_mounts_inherit(CGMounts *ret_mounts) {
+        _cleanup_fclose_ FILE *proc_self_mountinfo = NULL;
+        _cleanup_(cgroup_free_mounts) CGMounts mounts = {};
+        int r;
+
+        proc_self_mountinfo = fopen("/proc/self/mountinfo", "re");
+        if (!proc_self_mountinfo)
+                return -errno;
+
+        for (;;) {
+                _cleanup_free_ char *escmountpoint = NULL, *mountpoint = NULL, *fstype = NULL, *superopts = NULL, *fsopts = NULL;
+                char *name;
+                CGMountType type;
+                int k;
+
+                k = fscanf(proc_self_mountinfo,
+                           "%*s "       /* (1) mount id */
+                           "%*s "       /* (2) parent id */
+                           "%*s "       /* (3) major:minor */
+                           "%*s "       /* (4) root */
+                           "%ms "       /* (5) mount point */
+                           "%*s"        /* (6) per-mount options */
+                           "%*[^-]"     /* (7) optional fields */
+                           "- "         /* (8) separator */
+                           "%ms "       /* (9) file system type */
+                           "%*s"        /* (10) mount source */
+                           "%ms"        /* (11) per-superblock options */
+                           "%*[^\n]",   /* some rubbish at the end */
+                           &escmountpoint,
+                           &fstype,
+                           &superopts
+                );
+                if (k != 3) {
+                        if (k == EOF)
+                                break;
+
+                        continue;
+                }
+
+                r = cunescape(escmountpoint, UNESCAPE_RELAX, &mountpoint);
+                if (r < 0)
+                        return r;
+
+                name = path_startswith(mountpoint, "/sys/fs/cgroup");
+                if (!name)
+                        continue;
+
+                if (!filename_is_valid(name) && !isempty(name))
+                        continue;
+
+                if (streq(fstype, "tmpfs"))
+                        type = CGMOUNT_TMPFS;
+                else if(streq(fstype, "cgroup"))
+                        type = CGMOUNT_CGROUP1;
+                else if (streq(fstype, "cgroup2")) {
+                        if (!isempty(name) && !streq(name, "systemd"))
+                                /* Unfortunately, We need to disable cgroup v2 when outer_cgver=CGROUP_UNIFIED_UNKNOWN
+                                 * until cgroup_setup() can do something intelligent with it. */
+                                return log_error_errno(EINVAL, "Cannot use the cgroup v2 hierarchy unless running on a host with a recognized (systemd or unified) cgroup setup");
+                        type = CGMOUNT_CGROUP2;
+                } else
+                        continue;
+
+                r = mnt_split_optstr(superopts, NULL, NULL, &fsopts, 0, 0);
+                if (r < 0)
+                        return r;
+
+                if (!cgmount_add(&mounts, type, fsopts, name)) {
+                        return -ENOMEM;
+                }
+
+                if (type == CGMOUNT_TMPFS) {
+                        _cleanup_closedir_ DIR *dir;
+                        struct dirent *entry;
+
+                        dir = opendir(mountpoint);
+                        if (!dir)
+                                return log_error_errno(errno, "Failed to open directory %s: %m", mountpoint);
+
+                        FOREACH_DIRENT(entry, dir, break) {
+                                _cleanup_free_ char *target = NULL;
+                                r = dirent_ensure_type(dir, entry);
+                                if (r < 0)
+                                        return r;
+                                if (entry->d_type != DT_LNK)
+                                        continue;
+                                r = readlinkat_malloc(dirfd(dir), entry->d_name, &target);
+                                if (r < 0)
+                                        return r;
+                                if (!cgmount_add(&mounts, CGMOUNT_SYMLINK, target, entry->d_name))
+                                        return -ENOMEM;
+                        }
+                }
+        }
+
+        *ret_mounts = mounts;
+        mounts.mounts = NULL;
+        mounts.n = 0;
+
+        return 0;
+}
+
 /* Retrieve a list of cgroup v1 hierarchies. */
 static int get_v1_hierarchies(Set *subsystems) {
         _cleanup_fclose_ FILE *f = NULL;
@@ -522,6 +634,8 @@ int cgroup_decide_mounts(
                 bool use_cgns) {
 
         switch (inner_cgver) {
+        case CGROUP_UNIFIED_INHERIT:
+                return cgroup_decide_mounts_inherit(ret_mounts);
         case CGROUP_UNIFIED_NONE:
         case CGROUP_UNIFIED_SYSTEMD:
                 /* Historically, if use_cgns, then this ran inside the container; if !use_cgns, then it ran outside.
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index b8afd8c325..1de119867b 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -328,11 +328,14 @@ static int pick_cgroup_version(const char *directory, CGroupUnified outer) {
          * by checking libsystemd-shared). */
         switch (outer) {
         default:
+        case CGROUP_UNIFIED_INHERIT:
+                assert_not_reached("Invalid host cgroup version");
+                return -EINVAL;
         case CGROUP_UNIFIED_UNKNOWN:
-                assert_not_reached("Unknown host cgroup version");
+                arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_INHERIT;
                 break;
         case CGROUP_UNIFIED_NONE: /* cgroup v1-sd */
-                arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_NONE;
+                arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_INHERIT;
                 break;
         case CGROUP_UNIFIED_ALL: /* cgroup v2 */
                 /* Unified cgroup hierarchy support was added in 230. Unfortunately libsystemd-shared,
@@ -342,7 +345,7 @@ static int pick_cgroup_version(const char *directory, CGroupUnified outer) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to decide cgroup version to use: Failed to determine systemd version in container: %m");
                 if (r > 0)
-                        arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_ALL;
+                        arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_INHERIT;
                 else
                         arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_NONE;
                 break;
@@ -352,7 +355,7 @@ static int pick_cgroup_version(const char *directory, CGroupUnified outer) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to decide cgroup version to use: Failed to determine systemd version in container: %m");
                 if (r > 0)
-                        arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_SYSTEMD;
+                        arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_INHERIT;
                 else
                         arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_NONE;
                 break;
@@ -4076,6 +4079,9 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
         cg_unified_flush();
+        r = cg_version(&outer_cgver);
+        if (r < 0)
+                outer_cgver = CGROUP_UNIFIED_UNKNOWN;
 
         /* Make sure rename_process() in the stub init process can work */
         saved_argv = argv;
@@ -4090,13 +4096,6 @@ int main(int argc, char *argv[]) {
                 r = -EPERM;
                 goto finish;
         }
-
-        r = cg_version(&outer_cgver);
-        if (r < 0) {
-                log_error_errno(r, "Failed to determine whether the unified cgroups hierarchy is used: %m");
-                goto finish;
-        }
-
         r = determine_names();
         if (r < 0)
                 goto finish;
@@ -4265,6 +4264,12 @@ int main(int argc, char *argv[]) {
                         goto finish;
         }
 
+        if (outer_cgver == CGROUP_UNIFIED_UNKNOWN &&
+            arg_unified_cgroup_hierarchy != CGROUP_UNIFIED_INHERIT) {
+                r = log_error_errno(EINVAL, "Cannot set cgroup version of container unless running on a host with a recognized (systemd or unified) cgroup setup");
+                goto finish;
+        }
+
         interactive =
                 isatty(STDIN_FILENO) > 0 &&
                 isatty(STDOUT_FILENO) > 0;