Move hostfs mount to Kernel struct.

This is needed to set up host fds passed through a Unix socket. Note that
the host package depends on kernel, so we cannot set up the hostfs mount
directly in Kernel.Init as we do for sockfs and pipefs.

Also, adjust sockfs to make its setup look more like hostfs's and pipefs's.

PiperOrigin-RevId: 308274053

2 files changed, 19 insertions, 23 deletions

diff --git a/pkg/sentry/fsimpl/host/host.go b/pkg/sentry/fsimpl/host/host.go
index 7847e3cc2..a26b13067 100644
--- a/pkg/sentry/fsimpl/host/host.go
+++ b/pkg/sentry/fsimpl/host/host.go
@@ -42,7 +42,7 @@ type filesystemType struct{}
 
 // GetFilesystem implements FilesystemType.GetFilesystem.
 func (filesystemType) GetFilesystem(context.Context, *vfs.VirtualFilesystem, *auth.Credentials, string, vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
-	panic("cannot instaniate a host filesystem")
+	panic("host.filesystemType.GetFilesystem should never be called")
 }
 
 // Name implements FilesystemType.Name.
@@ -55,14 +55,14 @@ type filesystem struct {
 	kernfs.Filesystem
 }
 
-// NewMount returns a new disconnected mount in vfsObj that may be passed to ImportFD.
-func NewMount(vfsObj *vfs.VirtualFilesystem) (*vfs.Mount, error) {
+// NewFilesystem sets up and returns a new hostfs filesystem.
+//
+// Note that there should only ever be one instance of host.filesystem,
+// a global mount for host fds.
+func NewFilesystem(vfsObj *vfs.VirtualFilesystem) *vfs.Filesystem {
 	fs := &filesystem{}
-	fs.Init(vfsObj, &filesystemType{})
-	vfsfs := fs.VFSFilesystem()
-	// NewDisconnectedMount will take an additional reference on vfsfs.
-	defer vfsfs.DecRef()
-	return vfsObj.NewDisconnectedMount(vfsfs, nil, &vfs.MountOptions{})
+	fs.Init(vfsObj, filesystemType{})
+	return fs.VFSFilesystem()
 }
 
 // ImportFD sets up and returns a vfs.FileDescription from a donated fd.
diff --git a/pkg/sentry/fsimpl/sockfs/sockfs.go b/pkg/sentry/fsimpl/sockfs/sockfs.go
index 3f7ad1d65..632cfde88 100644
--- a/pkg/sentry/fsimpl/sockfs/sockfs.go
+++ b/pkg/sentry/fsimpl/sockfs/sockfs.go
@@ -24,26 +24,12 @@ import (
 	"gvisor.dev/gvisor/pkg/syserror"
 )
 
-// NewFilesystem creates a new sockfs filesystem.
-//
-// Note that there should only ever be one instance of sockfs.Filesystem,
-// backing a global socket mount.
-func NewFilesystem(vfsObj *vfs.VirtualFilesystem) *vfs.Filesystem {
-	fs, _, err := filesystemType{}.GetFilesystem(nil, vfsObj, nil, "", vfs.GetFilesystemOptions{})
-	if err != nil {
-		panic("failed to create sockfs filesystem")
-	}
-	return fs
-}
-
 // filesystemType implements vfs.FilesystemType.
 type filesystemType struct{}
 
 // GetFilesystem implements FilesystemType.GetFilesystem.
 func (fsType filesystemType) GetFilesystem(_ context.Context, vfsObj *vfs.VirtualFilesystem, _ *auth.Credentials, _ string, _ vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
-	fs := &filesystem{}
-	fs.Init(vfsObj, fsType)
-	return fs.VFSFilesystem(), nil, nil
+	panic("sockfs.filesystemType.GetFilesystem should never be called")
 }
 
 // Name implements FilesystemType.Name.
@@ -60,6 +46,16 @@ type filesystem struct {
 	kernfs.Filesystem
 }
 
+// NewFilesystem sets up and returns a new sockfs filesystem.
+//
+// Note that there should only ever be one instance of sockfs.Filesystem,
+// backing a global socket mount.
+func NewFilesystem(vfsObj *vfs.VirtualFilesystem) *vfs.Filesystem {
+	fs := &filesystem{}
+	fs.Init(vfsObj, filesystemType{})
+	return fs.VFSFilesystem()
+}
+
 // inode implements kernfs.Inode.
 //
 // TODO(gvisor.dev/issue/1476): Add device numbers to this inode (which are