Implement synthetic mountpoints for kernfs.

PiperOrigin-RevId: 330629897

pkg/sentry/fsimpl/kernfs/BUILD

@@ -83,6 +83,7 @@ go_library(
         "slot_list.go",
         "static_directory_refs.go",
         "symlink.go",
+        "synthetic_directory.go",
     ],
     visibility = ["//pkg/sentry:internal"],
     deps = [

pkg/sentry/fsimpl/kernfs/filesystem.go

@@ -360,7 +360,10 @@ func (fs *Filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 	defer rp.Mount().EndWrite()
 	childVFSD, err := parentInode.NewDir(ctx, pc, opts)
 	if err != nil {
-		return err
+		if !opts.ForSyntheticMountpoint || err == syserror.EEXIST {
+			return err
+		}
+		childVFSD = newSyntheticDirectory(rp.Credentials(), opts.Mode)
 	}
 	parentVFSD.Impl().(*Dentry).InsertChild(pc, childVFSD.Impl().(*Dentry))
 	return nil

pkg/sentry/fsimpl/kernfs/synthetic_directory.go

@@ -0,0 +1,102 @@
+// Copyright 2019 The gVisor 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 kernfs
+
+import (
+	"fmt"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// syntheticDirectory implements kernfs.Inode for a directory created by
+// MkdirAt(ForSyntheticMountpoint=true).
+//
+// +stateify savable
+type syntheticDirectory struct {
+	InodeAttrs
+	InodeNoStatFS
+	InodeNoopRefCount
+	InodeNoDynamicLookup
+	InodeNotSymlink
+	OrderedChildren
+
+	locks vfs.FileLocks
+}
+
+var _ Inode = (*syntheticDirectory)(nil)
+
+func newSyntheticDirectory(creds *auth.Credentials, perm linux.FileMode) *vfs.Dentry {
+	inode := &syntheticDirectory{}
+	inode.Init(creds, 0 /* devMajor */, 0 /* devMinor */, 0 /* ino */, perm)
+	d := &Dentry{}
+	d.Init(inode)
+	return &d.vfsd
+}
+
+func (dir *syntheticDirectory) Init(creds *auth.Credentials, devMajor, devMinor uint32, ino uint64, perm linux.FileMode) {
+	if perm&^linux.PermissionsMask != 0 {
+		panic(fmt.Sprintf("perm contains non-permission bits: %#o", perm))
+	}
+	dir.InodeAttrs.Init(creds, devMajor, devMinor, ino, linux.S_IFDIR|perm)
+	dir.OrderedChildren.Init(OrderedChildrenOptions{
+		Writable: true,
+	})
+}
+
+// Open implements Inode.Open.
+func (dir *syntheticDirectory) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	fd, err := NewGenericDirectoryFD(rp.Mount(), vfsd, &dir.OrderedChildren, &dir.locks, &opts, GenericDirectoryFDOptions{})
+	if err != nil {
+		return nil, err
+	}
+	return &fd.vfsfd, nil
+}
+
+// NewFile implements Inode.NewFile.
+func (dir *syntheticDirectory) NewFile(ctx context.Context, name string, opts vfs.OpenOptions) (*vfs.Dentry, error) {
+	return nil, syserror.EPERM
+}
+
+// NewDir implements Inode.NewDir.
+func (dir *syntheticDirectory) NewDir(ctx context.Context, name string, opts vfs.MkdirOptions) (*vfs.Dentry, error) {
+	if !opts.ForSyntheticMountpoint {
+		return nil, syserror.EPERM
+	}
+	subdird := newSyntheticDirectory(auth.CredentialsFromContext(ctx), opts.Mode&linux.PermissionsMask)
+	if err := dir.OrderedChildren.Insert(name, subdird); err != nil {
+		subdird.DecRef(ctx)
+		return nil, err
+	}
+	return subdird, nil
+}
+
+// NewLink implements Inode.NewLink.
+func (dir *syntheticDirectory) NewLink(ctx context.Context, name string, target Inode) (*vfs.Dentry, error) {
+	return nil, syserror.EPERM
+}
+
+// NewSymlink implements Inode.NewSymlink.
+func (dir *syntheticDirectory) NewSymlink(ctx context.Context, name, target string) (*vfs.Dentry, error) {
+	return nil, syserror.EPERM
+}
+
+// NewNode implements Inode.NewNode.
+func (dir *syntheticDirectory) NewNode(ctx context.Context, name string, opts vfs.MknodOptions) (*vfs.Dentry, error) {
+	return nil, syserror.EPERM
+}