Add basic plumbing for splice and stub implementation.

This does not actually implement an efficient splice or sendfile. Rather, it
adds a generic plumbing to the file internals so that this can be added. All
file implementations use the stub fileutil.NoSplice implementation, which
causes sendfile and splice to fall back to an internal copy.

A basic splice system call interface is added, along with a test.

PiperOrigin-RevId: 249335960
Change-Id: Ic5568be2af0a505c19e7aec66d5af2480ab0939b

2 files changed, 18 insertions, 1 deletions

diff --git a/pkg/sentry/fs/fsutil/file.go b/pkg/sentry/fs/fsutil/file.go
index e355d8594..9381963d0 100644
--- a/pkg/sentry/fs/fsutil/file.go
+++ b/pkg/sentry/fs/fsutil/file.go
@@ -223,6 +223,20 @@ func (FileNoIoctl) Ioctl(ctx context.Context, io usermem.IO, args arch.SyscallAr
 	return 0, syserror.ENOTTY
 }
 
+// FileNoSplice implements fs.FileOperations.ReadFrom and
+// fs.FileOperations.WriteTo for files that don't support splice.
+type FileNoSplice struct{}
+
+// WriteTo implements fs.FileOperations.WriteTo.
+func (FileNoSplice) WriteTo(context.Context, *fs.File, *fs.File, fs.SpliceOpts) (int64, error) {
+	return 0, syserror.ENOSYS
+}
+
+// ReadFrom implements fs.FileOperations.ReadFrom.
+func (FileNoSplice) ReadFrom(context.Context, *fs.File, *fs.File, fs.SpliceOpts) (int64, error) {
+	return 0, syserror.ENOSYS
+}
+
 // DirFileOperations implements most of fs.FileOperations for directories,
 // except for Readdir and UnstableAttr which the embedding type must implement.
 type DirFileOperations struct {
@@ -233,6 +247,7 @@ type DirFileOperations struct {
 	FileNoopFlush
 	FileNoopFsync
 	FileNoopRelease
+	FileNoSplice
 }
 
 // Read implements fs.FileOperations.Read
@@ -303,6 +318,7 @@ type NoReadWriteFile struct {
 	FileNoWrite              `state:"nosave"`
 	FileNotDirReaddir        `state:"nosave"`
 	FileUseInodeUnstableAttr `state:"nosave"`
+	FileNoSplice             `state:"nosave"`
 }
 
 var _ fs.FileOperations = (*NoReadWriteFile)(nil)
diff --git a/pkg/sentry/fs/fsutil/inode.go b/pkg/sentry/fs/fsutil/inode.go
index a22b6ce9c..925887335 100644
--- a/pkg/sentry/fs/fsutil/inode.go
+++ b/pkg/sentry/fs/fsutil/inode.go
@@ -250,16 +250,17 @@ func (i *InodeSimpleExtendedAttributes) Listxattr(_ *fs.Inode) (map[string]struc
 //
 // +stateify savable
 type staticFile struct {
-	waiter.AlwaysReady       `state:"nosave"`
 	FileGenericSeek          `state:"nosave"`
 	FileNoIoctl              `state:"nosave"`
 	FileNoMMap               `state:"nosave"`
+	FileNoSplice             `state:"nosave"`
 	FileNoopFsync            `state:"nosave"`
 	FileNoopFlush            `state:"nosave"`
 	FileNoopRelease          `state:"nosave"`
 	FileNoopWrite            `state:"nosave"`
 	FileNotDirReaddir        `state:"nosave"`
 	FileUseInodeUnstableAttr `state:"nosave"`
+	waiter.AlwaysReady       `state:"nosave"`
 
 	FileStaticContentReader
 }