Fix lock-ordering violation in Create by logging BaseName instead of FullName.

Dirent.FullName takes the global renameMu, but can be called during Create,
which itself takes dirent.mu and dirent.dirMu, which is a lock-order violation:

Dirent.Create
  d.dirMu.Lock
  d.mu.Lock
  Inode.Create
    gofer.inodeOperations.Create
      gofer.NewFile
        Dirent.FullName
          d.renameMu.RLock

We only use the FullName here for logging, and in this case we can get by with
logging only the BaseName.

A `BaseName` method was added to Dirent, which simply returns the name, taking
d.parent.mu as required.

In the Create pathway, we can't call d.BaseName() because taking d.parent.mu
after d.mu violates the lock order. But we already know the base name of the
file we just created, so that's OK.

In the Open/GetFile pathway, we are free to call d.BaseName() because the other
dirent locks are not held.

PiperOrigin-RevId: 205112278
Change-Id: Ib45c734081aecc9b225249a65fa8093eb4995f10

pkg/sentry/fs/dirent.go

@@ -334,6 +334,17 @@ func (d *Dirent) SyncAll(ctx context.Context) {
 	}
 }
 
+// BaseName returns the base name of the dirent.
+func (d *Dirent) BaseName() string {
+	p := d.parent
+	if p == nil {
+		return d.name
+	}
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	return d.name
+}
+
 // FullName returns the fully-qualified name and a boolean value representing
 // whether this Dirent was a descendant of root.
 // If the root argument is nil it is assumed to be the root of the Dirent tree.

pkg/sentry/fs/gofer/file.go

@@ -57,7 +57,14 @@ type fileOperations struct {
 var _ fs.FileOperations = (*fileOperations)(nil)
 
 // NewFile returns a file. NewFile is not appropriate with host pipes and sockets.
-func NewFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags, i *inodeOperations, handles *handles) *fs.File {
+//
+// The `name` argument is only used to log a warning if we are returning a
+// writeable+executable file. (A metric counter is incremented in this case as
+// well.) Note that we cannot call d.BaseName() directly in this function,
+// because that would lead to a lock order violation, since this is called in
+// d.Create which holds d.mu, while d.BaseName() takes d.parent.mu, and the two
+// locks must be taken in the opposite order.
+func NewFile(ctx context.Context, dirent *fs.Dirent, name string, flags fs.FileFlags, i *inodeOperations, handles *handles) *fs.File {
 	// Remote file systems enforce readability/writability at an offset,
 	// see fs/9p/vfs_inode.c:v9fs_vfs_atomic_open -> fs/open.c:finish_open.
 	flags.Pread = true
@@ -70,7 +77,6 @@ func NewFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags, i *inod
 	}
 	if flags.Write {
 		if err := dirent.Inode.CheckPermission(ctx, fs.PermMask{Execute: true}); err == nil {
-			name, _ := dirent.FullName(fs.RootFromContext(ctx))
 			openedWX.Increment()
 			log.Warningf("Opened a writable executable: %q", name)
 		}

pkg/sentry/fs/gofer/gofer_test.go

@@ -545,6 +545,7 @@ func TestPreadv(t *testing.T) {
 		f := NewFile(
 			ctx,
 			fs.NewDirent(rootInode, ""),
+			"",
 			fs.FileFlags{Read: true},
 			rootInode.InodeOperations.(*inodeOperations),
 			&handles{File: contextFile{file: openFile}},
@@ -751,6 +752,7 @@ func TestPwritev(t *testing.T) {
 		f := NewFile(
 			ctx,
 			fs.NewDirent(rootInode, ""),
+			"",
 			fs.FileFlags{Write: true},
 			rootInode.InodeOperations.(*inodeOperations),
 			&handles{File: contextFile{file: openFile}},

pkg/sentry/fs/gofer/inode.go

@@ -391,7 +391,7 @@ func (i *inodeOperations) getFilePipe(ctx context.Context, d *fs.Dirent, flags f
 	if err != nil {
 		return nil, err
 	}
-	return NewFile(ctx, d, flags, i, h), nil
+	return NewFile(ctx, d, d.BaseName(), flags, i, h), nil
 }
 
 // errNotHostFile indicates that the file is not a host file.
@@ -430,7 +430,7 @@ func (i *inodeOperations) getFileDefault(ctx context.Context, d *fs.Dirent, flag
 		if err != nil {
 			return nil, err
 		}
-		return NewFile(ctx, d, flags, i, h), nil
+		return NewFile(ctx, d, d.BaseName(), flags, i, h), nil
 	}
 
 	h, ok := i.fileState.getCachedHandles(ctx, flags, d.Inode.MountSource)
@@ -443,7 +443,7 @@ func (i *inodeOperations) getFileDefault(ctx context.Context, d *fs.Dirent, flag
 	}
 	i.fileState.setHandlesForCachedIO(flags, h)
 
-	return NewFile(ctx, d, flags, i, h), nil
+	return NewFile(ctx, d, d.BaseName(), flags, i, h), nil
 }
 
 // SetPermissions implements fs.InodeOperations.SetPermissions.

pkg/sentry/fs/gofer/path.go

@@ -127,7 +127,7 @@ func (i *inodeOperations) Create(ctx context.Context, dir *fs.Inode, name string
 	if iops.session().cachePolicy.usePageCache(d.Inode) {
 		iops.fileState.setHandlesForCachedIO(flags, h)
 	}
-	return NewFile(ctx, d, flags, iops, h), nil
+	return NewFile(ctx, d, name, flags, iops, h), nil
 }
 
 // CreateLink uses Create to create a symlink between oldname and newname.