Add fh support for revise attr and fstat(2) test

According to Linux 4.4's FUSE behavior, the flags and fh attributes in
FUSE_GETATTR are only used in read, write, and lseek. fstat(2) doesn't
use them either. Add tests to ensure the requests sent from FUSE module
are consistent with Linux's.

Updates #3655

3 files changed, 16 insertions, 12 deletions

diff --git a/pkg/abi/linux/fuse.go b/pkg/abi/linux/fuse.go
index e49a92fb2..f0bef1e8e 100644
--- a/pkg/abi/linux/fuse.go
+++ b/pkg/abi/linux/fuse.go
@@ -227,6 +227,11 @@ type FUSEInitOut struct {
 	_ [8]uint32
 }
 
+// FUSE_GETATTR_FH is currently the only flag of FUSEGetAttrIn.GetAttrFlags.
+// If it is set, the file handle (FUSEGetAttrIn.Fh) is used to indicate the
+// object instead of the node id attribute in the request header.
+const FUSE_GETATTR_FH = (1 << 0)
+
 // FUSEGetAttrIn is the request sent by the kernel to the daemon,
 // to get the attribute of a inode.
 //
diff --git a/pkg/sentry/fsimpl/fuse/fusefs.go b/pkg/sentry/fsimpl/fuse/fusefs.go
index 8cf13dcb6..821048d87 100644
--- a/pkg/sentry/fsimpl/fuse/fusefs.go
+++ b/pkg/sentry/fsimpl/fuse/fusefs.go
@@ -609,9 +609,9 @@ func statFromFUSEAttr(attr linux.FUSEAttr, mask, devMinor uint32) linux.Statx {
 }
 
 // getAttr gets the attribute of this inode by issuing a FUSE_GETATTR request
-// or read from local cache.
-// It updates the corresponding attributes if necessary.
-func (i *inode) getAttr(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOptions) (linux.FUSEAttr, error) {
+// or read from local cache. It updates the corresponding attributes if
+// necessary.
+func (i *inode) getAttr(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOptions, flags uint32, fh uint64) (linux.FUSEAttr, error) {
 	attributeVersion := atomic.LoadUint64(&i.fs.conn.attributeVersion)
 
 	// TODO(gvisor.dev/issue/3679): send the request only if
@@ -631,11 +631,10 @@ func (i *inode) getAttr(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOp
 
 	creds := auth.CredentialsFromContext(ctx)
 
-	var in linux.FUSEGetAttrIn
-	// We don't set any attribute in the request, because in VFS2 fstat(2) will
-	// finally be translated into vfs.FilesystemImpl.StatAt() (see
-	// pkg/sentry/syscalls/linux/vfs2/stat.go), resulting in the same flow
-	// as stat(2). Thus GetAttrFlags and Fh variable will never be used in VFS2.
+	in := linux.FUSEGetAttrIn{
+		GetAttrFlags: flags,
+		Fh:           fh,
+	}
 	req, err := i.fs.conn.NewRequest(creds, uint32(task.ThreadID()), i.NodeID, linux.FUSE_GETATTR, &in)
 	if err != nil {
 		return linux.FUSEAttr{}, err
@@ -676,17 +675,17 @@ func (i *inode) getAttr(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOp
 // reviseAttr attempts to update the attributes for internal purposes
 // by calling getAttr with a pre-specified mask.
 // Used by read, write, lseek.
-func (i *inode) reviseAttr(ctx context.Context) error {
+func (i *inode) reviseAttr(ctx context.Context, flags uint32, fh uint64) error {
 	// Never need atime for internal purposes.
 	_, err := i.getAttr(ctx, i.fs.VFSFilesystem(), vfs.StatOptions{
 		Mask: linux.STATX_BASIC_STATS &^ linux.STATX_ATIME,
-	})
+	}, flags, fh)
 	return err
 }
 
 // Stat implements kernfs.Inode.Stat.
 func (i *inode) Stat(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOptions) (linux.Statx, error) {
-	attr, err := i.getAttr(ctx, fs, opts)
+	attr, err := i.getAttr(ctx, fs, opts, 0, 0)
 	if err != nil {
 		return linux.Statx{}, err
 	}
diff --git a/pkg/sentry/fsimpl/fuse/regular_file.go b/pkg/sentry/fsimpl/fuse/regular_file.go
index 193e77392..5bdd096c3 100644
--- a/pkg/sentry/fsimpl/fuse/regular_file.go
+++ b/pkg/sentry/fsimpl/fuse/regular_file.go
@@ -65,7 +65,7 @@ func (fd *regularFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offs
 
 	// Reading beyond EOF, update file size if outdated.
 	if uint64(offset+size) > atomic.LoadUint64(&inode.size) {
-		if err := inode.reviseAttr(ctx); err != nil {
+		if err := inode.reviseAttr(ctx, linux.FUSE_GETATTR_FH, fd.Fh); err != nil {
 			return 0, err
 		}
 		// If the offset after update is still too large, return error.