mount: support idmapped mount points

This patch introduces idmapped mounts support for
container rootfs.

The idmapped mounts support was merged in Linux kernel 5.12
torvalds/linux@7d6beb7.
This functionality allows to address chown overhead for containers that
use user namespace.

The changes are based on experimental patchset published by
Mauricio Vásquez #4734.
Current version reiplements support of idmapped mounts using Golang.

Performance measurement results:
Image           idmapped mount  recursive chown
BusyBox         00.135          04.964
Ubuntu          00.171          15.713
Fedora          00.143          38.799

Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
Signed-off-by: Artem Kuzin <artem.kuzin@huawei.com>
Signed-off-by: Alexey Perevalov <alexey.perevalov@huawei.com>
Signed-off-by: Ilya Hanov <ilya.hanov@huawei-partners.com>

sys/userns_unsafe_linux.go

@@ -0,0 +1,65 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package sys
+
+import (
+	"runtime"
+	"syscall"
+	"unsafe"
+)
+
+// ProcSyncType is used for synchronization
+// between parent and child processes.
+type ProcSyncType uint8
+
+const (
+	// ProcSyncExit tells child "it's time to exit".
+	ProcSyncExit ProcSyncType = 0x1
+)
+
+//go:norace
+//go:noinline
+func ForkUserns(pipeMap [2]int) (pid uintptr, errno syscall.Errno) {
+	var sync ProcSyncType
+
+	beforeFork()
+	if runtime.GOARCH == "s390x" {
+		pid, _, errno = syscall.RawSyscall6(uintptr(syscall.SYS_CLONE), 0, syscall.CLONE_NEWUSER|uintptr(syscall.SIGCHLD), 0, 0, 0, 0)
+	} else {
+		pid, _, errno = syscall.RawSyscall6(uintptr(syscall.SYS_CLONE), syscall.CLONE_NEWUSER|uintptr(syscall.SIGCHLD), 0, 0, 0, 0, 0)
+	}
+	if errno != 0 || pid != 0 {
+		afterFork()
+		return pid, errno
+	}
+
+	afterForkInChild()
+	if _, _, errno = syscall.RawSyscall(syscall.SYS_CLOSE, uintptr(pipeMap[1]), 0, 0); errno != 0 {
+		goto err
+	}
+	if _, _, errno = syscall.RawSyscall6(syscall.SYS_PRCTL, syscall.PR_SET_PDEATHSIG, uintptr(syscall.SIGKILL), 0, 0, 0, 0); errno != 0 {
+		goto err
+	}
+	// wait for parent's signal
+	if _, _, errno = syscall.RawSyscall6(syscall.SYS_READ, uintptr(pipeMap[0]), uintptr(unsafe.Pointer(&sync)), unsafe.Sizeof(sync), 0, 0, 0); errno != 0 || sync != ProcSyncExit {
+		goto err
+	}
+
+err:
+	syscall.RawSyscall6(syscall.SYS_EXIT, uintptr(errno), 0, 0, 0, 0, 0)
+	panic("unreachable")
+}