path: root/ext/fs.go

package ext

import (
	"os"
	"runtime"
	"syscall"
	"unsafe"
)

// include/uapi/linux/mount.h

/*
 * move_mount() flags.
 */
const (
	MOVE_MOUNT_F_SYMLINKS   = 1 << iota /* Follow symlinks on from path */
	MOVE_MOUNT_F_AUTOMOUNTS             /* Follow automounts on from path */
	MOVE_MOUNT_F_EMPTY_PATH             /* Empty from path permitted */
	_
	MOVE_MOUNT_T_SYMLINKS   /* Follow symlinks on to path */
	MOVE_MOUNT_T_AUTOMOUNTS /* Follow automounts on to path */
	MOVE_MOUNT_T_EMPTY_PATH /* Empty to path permitted */
	_
	MOVE_MOUNT_SET_GROUP /* Set sharing group instead */
	MOVE_MOUNT_BENEATH   /* Mount beneath top mount */
)

/*
 * fsopen() flags.
 */
const (
	FSOPEN_CLOEXEC = 1 << iota
)

/*
 * fspick() flags.
 */
const (
	FSPICK_CLOEXEC = 1 << iota
	FSPICK_SYMLINK_NOFOLLOW
	FSPICK_NO_AUTOMOUNT
	FSPICK_EMPTY_PATH
)

/*
 * The type of fsconfig() call made.
 */
const (
	FSCONFIG_SET_FLAG        = iota /* Set parameter, supplying no value */
	FSCONFIG_SET_STRING             /* Set parameter, supplying a string value */
	FSCONFIG_SET_BINARY             /* Set parameter, supplying a binary blob value */
	FSCONFIG_SET_PATH               /* Set parameter, supplying an object by path */
	FSCONFIG_SET_PATH_EMPTY         /* Set parameter, supplying an object by (empty) path */
	FSCONFIG_SET_FD                 /* Set parameter, supplying an object by fd */
	FSCONFIG_CMD_CREATE             /* Create new or reuse existing superblock */
	FSCONFIG_CMD_RECONFIGURE        /* Invoke superblock reconfiguration */
	FSCONFIG_CMD_CREATE_EXCL        /* Create new superblock, fail if reusing existing superblock */
)

/*
 * fsmount() flags.
 */
const (
	FSMOUNT_CLOEXEC = 1 << iota
)

/*
 * Mount attributes.
 */
const (
	MOUNT_ATTR_RDONLY      = 0x00000001 /* Mount read-only */
	MOUNT_ATTR_NOSUID      = 0x00000002 /* Ignore suid and sgid bits */
	MOUNT_ATTR_NODEV       = 0x00000004 /* Disallow access to device special files */
	MOUNT_ATTR_NOEXEC      = 0x00000008 /* Disallow program execution */
	MOUNT_ATTR__ATIME      = 0x00000070 /* Setting on how atime should be updated */
	MOUNT_ATTR_RELATIME    = 0x00000000 /* - Update atime relative to mtime/ctime. */
	MOUNT_ATTR_NOATIME     = 0x00000010 /* - Do not update access times. */
	MOUNT_ATTR_STRICTATIME = 0x00000020 /* - Always perform atime updates */
	MOUNT_ATTR_NODIRATIME  = 0x00000080 /* Do not update directory access times */
	MOUNT_ATTR_IDMAP       = 0x00100000 /* Idmap mount to @userns_fd in struct mount_attr. */
	MOUNT_ATTR_NOSYMFOLLOW = 0x00200000 /* Do not follow symlinks */
)

// FS provides low-level wrappers around the suite of file-descriptor-based
// mount facilities in Linux.
type FS struct {
	fd uintptr
	c  runtime.Cleanup
}

// newFS allocates a new [FS] for the specified fd.
func newFS(fd uintptr) *FS {
	fs := FS{fd: fd}
	fs.c = runtime.AddCleanup(&fs, func(fd uintptr) {
		_ = syscall.Close(int(fd))
	}, fd)
	return &fs
}

// Close closes the underlying filesystem context.
func (fs *FS) Close() error {
	if fs == nil {
		return syscall.EINVAL
	}
	err := syscall.Close(int(fs.fd))
	fs.c.Stop()
	return err
}

// OpenFS creates a new filesystem context.
func OpenFS(fsname string, flags int) (fs *FS, err error) {
	var s *byte
	s, err = syscall.BytePtrFromString(fsname)
	if err != nil {
		return
	}
	fd, _, errno := syscall.Syscall(
		SYS_FSOPEN,
		uintptr(unsafe.Pointer(s)),
		uintptr(flags|FSOPEN_CLOEXEC),
		0,
	)
	if errno != 0 {
		err = os.NewSyscallError("fsopen", errno)
	} else {
		fs = newFS(fd)
	}
	return
}

// PickFS selects filesystem for reconfiguration.
func PickFS(dirfd int, pathname string, flags int) (fs *FS, err error) {
	var s *byte
	s, err = syscall.BytePtrFromString(pathname)
	if err != nil {
		return
	}
	fd, _, errno := syscall.Syscall(
		SYS_FSPICK,
		uintptr(dirfd),
		uintptr(unsafe.Pointer(s)),
		uintptr(flags|FSPICK_CLOEXEC),
	)
	if errno != 0 {
		err = os.NewSyscallError("fspick", errno)
	} else {
		fs = newFS(fd)
	}
	return
}

// config configures new or existing filesystem context.
func (fs *FS) config(cmd uint, key *byte, value unsafe.Pointer, aux int) (err error) {
	_, _, errno := syscall.Syscall6(
		SYS_FSCONFIG,
		fs.fd,
		uintptr(cmd),
		uintptr(unsafe.Pointer(key)),
		uintptr(value),
		uintptr(aux),
		0,
	)
	if errno != 0 {
		err = os.NewSyscallError("fsconfig", errno)
	}
	return
}

// SetFlag sets the flag parameter named by key. ([FSCONFIG_SET_FLAG])
func (fs *FS) SetFlag(key string) (err error) {
	var s *byte
	s, err = syscall.BytePtrFromString(key)
	if err != nil {
		return
	}

	return fs.config(FSCONFIG_SET_FLAG, s, nil, 0)
}

// SetString sets the string parameter named by key to the value specified by
// value. ([FSCONFIG_SET_STRING])
func (fs *FS) SetString(key, value string) (err error) {
	var s0 *byte
	s0, err = syscall.BytePtrFromString(key)
	if err != nil {
		return
	}

	var s1 *byte
	s1, err = syscall.BytePtrFromString(value)
	if err != nil {
		return
	}

	return fs.config(FSCONFIG_SET_STRING, s0, unsafe.Pointer(s1), 0)
}

// mount instantiates mount object from filesystem context.
func (fs *FS) mount(flags, attrFlags int) (fsfd int, err error) {
	r, _, errno := syscall.Syscall(
		SYS_FSMOUNT,
		fs.fd,
		uintptr(flags|FSMOUNT_CLOEXEC),
		uintptr(attrFlags),
	)
	fsfd = int(r)
	if errno != 0 {
		err = os.NewSyscallError("fsmount", errno)
	}
	return
}

// MoveMount moves or attaches mount object to filesystem.
func MoveMount(
	fromDirfd int,
	fromPathname string,
	toDirfd int,
	toPathname string,
	flags int,
) (err error) {
	var s0 *byte
	s0, err = syscall.BytePtrFromString(fromPathname)
	if err != nil {
		return
	}

	var s1 *byte
	s1, err = syscall.BytePtrFromString(toPathname)
	if err != nil {
		return
	}

	_, _, errno := syscall.Syscall6(
		SYS_MOVE_MOUNT,
		uintptr(fromDirfd),
		uintptr(unsafe.Pointer(s0)),
		uintptr(toDirfd),
		uintptr(unsafe.Pointer(s1)),
		uintptr(flags),
		0,
	)
	if errno != 0 {
		err = os.NewSyscallError("move_mount", errno)
	}
	return
}

// Mount attaches the underlying filesystem context to the specified pathname.
func (fs *FS) Mount(pathname string, attrFlags int) error {
	if err := fs.config(FSCONFIG_CMD_CREATE_EXCL, nil, nil, 0); err != nil {
		return err
	}
	fd, err := fs.mount(0, attrFlags)
	if err != nil {
		return err
	}
	err = MoveMount(
		fd, "",
		-1, pathname,
		MOVE_MOUNT_F_EMPTY_PATH,
	)
	closeErr := syscall.Close(fd)
	if err == nil {
		err = closeErr
	}
	return err
}