Return correct name for imported host files

Implement PrependPath() in host.filesystem to correctly format
name for host files.

Updates #1672

PiperOrigin-RevId: 309959135

pkg/sentry/fsimpl/devpts/devpts.go

@@ -59,7 +59,7 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 // master inode. It returns the filesystem and root Dentry.
 func (fstype FilesystemType) newFilesystem(vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials) (*kernfs.Filesystem, *kernfs.Dentry) {
 	fs := &kernfs.Filesystem{}
-	fs.Init(vfsObj, fstype)
+	fs.VFSFilesystem().Init(vfsObj, fstype, fs)
 
 	// Construct the root directory. This is always inode id 1.
 	root := &rootInode{

pkg/sentry/fsimpl/host/BUILD

@@ -20,6 +20,7 @@ go_library(
         "//pkg/abi/linux",
         "//pkg/context",
         "//pkg/fdnotifier",
+        "//pkg/fspath",
         "//pkg/log",
         "//pkg/refs",
         "//pkg/sentry/arch",

pkg/sentry/fsimpl/host/host.go

@@ -25,6 +25,7 @@ import (
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/fdnotifier"
+	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/refs"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
@@ -39,37 +40,9 @@ import (
 	"gvisor.dev/gvisor/pkg/waiter"
 )
 
-// filesystemType implements vfs.FilesystemType.
-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("host.filesystemType.GetFilesystem should never be called")
-}
-
-// Name implements FilesystemType.Name.
-func (filesystemType) Name() string {
-	return "none"
-}
-
-// filesystem implements vfs.FilesystemImpl.
-type filesystem struct {
-	kernfs.Filesystem
-}
-
-// 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{})
-	return fs.VFSFilesystem()
-}
-
 // ImportFD sets up and returns a vfs.FileDescription from a donated fd.
 func ImportFD(ctx context.Context, mnt *vfs.Mount, hostFD int, isTTY bool) (*vfs.FileDescription, error) {
-	fs, ok := mnt.Filesystem().Impl().(*kernfs.Filesystem)
+	fs, ok := mnt.Filesystem().Impl().(*filesystem)
 	if !ok {
 		return nil, fmt.Errorf("can't import host FDs into filesystems of type %T", mnt.Filesystem().Impl())
 	}
@@ -119,12 +92,47 @@ func ImportFD(ctx context.Context, mnt *vfs.Mount, hostFD int, isTTY bool) (*vfs
 
 	d := &kernfs.Dentry{}
 	d.Init(i)
+
 	// i.open will take a reference on d.
 	defer d.DecRef()
-
 	return i.open(ctx, d.VFSDentry(), mnt)
 }
 
+// filesystemType implements vfs.FilesystemType.
+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("host.filesystemType.GetFilesystem should never be called")
+}
+
+// Name implements FilesystemType.Name.
+func (filesystemType) Name() string {
+	return "none"
+}
+
+// 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.VFSFilesystem().Init(vfsObj, filesystemType{}, fs)
+	return fs.VFSFilesystem()
+}
+
+// filesystem implements vfs.FilesystemImpl.
+type filesystem struct {
+	kernfs.Filesystem
+}
+
+func (fs *filesystem) PrependPath(ctx context.Context, vfsroot, vd vfs.VirtualDentry, b *fspath.Builder) error {
+	d := vd.Dentry().Impl().(*kernfs.Dentry)
+	inode := d.Inode().(*inode)
+	b.PrependComponent(fmt.Sprintf("host:[%d]", inode.ino))
+	return vfs.PrependPathSyntheticError{}
+}
+
 // inode implements kernfs.Inode.
 type inode struct {
 	kernfs.InodeNotDirectory

pkg/sentry/fsimpl/kernfs/kernfs.go

@@ -132,13 +132,6 @@ func (fs *Filesystem) processDeferredDecRefsLocked() {
 	fs.droppedDentriesMu.Unlock()
 }
 
-// Init initializes a kernfs filesystem. This should be called from during
-// vfs.FilesystemType.NewFilesystem for the concrete filesystem embedding
-// kernfs.
-func (fs *Filesystem) Init(vfsObj *vfs.VirtualFilesystem, fsType vfs.FilesystemType) {
-	fs.vfsfs.Init(vfsObj, fsType, fs)
-}
-
 // VFSFilesystem returns the generic vfs filesystem object.
 func (fs *Filesystem) VFSFilesystem() *vfs.Filesystem {
 	return &fs.vfsfs
@@ -261,6 +254,11 @@ func (d *Dentry) insertChildLocked(name string, child *Dentry) {
 	d.children[name] = child
 }
 
+// Inode returns the dentry's inode.
+func (d *Dentry) Inode() Inode {
+	return d.inode
+}
+
 // The Inode interface maps filesystem-level operations that operate on paths to
 // equivalent operations on specific filesystem nodes.
 //

pkg/sentry/fsimpl/kernfs/kernfs_test.go

@@ -195,7 +195,7 @@ func (fsType) Name() string {
 
 func (fst fsType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opt vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
 	fs := &filesystem{}
-	fs.Init(vfsObj, &fst)
+	fs.VFSFilesystem().Init(vfsObj, &fst, fs)
 	root := fst.rootFn(creds, fs)
 	return fs.VFSFilesystem(), root.VFSDentry(), nil
 }

pkg/sentry/fsimpl/pipefs/pipefs.go

@@ -40,25 +40,19 @@ func (filesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFile
 	panic("pipefs.filesystemType.GetFilesystem should never be called")
 }
 
-// filesystem implements vfs.FilesystemImpl.
-type filesystem struct {
-	kernfs.Filesystem
+// TODO(gvisor.dev/issue/1193):
+//
+// - kernfs does not provide a way to implement statfs, from which we
+// should indicate PIPEFS_MAGIC.
+//
+// - kernfs does not provide a way to override names for
+// vfs.FilesystemImpl.PrependPath(); pipefs inodes should use synthetic
+// name fmt.Sprintf("pipe:[%d]", inode.ino).
 
-	// TODO(gvisor.dev/issue/1193):
-	//
-	// - kernfs does not provide a way to implement statfs, from which we
-	// should indicate PIPEFS_MAGIC.
-	//
-	// - kernfs does not provide a way to override names for
-	// vfs.FilesystemImpl.PrependPath(); pipefs inodes should use synthetic
-	// name fmt.Sprintf("pipe:[%d]", inode.ino).
-}
-
-// NewFilesystem sets up and returns a new vfs.Filesystem implemented by
-// pipefs.
+// NewFilesystem sets up and returns a new vfs.Filesystem implemented by pipefs.
 func NewFilesystem(vfsObj *vfs.VirtualFilesystem) *vfs.Filesystem {
-	fs := &filesystem{}
-	fs.Init(vfsObj, filesystemType{})
+	fs := &kernfs.Filesystem{}
+	fs.VFSFilesystem().Init(vfsObj, filesystemType{}, fs)
 	return fs.VFSFilesystem()
 }
 

pkg/sentry/fsimpl/sockfs/sockfs.go

@@ -41,18 +41,13 @@ func (filesystemType) Name() string {
 	return "sockfs"
 }
 
-// filesystem implements vfs.FilesystemImpl.
-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{})
+	fs := &kernfs.Filesystem{}
+	fs.VFSFilesystem().Init(vfsObj, filesystemType{}, fs)
 	return fs.VFSFilesystem()
 }
 

pkg/sentry/fsimpl/sys/sys.go

@@ -47,7 +47,7 @@ func (FilesystemType) Name() string {
 // GetFilesystem implements vfs.FilesystemType.GetFilesystem.
 func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
 	fs := &filesystem{}
-	fs.Filesystem.Init(vfsObj, &fsType)
+	fs.VFSFilesystem().Init(vfsObj, &fsType, fs)
 	k := kernel.KernelFromContext(ctx)
 	maxCPUCores := k.ApplicationCores()
 	defaultSysDirMode := linux.FileMode(0755)