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/*
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/containerd/containerd/log"
"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
err error
errno, status 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()
var pipefds [2]int
if err := syscall.Pipe2(pipefds[:], syscall.O_CLOEXEC); err != nil {
return errors.Wrap(err, "failed to open pipe")
}
defer func() {
// close both ends of the pipe in a deferred function, since open file
// descriptor table is shared with child
syscall.Close(pipefds[0])
syscall.Close(pipefds[1])
}()
pid, errno = forkAndMountat(dirfd,
uintptr(unsafe.Pointer(sourceP)),
uintptr(unsafe.Pointer(targetP)),
uintptr(unsafe.Pointer(fstypeP)),
flags,
uintptr(unsafe.Pointer(dataP)),
pipefds[1],
)
if errno != 0 {
return errors.Wrap(errno, "failed to fork thread")
}
defer func() {
_, err := unix.Wait4(int(pid), nil, 0, nil)
for err == syscall.EINTR {
_, err = unix.Wait4(int(pid), nil, 0, nil)
}
if err != nil {
log.L.WithError(err).Debugf("failed to find pid=%d process", pid)
}
}()
_, _, errno = syscall.RawSyscall(syscall.SYS_READ,
uintptr(pipefds[0]),
uintptr(unsafe.Pointer(&status)),
unsafe.Sizeof(status))
if errno != 0 {
return errors.Wrap(errno, "failed to read pipe")
}
if status != 0 {
return errors.Wrap(status, "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, pipefd int) (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:
_, _, errno = syscall.RawSyscall(syscall.SYS_WRITE, uintptr(pipefd), uintptr(unsafe.Pointer(&errno)), unsafe.Sizeof(errno))
syscall.RawSyscall(syscall.SYS_EXIT, uintptr(errno), 0, 0)
panic("unreachable")
}
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