Complete vfs2 implementation of fallocate.

This change includes overlay, special regular gofer files, and hostfs.

Fixes #3589.

PiperOrigin-RevId: 332330860

2 files changed, 8 insertions, 1 deletions

diff --git a/pkg/sentry/vfs/file_description.go b/pkg/sentry/vfs/file_description.go
index 2b29a3c3f..73bb36d3e 100644
--- a/pkg/sentry/vfs/file_description.go
+++ b/pkg/sentry/vfs/file_description.go
@@ -326,6 +326,9 @@ type FileDescriptionImpl interface {
 	// Allocate grows the file to offset + length bytes.
 	// Only mode == 0 is supported currently.
 	//
+	// Allocate should return EISDIR on directories, ESPIPE on pipes, and ENODEV on
+	// other files where it is not supported.
+	//
 	// Preconditions: The FileDescription was opened for writing.
 	Allocate(ctx context.Context, mode, offset, length uint64) error
 
diff --git a/pkg/sentry/vfs/file_description_impl_util.go b/pkg/sentry/vfs/file_description_impl_util.go
index 68b80a951..78da16bac 100644
--- a/pkg/sentry/vfs/file_description_impl_util.go
+++ b/pkg/sentry/vfs/file_description_impl_util.go
@@ -57,7 +57,11 @@ func (FileDescriptionDefaultImpl) StatFS(ctx context.Context) (linux.Statfs, err
 }
 
 // Allocate implements FileDescriptionImpl.Allocate analogously to
-// fallocate called on regular file, directory or FIFO in Linux.
+// fallocate called on an invalid type of file in Linux.
+//
+// Note that directories can rely on this implementation even though they
+// should technically return EISDIR. Allocate should never be called for a
+// directory, because it requires a writable fd.
 func (FileDescriptionDefaultImpl) Allocate(ctx context.Context, mode, offset, length uint64) error {
 	return syserror.ENODEV
 }