Support multiple FUSE kernel versions of FUSE_INIT response struct

The fuse_init_out struct changes in different FUSE kernel versions. A
FUSE server may implement older versions of fuse_init_out, but they
share common attributes from the beginning. Implement variable-length
marshallable interface to support older versions of ABI.

Fixes #3707

2 files changed, 8 insertions, 3 deletions

diff --git a/pkg/sentry/fsimpl/fuse/connection.go b/pkg/sentry/fsimpl/fuse/connection.go
index f7d1a5c52..0e91bb18e 100644
--- a/pkg/sentry/fsimpl/fuse/connection.go
+++ b/pkg/sentry/fsimpl/fuse/connection.go
@@ -345,6 +345,11 @@ func (r *Response) Error() error {
 	return error(sysErrNo)
 }
 
+// DataLen returns the size of the response without the header.
+func (r *Response) DataLen() uint32 {
+	return r.hdr.Len - uint32(r.hdr.SizeBytes())
+}
+
 // UnmarshalPayload unmarshals the response data into m.
 func (r *Response) UnmarshalPayload(m marshal.Marshallable) error {
 	hdrLen := r.hdr.SizeBytes()
diff --git a/pkg/sentry/fsimpl/fuse/init.go b/pkg/sentry/fsimpl/fuse/init.go
index 2ff2542b6..6384cbbdb 100644
--- a/pkg/sentry/fsimpl/fuse/init.go
+++ b/pkg/sentry/fsimpl/fuse/init.go
@@ -76,12 +76,12 @@ func (conn *connection) InitRecv(res *Response, hasSysAdminCap bool) error {
 		return err
 	}
 
-	var out linux.FUSEInitOut
-	if err := res.UnmarshalPayload(&out); err != nil {
+	initRes := linux.FUSEInitRes{Len: res.DataLen()}
+	if err := res.UnmarshalPayload(&initRes); err != nil {
 		return err
 	}
 
-	return conn.initProcessReply(&out, hasSysAdminCap)
+	return conn.initProcessReply(&initRes.InitOut, hasSysAdminCap)
 }
 
 // Process the FUSE_INIT reply from the FUSE server.