libct/cg/sd: use systemd version when generating dev props

Commit 343951a added a call to os.Stat for the device path
when generating systemd device properties, to avoid systemd warning for
non-existing devices. The idea was, since systemd uses stat(2) to look
up device properties for a given path, it will fail anyway. In addition,
this allowed to suppress a warning like this from systemd:

> Couldn't stat device /dev/char/10:200

NOTE that this was done because:
 - systemd could not add the rule anyway;
 - runs puts its own set of rules on top of what systemd does.

Apparently, the above change broke some setups, resulting in inability
to use e.g. /dev/null inside a container. My guess is this is because
in cgroup v2 we add a second eBPF program, which is not used if the
first one (added by systemd) returns "access denied".

Next, commit 3b95828 fixed that by adding a call to os.Stat for
"/sys/"+path (meaning, if "/dev/char/10:200" does not exist, we retry
with "/sys/dev/char/10:200", and if it exists, proceed with adding a
device rule with the original (non-"/sys") path).

How that second fix ever worked was a mystery, because the path we gave
to systemd still doesn't exist.

Well, I think now I know.

Since systemd v240 (commit 74c48bf5a8005f20) device access rules
specified as /dev/{block|char}/MM:mm are no longer looked up on the
filesystem, instead, if possible, those are parsed from the string.

So, we need to do different things, depending on systemd version:

 - for systemd >= v240, use the /dev/{char,block}/MM:mm as is, without
   doing stat() -- since systemd doesn't do stat() either;
 - for older version, check if the path exists, and skip passing it on
   to systemd otherwise.
 - the check for /sys/dev/{block,char}/MM:mm is not needed in either
   case.

Pass the systemd version to the function that generates the rules, and
fix it accordingly.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/devices/systemd.go

@@ -17,7 +17,7 @@ import (
 
 // systemdProperties takes the configured device rules and generates a
 // corresponding set of systemd properties to configure the devices correctly.
-func systemdProperties(r *configs.Resources) ([]systemdDbus.Property, error) {
+func systemdProperties(r *configs.Resources, sdVer int) ([]systemdDbus.Property, error) {
 	if r.SkipDevices {
 		return nil, nil
 	}
@@ -78,9 +78,10 @@ func systemdProperties(r *configs.Resources) ([]systemdDbus.Property, error) {
 		// trickery to convert things:
 		//
 		//  * Concrete rules with non-wildcard major/minor numbers have to use
-		//    /dev/{block,char} paths. This is slightly odd because it means
-		//    that we cannot add whitelist rules for devices that don't exist,
-		//    but there's not too much we can do about that.
+		//    /dev/{block,char}/MAJOR:minor paths. Before v240, systemd uses
+		//    stat(2) on such paths to look up device properties, meaning we
+		//    cannot add whitelist rules for devices that don't exist. Since v240,
+		//    device properties are parsed from the path string.
 		//
 		//    However, path globbing is not support for path-based rules so we
 		//    need to handle wildcards in some other manner.
@@ -128,21 +129,14 @@ func systemdProperties(r *configs.Resources) ([]systemdDbus.Property, error) {
 			case devices.CharDevice:
 				entry.Path = fmt.Sprintf("/dev/char/%d:%d", rule.Major, rule.Minor)
 			}
-			// systemd will issue a warning if the path we give here doesn't exist.
-			// Since all of this logic is best-effort anyway (we manually set these
-			// rules separately to systemd) we can safely skip entries that don't
-			// have a corresponding path.
-			if _, err := os.Stat(entry.Path); err != nil {
-				// Also check /sys/dev so that we don't depend on /dev/{block,char}
-				// being populated. (/dev/{block,char} is populated by udev, which
-				// isn't strictly required for systemd). Ironically, this happens most
-				// easily when starting containerd within a runc created container
-				// itself.
-
-				// We don't bother with securejoin here because we create entry.Path
-				// right above here, so we know it's safe.
-				if _, err := os.Stat("/sys" + entry.Path); err != nil {
-					logrus.Warnf("skipping device %s for systemd: %s", entry.Path, err)
+			if sdVer < 240 {
+				// Old systemd versions use stat(2) on path to find out device major:minor
+				// numbers and type. If the path doesn't exist, it will not add the rule,
+				// emitting a warning instead.
+				// Since all of this logic is best-effort anyway (we manually set these
+				// rules separately to systemd) we can safely skip entries that don't
+				// have a corresponding path.
+				if _, err := os.Stat(entry.Path); err != nil {
 					continue
 				}
 			}

libcontainer/cgroups/systemd/common.go

@@ -33,7 +33,7 @@ var (
 	isRunningSystemdOnce sync.Once
 	isRunningSystemd     bool
 
-	GenerateDeviceProps func(*configs.Resources) ([]systemdDbus.Property, error)
+	GenerateDeviceProps func(r *configs.Resources, sdVer int) ([]systemdDbus.Property, error)
 )
 
 // NOTE: This function comes from package github.com/coreos/go-systemd/util
@@ -342,13 +342,13 @@ func addCpuset(cm *dbusConnManager, props *[]systemdDbus.Property, cpus, mems st
 
 // generateDeviceProperties takes the configured device rules and generates a
 // corresponding set of systemd properties to configure the devices correctly.
-func generateDeviceProperties(r *configs.Resources) ([]systemdDbus.Property, error) {
+func generateDeviceProperties(r *configs.Resources, cm *dbusConnManager) ([]systemdDbus.Property, error) {
 	if GenerateDeviceProps == nil {
 		if len(r.Devices) > 0 {
 			return nil, cgroups.ErrDevicesUnsupported
 		}
 		return nil, nil
 	}
 
-	return GenerateDeviceProps(r)
+	return GenerateDeviceProps(r, systemdVersion(cm))
 }

libcontainer/cgroups/systemd/v1.go

@@ -75,7 +75,7 @@ var legacySubsystems = []subsystem{
 func genV1ResourcesProperties(r *configs.Resources, cm *dbusConnManager) ([]systemdDbus.Property, error) {
 	var properties []systemdDbus.Property
 
-	deviceProperties, err := generateDeviceProperties(r)
+	deviceProperties, err := generateDeviceProperties(r, cm)
 	if err != nil {
 		return nil, err
 	}

libcontainer/cgroups/systemd/v2.go

@@ -214,7 +214,7 @@ func genV2ResourcesProperties(dirPath string, r *configs.Resources, cm *dbusConn
 	//       aren't the end of the world, but it is a bit concerning. However
 	//       it's unclear if systemd removes all eBPF programs attached when
 	//       doing SetUnitProperties...
-	deviceProperties, err := generateDeviceProperties(r)
+	deviceProperties, err := generateDeviceProperties(r, cm)
 	if err != nil {
 		return nil, err
 	}