memfs fixes.

- Unexport Filesystem/Dentry/Inode.

- Support SEEK_CUR in directoryFD.Seek().

- Hold Filesystem.mu before touching directoryFD.off in
directoryFD.Seek().

- Remove deleted Dentries from their parent directory.childLists.

- Remove invalid FIXMEs.

PiperOrigin-RevId: 262400633

1 files changed, 32 insertions, 23 deletions

diff --git a/pkg/sentry/fsimpl/memfs/directory.go b/pkg/sentry/fsimpl/memfs/directory.go
index b0c3ea39a..c52dc781c 100644
--- a/pkg/sentry/fsimpl/memfs/directory.go
+++ b/pkg/sentry/fsimpl/memfs/directory.go
@@ -23,23 +23,23 @@ import (
 )
 
 type directory struct {
-	inode Inode
+	inode inode
 
 	// childList is a list containing (1) child Dentries and (2) fake Dentries
 	// (with inode == nil) that represent the iteration position of
 	// directoryFDs. childList is used to support directoryFD.IterDirents()
-	// efficiently. childList is protected by Filesystem.mu.
+	// efficiently. childList is protected by filesystem.mu.
 	childList dentryList
 }
 
-func (fs *Filesystem) newDirectory(creds *auth.Credentials, mode uint16) *Inode {
+func (fs *filesystem) newDirectory(creds *auth.Credentials, mode uint16) *inode {
 	dir := &directory{}
 	dir.inode.init(dir, fs, creds, mode)
 	dir.inode.nlink = 2 // from "." and parent directory or ".." for root
 	return &dir.inode
 }
 
-func (i *Inode) isDir() bool {
+func (i *inode) isDir() bool {
 	_, ok := i.impl.(*directory)
 	return ok
 }
@@ -48,8 +48,8 @@ type directoryFD struct {
 	fileDescription
 	vfs.DirectoryFileDescriptionDefaultImpl
 
-	// Protected by Filesystem.mu.
-	iter *Dentry
+	// Protected by filesystem.mu.
+	iter *dentry
 	off  int64
 }
 
@@ -68,7 +68,7 @@ func (fd *directoryFD) Release() {
 // IterDirents implements vfs.FileDescriptionImpl.IterDirents.
 func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
 	fs := fd.filesystem()
-	d := fd.vfsfd.VirtualDentry().Dentry()
+	vfsd := fd.vfsfd.VirtualDentry().Dentry()
 
 	fs.mu.Lock()
 	defer fs.mu.Unlock()
@@ -77,7 +77,7 @@ func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallba
 		if !cb.Handle(vfs.Dirent{
 			Name: ".",
 			Type: linux.DT_DIR,
-			Ino:  d.Impl().(*Dentry).inode.ino,
+			Ino:  vfsd.Impl().(*dentry).inode.ino,
 			Off:  0,
 		}) {
 			return nil
@@ -85,7 +85,7 @@ func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallba
 		fd.off++
 	}
 	if fd.off == 1 {
-		parentInode := d.ParentOrSelf().Impl().(*Dentry).inode
+		parentInode := vfsd.ParentOrSelf().Impl().(*dentry).inode
 		if !cb.Handle(vfs.Dirent{
 			Name: "..",
 			Type: parentInode.direntType(),
@@ -97,12 +97,12 @@ func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallba
 		fd.off++
 	}
 
-	dir := d.Impl().(*Dentry).inode.impl.(*directory)
-	var child *Dentry
+	dir := vfsd.Impl().(*dentry).inode.impl.(*directory)
+	var child *dentry
 	if fd.iter == nil {
 		// Start iteration at the beginning of dir.
 		child = dir.childList.Front()
-		fd.iter = &Dentry{}
+		fd.iter = &dentry{}
 	} else {
 		// Continue iteration from where we left off.
 		child = fd.iter.Next()
@@ -130,32 +130,41 @@ func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallba
 
 // Seek implements vfs.FileDescriptionImpl.Seek.
 func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
-	if whence != linux.SEEK_SET {
-		// TODO: Linux also allows SEEK_CUR.
+	fs := fd.filesystem()
+	fs.mu.Lock()
+	defer fs.mu.Unlock()
+
+	switch whence {
+	case linux.SEEK_SET:
+		// Use offset as given.
+	case linux.SEEK_CUR:
+		offset += fd.off
+	default:
 		return 0, syserror.EINVAL
 	}
 	if offset < 0 {
 		return 0, syserror.EINVAL
 	}
 
+	// If the offset isn't changing (e.g. due to lseek(0, SEEK_CUR)), don't
+	// seek even if doing so might reposition the iterator due to concurrent
+	// mutation of the directory. Compare fs/libfs.c:dcache_dir_lseek().
+	if fd.off == offset {
+		return offset, nil
+	}
+
 	fd.off = offset
 	// Compensate for "." and "..".
-	var remChildren int64
-	if offset < 2 {
-		remChildren = 0
-	} else {
+	remChildren := int64(0)
+	if offset >= 2 {
 		remChildren = offset - 2
 	}
 
-	fs := fd.filesystem()
 	dir := fd.inode().impl.(*directory)
 
-	fs.mu.Lock()
-	defer fs.mu.Unlock()
-
 	// Ensure that fd.iter exists and is not linked into dir.childList.
 	if fd.iter == nil {
-		fd.iter = &Dentry{}
+		fd.iter = &dentry{}
 	} else {
 		dir.childList.Remove(fd.iter)
 	}