Fix failed exec after systemctl daemon-reload

A regression reported for runc v1.1.3 says that "runc exec -t" fails
after doing "systemctl daemon-reload":

> exec failed: unable to start container process: open /dev/pts/0: operation not permitted: unknown

Apparently, with commit 7219387 we are no longer adding
"DeviceAllow=char-pts rwm" rule (because os.Stat("char-pts") returns
ENOENT).

The bug can only be seen after "systemctl daemon-reload" because runc
also applies the same rules manually (by writing to devices.allow for
cgroup v1), and apparently reloading systemd leads to re-applying the
rules that systemd has (thus removing the char-pts access).

The fix is to do os.Stat only for "/dev" paths.

Also, emit a warning that the path was skipped. Since the original idea
was to emit less warnings, demote the level to debug.

Note this also fixes the issue of not adding "m" permission for block-*
and char-* devices.

A test case is added, which reliably fails before the fix
on both cgroup v1 and v2.

Fixes: opencontainers#3551
Fixes: 7219387
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/devices/systemd.go

@@ -128,14 +128,16 @@ 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 {
+				logrus.Debugf("skipping device %s for systemd: %s", entry.Path, err)
+				continue
+			}
 		}
-		// 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 {
-			deviceAllowList = append(deviceAllowList, entry)
-		}
+		deviceAllowList = append(deviceAllowList, entry)
 	}
 
 	properties = append(properties, newProp("DeviceAllow", deviceAllowList))

tests/integration/dev.bats

@@ -128,3 +128,19 @@ function teardown() {
 	runc exec test_allow_block sh -c 'fdisk -l '"$device"''
 	[ "$status" -eq 0 ]
 }
+
+# https://github.com/opencontainers/runc/issues/3551
+@test "runc exec vs systemctl daemon-reload" {
+	requires systemd root
+
+	runc run -d --console-socket "$CONSOLE_SOCKET" test_exec
+	[ "$status" -eq 0 ]
+
+	runc exec -t test_exec sh -c "ls -l /proc/self/fd/0; echo 123"
+	[ "$status" -eq 0 ]
+
+	systemctl daemon-reload
+
+	runc exec -t test_exec sh -c "ls -l /proc/self/fd/0; echo 123"
+	[ "$status" -eq 0 ]
+}