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

pkg/abi/linux/fuse.go

@@ -15,6 +15,7 @@
 package linux
 
 import (
+	"gvisor.dev/gvisor/pkg/usermem"
 	"gvisor.dev/gvisor/tools/go_marshal/marshal"
 	"gvisor.dev/gvisor/tools/go_marshal/primitive"
 )
@@ -262,6 +263,63 @@ type FUSEInitOut struct {
 	_ [8]uint32
 }
 
+// FUSEInitRes is a variable-length wrapper of FUSEInitOut. The FUSE server may
+// implement older version of FUSE protocol, which contains a FUSEInitOut with
+// less attributes.
+//
+// Dynamically-sized objects cannot be marshalled.
+type FUSEInitRes struct {
+	marshal.StubMarshallable
+
+	// InitOut contains the response from the FUSE server.
+	InitOut FUSEInitOut
+
+	// Len is the total length of bytes of the response.
+	Len uint32
+}
+
+// UnMarshalBytes deserializes src to the InitOut attribute in a FUSEInitRes.
+func (r *FUSEInitRes) UnmarshalBytes(src []byte) {
+	out := &r.InitOut
+
+	// Introduced before FUSE kernel version 7.13.
+	out.Major = uint32(usermem.ByteOrder.Uint32(src[:4]))
+	src = src[4:]
+	out.Minor = uint32(usermem.ByteOrder.Uint32(src[:4]))
+	src = src[4:]
+	out.MaxReadahead = uint32(usermem.ByteOrder.Uint32(src[:4]))
+	src = src[4:]
+	out.Flags = uint32(usermem.ByteOrder.Uint32(src[:4]))
+	src = src[4:]
+	out.MaxBackground = uint16(usermem.ByteOrder.Uint16(src[:2]))
+	src = src[2:]
+	out.CongestionThreshold = uint16(usermem.ByteOrder.Uint16(src[:2]))
+	src = src[2:]
+	out.MaxWrite = uint32(usermem.ByteOrder.Uint32(src[:4]))
+	src = src[4:]
+
+	// Introduced in FUSE kernel version 7.23.
+	if len(src) >= 4 {
+		out.TimeGran = uint32(usermem.ByteOrder.Uint32(src[:4]))
+		src = src[4:]
+	}
+	// Introduced in FUSE kernel version 7.28.
+	if len(src) >= 2 {
+		out.MaxPages = uint16(usermem.ByteOrder.Uint16(src[:2]))
+		src = src[2:]
+	}
+	// Introduced in FUSE kernel version 7.31.
+	if len(src) >= 2 {
+		out.MapAlignment = uint16(usermem.ByteOrder.Uint16(src[:2]))
+		src = src[2:]
+	}
+}
+
+// SizeBytes is the size of the payload of the FUSE_INIT response.
+func (r *FUSEInitRes) SizeBytes() int {
+	return int(r.Len)
+}
+
 // FUSEGetAttrIn is the request sent by the kernel to the daemon,
 // to get the attribute of a inode.
 //

pkg/sentry/fsimpl/fuse/connection.go

@@ -346,6 +346,11 @@ 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()

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.