Add mount functionality to 'Directory' (#177)

This is needed for 'chroot' runners that must mount the special
filesystems '/proc' and '/sys' in the input root.
The necessary syscalls are not yet merged into the 'sys' package,
so this includes a patch while we work to upstream the code.

Co-authored-by: Nils Wireklint <nils@meroton.com>

BUILD.bazel

@@ -1,6 +1,6 @@
 load("@bazel_gazelle//:def.bzl", "gazelle")
 load("@com_github_bazelbuild_buildtools//buildifier:def.bzl", "buildifier")
-load("@npm//:defs.bzl", "npm_link_all_packages", "npm_link_targets")
+load("@npm//:defs.bzl", "npm_link_all_packages")
 
 # gazelle:prefix github.com/buildbarn/bb-storage
 # gazelle:resolve proto build/bazel/remote/execution/v2/remote_execution.proto @com_github_bazelbuild_remote_apis//build/bazel/remote/execution/v2:remote_execution_proto

go_dependencies.bzl

@@ -1489,6 +1489,7 @@ def go_dependencies():
     go_repository(
         name = "org_golang_x_sys",
         importpath = "golang.org/x/sys",
+        patches = ["//:patches/org_golang_x_sys/golang-issue-59357.diff"],
         sum = "h1:Vz7Qs629MkJkGyHxUlRHizWJRG2j8fbQKjELVSNhy7Q=",
         version = "v0.14.0",
     )

patches/org_golang_x_sys/golang-issue-59357.diff

@@ -0,0 +1,181 @@
+From 9313951415ca77dcf5e84ba5fa41483b5c16d121 Mon Sep 17 00:00:00 2001
+From: Ilya Hanov <ilya.hanov@huawei-partners.com>
+Date: Sat, 30 Sep 2023 12:02:48 +0300
+Subject: [PATCH] unix: add API for fsconfig system call
+
+Fixes golang/go#59537
+
+Change-Id: I8d806ace3adad423c633813455d8f758706cee1d
+---
+
+diff --git unix/linux/types.go unix/linux/types.go
+index 25b9279..b9419f3 100644
+--- unix/linux/types.go
++++ unix/linux/types.go
+@@ -972,6 +972,15 @@
+ 	FSPICK_EMPTY_PATH       = C.FSPICK_EMPTY_PATH
+
+ 	FSMOUNT_CLOEXEC = C.FSMOUNT_CLOEXEC
++
++	FSCONFIG_SET_FLAG        = C.FSCONFIG_SET_FLAG
++	FSCONFIG_SET_STRING      = C.FSCONFIG_SET_STRING
++	FSCONFIG_SET_BINARY      = C.FSCONFIG_SET_BINARY
++	FSCONFIG_SET_PATH        = C.FSCONFIG_SET_PATH
++	FSCONFIG_SET_PATH_EMPTY  = C.FSCONFIG_SET_PATH_EMPTY
++	FSCONFIG_SET_FD          = C.FSCONFIG_SET_FD
++	FSCONFIG_CMD_CREATE      = C.FSCONFIG_CMD_CREATE
++	FSCONFIG_CMD_RECONFIGURE = C.FSCONFIG_CMD_RECONFIGURE
+ )
+
+ type OpenHow C.struct_open_how
+diff --git unix/syscall_linux.go unix/syscall_linux.go
+index d844b16..80aadf2 100644
+--- unix/syscall_linux.go
++++ unix/syscall_linux.go
+@@ -1840,6 +1840,105 @@
+ //sys	Fsmount(fd int, flags int, mountAttrs int) (fsfd int, err error)
+ //sys	Fsopen(fsName string, flags int) (fd int, err error)
+ //sys	Fspick(dirfd int, pathName string, flags int) (fd int, err error)
++
++//sys	fsconfig(fd int, cmd uint, key *byte, value *byte, aux int) (err error)
++
++func fsconfigCommon(fd int, cmd uint, key string, value *byte, aux int) (err error) {
++	var keyp *byte
++	if keyp, err = BytePtrFromString(key); err != nil {
++		return
++	}
++	return fsconfig(fd, cmd, keyp, value, aux)
++}
++
++// FsconfigSetFlag is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_SET_FLAG.
++//
++// fd is the filesystem context to act upon.
++// key the parameter key to set.
++func FsconfigSetFlag(fd int, key string) (err error) {
++	return fsconfigCommon(fd, FSCONFIG_SET_FLAG, key, nil, 0)
++}
++
++// FsconfigSetString is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_SET_STRING.
++//
++// fd is the filesystem context to act upon.
++// key the parameter key to set.
++// value is the parameter value to set.
++func FsconfigSetString(fd int, key string, value string) (err error) {
++	var valuep *byte
++	if valuep, err = BytePtrFromString(value); err != nil {
++		return
++	}
++	return fsconfigCommon(fd, FSCONFIG_SET_STRING, key, valuep, 0)
++}
++
++// FsconfigSetBinary is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_SET_BINARY.
++//
++// fd is the filesystem context to act upon.
++// key the parameter key to set.
++// value is the parameter value to set.
++func FsconfigSetBinary(fd int, key string, value []byte) (err error) {
++	if len(value) == 0 {
++		return EINVAL
++	}
++	return fsconfigCommon(fd, FSCONFIG_SET_BINARY, key, &value[0], len(value))
++}
++
++// FsconfigSetPath is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_SET_PATH.
++//
++// fd is the filesystem context to act upon.
++// key the parameter key to set.
++// path is a non-empty path for specified key.
++// atfd is a file descriptor at which to start lookup from or AT_FDCWD.
++func FsconfigSetPath(fd int, key string, path string, atfd int) (err error) {
++	var valuep *byte
++	if valuep, err = BytePtrFromString(path); err != nil {
++		return
++	}
++	return fsconfigCommon(fd, FSCONFIG_SET_PATH, key, valuep, atfd)
++}
++
++// FsconfigSetPathEmpty is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_SET_PATH_EMPTY. The same as
++// FconfigSetPath but with AT_PATH_EMPTY implied.
++func FsconfigSetPathEmpty(fd int, key string, path string, atfd int) (err error) {
++	var valuep *byte
++	if valuep, err = BytePtrFromString(path); err != nil {
++		return
++	}
++	return fsconfigCommon(fd, FSCONFIG_SET_PATH_EMPTY, key, valuep, atfd)
++}
++
++// FsconfigSetFd is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_SET_FD.
++//
++// fd is the filesystem context to act upon.
++// key the parameter key to set.
++// value is a file descriptor to be assigned to specified key.
++func FsconfigSetFd(fd int, key string, value int) (err error) {
++	return fsconfigCommon(fd, FSCONFIG_SET_FD, key, nil, value)
++}
++
++// FsconfigCreate is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_CMD_CREATE.
++//
++// fd is the filesystem context to act upon.
++func FsconfigCreate(fd int) (err error) {
++	return fsconfig(fd, FSCONFIG_CMD_CREATE, nil, nil, 0)
++}
++
++// FsconfigReconfigure is equivalent to fsconfig(2) called
++// with cmd == FSCONFIG_CMD_RECONFIGURE.
++//
++// fd is the filesystem context to act upon.
++func FsconfigReconfigure(fd int) (err error) {
++	return fsconfig(fd, FSCONFIG_CMD_RECONFIGURE, nil, nil, 0)
++}
++
+ //sys	Getdents(fd int, buf []byte) (n int, err error) = SYS_GETDENTS64
+ //sysnb	Getpgid(pid int) (pgid int, err error)
+
+diff --git unix/zsyscall_linux.go unix/zsyscall_linux.go
+index 14ab34a..dd1720f 100644
+--- unix/zsyscall_linux.go
++++ unix/zsyscall_linux.go
+@@ -892,6 +892,16 @@
+
+ // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
++func fsconfig(fd int, cmd uint, key *byte, value *byte, aux int) (err error) {
++	_, _, e1 := Syscall6(SYS_FSCONFIG, uintptr(fd), uintptr(cmd), uintptr(unsafe.Pointer(key)), uintptr(unsafe.Pointer(value)), uintptr(aux), 0)
++	if e1 != 0 {
++		err = errnoErr(e1)
++	}
++	return
++}
++
++// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
++
+ func Getdents(fd int, buf []byte) (n int, err error) {
+ 	var _p0 unsafe.Pointer
+ 	if len(buf) > 0 {
+diff --git unix/ztypes_linux.go unix/ztypes_linux.go
+index 18aa70b..3c9bfa2 100644
+--- unix/ztypes_linux.go
++++ unix/ztypes_linux.go
+@@ -833,6 +833,15 @@
+ 	FSPICK_EMPTY_PATH       = 0x8
+
+ 	FSMOUNT_CLOEXEC = 0x1
++
++	FSCONFIG_SET_FLAG        = 0x0
++	FSCONFIG_SET_STRING      = 0x1
++	FSCONFIG_SET_BINARY      = 0x2
++	FSCONFIG_SET_PATH        = 0x3
++	FSCONFIG_SET_PATH_EMPTY  = 0x4
++	FSCONFIG_SET_FD          = 0x5
++	FSCONFIG_CMD_CREATE      = 0x6
++	FSCONFIG_CMD_RECONFIGURE = 0x7
+ )
+
+ type OpenHow struct {

pkg/filesystem/directory.go

@@ -112,6 +112,10 @@ type Directory interface {
 	// Function that base types may use to implement calls that
 	// require double dispatching, such as hardlinking and renaming.
 	Apply(arg interface{}) error
+
+	// Mount and Unmount.
+	Mount(mountpoint path.Component, source, fstype string) error
+	Unmount(mountpoint path.Component) error
 }
 
 // DirectoryCloser is a Directory handle that can be released.

pkg/filesystem/local_directory_darwin.go

@@ -23,3 +23,7 @@ func (d *localDirectory) Mknod(name path.Component, perm os.FileMode, deviceNumb
 func clonefileImpl(oldFD int, oldName string, newFD int, newName string) error {
 	return unix.Clonefileat(oldFD, oldName, newFD, newName, unix.CLONE_NOFOLLOW)
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source, fstype string) error {
+	return status.Error(codes.Unimplemented, "Mount is not supported")
+}

pkg/filesystem/local_directory_freebsd.go

@@ -24,3 +24,7 @@ func (d *localDirectory) Mknod(name path.Component, perm os.FileMode, deviceNumb
 func clonefileImpl(oldFD int, oldName string, newFD int, newName string) error {
 	return status.Error(codes.Unimplemented, "Clonefile is only supported on Darwin")
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source, fstype string) error {
+	return status.Error(codes.Unimplemented, "Mount is not supported")
+}

pkg/filesystem/local_directory_linux.go

@@ -8,6 +8,7 @@ import (
 	"runtime"
 
 	"github.com/buildbarn/bb-storage/pkg/filesystem/path"
+	"github.com/buildbarn/bb-storage/pkg/util"
 
 	"golang.org/x/sys/unix"
 	"google.golang.org/grpc/codes"
@@ -34,3 +35,38 @@ func (d *localDirectory) Mknod(name path.Component, perm os.FileMode, deviceNumb
 func clonefileImpl(oldFD int, oldName string, newFD int, newName string) error {
 	return status.Error(codes.Unimplemented, "Clonefile is only supported on Darwin")
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source, fstype string) error {
+	mountname := mountpoint.String()
+	fd, err := unix.Fsopen(fstype, unix.FSOPEN_CLOEXEC)
+	if err != nil {
+		return util.StatusWrapf(err, "Fsopen '%s'", fstype)
+	}
+	defer unix.Close(fd)
+
+	err = unix.FsconfigSetString(fd, "source", source)
+	if err != nil {
+		return util.StatusWrapf(err, "Fsconfig source '%s'", source)
+	}
+
+	err = unix.FsconfigCreate(fd)
+	if err != nil {
+		return util.StatusWrap(err, "Fsconfig create")
+	}
+
+	mfd, err := unix.Fsmount(fd, unix.FSMOUNT_CLOEXEC, unix.MS_NOEXEC)
+	if err != nil {
+		return util.StatusWrap(err, "Fsmount")
+	}
+	// NB: `Fsmount` creates a file descriptor to the mount object, that can be
+	// used to move it again. But we will not do so, so it is best to close it.
+	// Unmount will fail with `EBUSY` if it is left open.
+	defer unix.Close(mfd)
+
+	err = unix.MoveMount(mfd, "", d.fd, mountname, unix.MOVE_MOUNT_F_EMPTY_PATH)
+	if err != nil {
+		return util.StatusWrapf(err, "Movemount mountname '%s' in file descriptor %d", mountname, d.fd)
+	}
+
+	return nil
+}

pkg/filesystem/local_directory_unix.go

@@ -268,7 +268,7 @@ func (d *localDirectory) Remove(name path.Component) error {
 
 var workingDirectoryLock sync.Mutex
 
-func (d *localDirectory) unmount(name path.Component) error {
+func (d *localDirectory) Unmount(name path.Component) error {
 	defer runtime.KeepAlive(d)
 
 	// POSIX systems provide no umountat() system call that permits
@@ -302,7 +302,7 @@ func (d *localDirectory) removeAllChildren(parentDeviceNumber rawDeviceNumber) e
 		// unmount until the remaining directory is on the same
 		// file system.
 		for parentDeviceNumber != childDeviceNumber {
-			if err := d.unmount(component); err != nil {
+			if err := d.Unmount(component); err != nil {
 				return err
 			}
 			fileType, childDeviceNumber, _, err = d.lstat(component)

pkg/filesystem/local_directory_windows.go

@@ -978,3 +978,11 @@ func (d *localDirectory) Apply(arg interface{}) error {
 		return syscall.EXDEV
 	}
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source, fstype string) error {
+	return status.Error(codes.Unimplemented, "Mount is not supported")
+}
+
+func (d *localDirectory) Unmount(mountpoint path.Component) error {
+	return status.Error(codes.Unimplemented, "Unmount is not supported")
+}