Support >= 128 layers in overlayfs snapshots

Auto-detect longest common dir in lowerdir option and compact it if the
option size is hitting one page size. If does, Use chdir + CLONE to do
mount thing to avoid hitting one page argument buffer in linux kernel
mount.

Signed-off-by: Wei Fu <fhfuwei@163.com>

sys/mount_linux.go

@@ -0,0 +1,119 @@
+/*
+   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"
+
+	"github.com/pkg/errors"
+	"golang.org/x/sys/unix"
+)
+
+// FMountat performs mount from the provided directory.
+func FMountat(dirfd uintptr, source, target, fstype string, flags uintptr, data string) error {
+	var (
+		sourceP, targetP, fstypeP, dataP *byte
+		pid                              uintptr
+		ws                               unix.WaitStatus
+		err                              error
+		errno                            syscall.Errno
+	)
+
+	sourceP, err = syscall.BytePtrFromString(source)
+	if err != nil {
+		return err
+	}
+
+	targetP, err = syscall.BytePtrFromString(target)
+	if err != nil {
+		return err
+	}
+
+	fstypeP, err = syscall.BytePtrFromString(fstype)
+	if err != nil {
+		return err
+	}
+
+	if data != "" {
+		dataP, err = syscall.BytePtrFromString(data)
+		if err != nil {
+			return err
+		}
+	}
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	pid, errno = forkAndMountat(dirfd,
+		uintptr(unsafe.Pointer(sourceP)),
+		uintptr(unsafe.Pointer(targetP)),
+		uintptr(unsafe.Pointer(fstypeP)),
+		flags,
+		uintptr(unsafe.Pointer(dataP)))
+
+	if errno != 0 {
+		return errors.Wrap(errno, "failed to fork thread")
+	}
+
+	_, err = unix.Wait4(int(pid), &ws, 0, nil)
+	for err == syscall.EINTR {
+		_, err = unix.Wait4(int(pid), &ws, 0, nil)
+	}
+
+	if err != nil {
+		return errors.Wrapf(err, "failed to find pid=%d process", pid)
+	}
+
+	errno = syscall.Errno(ws.ExitStatus())
+	if errno != 0 {
+		return errors.Wrap(errno, "failed to mount")
+	}
+	return nil
+}
+
+// forkAndMountat will fork thread, change working dir and mount.
+//
+// precondition: the runtime OS thread must be locked.
+func forkAndMountat(dirfd uintptr, source, target, fstype, flags, data uintptr) (pid uintptr, errno syscall.Errno) {
+	// block signal during clone
+	beforeFork()
+
+	// the cloned thread shares the open file descriptor, but the thread
+	// never be reused by runtime.
+	pid, _, errno = syscall.RawSyscall6(syscall.SYS_CLONE, uintptr(syscall.SIGCHLD)|syscall.CLONE_FILES, 0, 0, 0, 0, 0)
+	if errno != 0 || pid != 0 {
+		// restore all signals
+		afterFork()
+		return
+	}
+
+	// restore all signals
+	afterForkInChild()
+
+	// change working dir
+	_, _, errno = syscall.RawSyscall(syscall.SYS_FCHDIR, dirfd, 0, 0)
+	if errno != 0 {
+		goto childerr
+	}
+	_, _, errno = syscall.RawSyscall6(syscall.SYS_MOUNT, source, target, fstype, flags, data, 0)
+
+childerr:
+	syscall.RawSyscall(syscall.SYS_EXIT, uintptr(errno), 0, 0)
+	panic("unreachable")
+}