问题
I've been playing around with ABC bytecode and was hoping someone could clear up a point of confusion for me. I have a simple flash file that places a clip on the stage and has a tiny script to update its position on each frame. The code looks something like:
package
{
import flash.display.MovieClip;
import flash.events.Event;
public class RedCircle extends MovieClip
{
public function RedCircle()
{
this.addEventListener(Event.ENTER_FRAME, moveit);
}
function moveit(e:Event)
{
this.x -=1;
}
}
}
Which compiles to something like:
protected package protected RedCircle
{
class RedCircle extends flash.display.MovieClip
{
static () : Void
{
getlocal_0();
pushscope();
returnvoid();
}
RedCircle () : Void
{
getlocal_0();
pushscope();
getlocal_0();
constructsuper(0);
getlocal_0();
getlex(flash.events.Event);
getproperty(ENTER_FRAME);
getlex(internal .moveit); // ###1
callpropvoid(addEventListener, 2);
returnvoid();
}
function (anonymous) (flash.events.Event param1) : Void // ###2
{
getlocal_0();
pushscope();
getlocal_0();
getlocal_0();
getproperty(x);
decrement();
setproperty(x);
returnvoid();
}
}
}
My question is how does the 'getlex' operation work (I've marked it with ###1). It is passed a multiname which references the 'moveit' method of the class. Unfortunately, the 'name' field in method info seems never ever to get used by the compiler. All methods have the empty string as its name (Shown above as the unnamed function at ###2).
How does the flash player link the multiname to the unnamed method? There appears to be no provision for this in the AVM2 spec.
I know it's possible because commercial decompilers like sothink manage to determine the method name. I'm just not sure how they do it, or how the code could ever work.
回答1:
I'm not sure why your decompiler shows the method as (anonymous).
Here is a dump of the abcData:
abcFile{
minor_version (17): 16
major_version (19): 46
constant_pool{
int_count (21): 0
[0]: zero (not included in abcFile)
uint_count (22): 0
[0]: zero (not included in abcFile)
double_count (23): 0
[0]: NaN (not included in abcFile)
string_count (24): 17
string_info[0]{
name: * (not included in abcFile)
}
string_info[1]{
size (25): 12
name (26): "flash.events"
}
string_info[2]{
size (38): 5
name (39): "Event"
}
string_info[3]{
size (44): 0
name (45): ""
}
string_info[4]{
size (45): 9
name (46): "RedCircle"
}
string_info[5]{
size (55): 13
name (56): "flash.display"
}
string_info[6]{
size (69): 9
name (70): "MovieClip"
}
string_info[7]{
size (79): 6
name (80): "moveit"
}
string_info[8]{
size (86): 11
name (87): "ENTER_FRAME"
}
string_info[9]{
size (98): 16
name (99): "addEventListener"
}
string_info[10]{
size (115): 1
name (116): "x"
}
string_info[11]{
size (117): 6
name (118): "Object"
}
string_info[12]{
size (124): 15
name (125): "EventDispatcher"
}
string_info[13]{
size (140): 13
name (141): "DisplayObject"
}
string_info[14]{
size (154): 17
name (155): "InteractiveObject"
}
string_info[15]{
size (172): 22
name (173): "DisplayObjectContainer"
}
string_info[16]{
size (195): 6
name (196): "Sprite"
}
namespace_count (202): 6
namespace_info[0]{
kind: * (not included in abcFile)
}
namespace_info[1]{
kind (203): CONSTANT_PackageNamespace
name (204): 1
}
namespace_info[2]{
kind (205): CONSTANT_PackageNamespace
name (206): 3
}
namespace_info[3]{
kind (207): CONSTANT_PackageNamespace
name (208): 5
}
namespace_info[4]{
kind (209): CONSTANT_ProtectedNamespace
name (210): 4
}
namespace_info[5]{
kind (211): CONSTANT_PackageInternalNs
name (212): 3
}
ns_set_count (213): 0
ns_set_info[0]{
ns: 0 (not included in abcFile)
}
multiname_count (214): 14
multiname_info[0]{
kind: 0 (not included in abcFile)
}
multiname_info[1]{
kind (216): CONSTANT_QName
multiname_kind_QNAME{
ns (216): 1
name (217): 2 ("Event")
}
}
multiname_info[2]{
kind (219): CONSTANT_QName
multiname_kind_QNAME{
ns (219): 2
name (220): 4 ("RedCircle")
}
}
multiname_info[3]{
kind (222): CONSTANT_QName
multiname_kind_QNAME{
ns (222): 3
name (223): 6 ("MovieClip")
}
}
multiname_info[4]{
kind (225): CONSTANT_QName
multiname_kind_QNAME{
ns (225): 5
name (226): 7 ("moveit")
}
}
multiname_info[5]{
kind (228): CONSTANT_QName
multiname_kind_QNAME{
ns (228): 2
name (229): 8 ("ENTER_FRAME")
}
}
multiname_info[6]{
kind (231): CONSTANT_QName
multiname_kind_QNAME{
ns (231): 2
name (232): 9 ("addEventListener")
}
}
multiname_info[7]{
kind (234): CONSTANT_QName
multiname_kind_QNAME{
ns (234): 2
name (235): 10 ("x")
}
}
multiname_info[8]{
kind (237): CONSTANT_QName
multiname_kind_QNAME{
ns (237): 2
name (238): 11 ("Object")
}
}
multiname_info[9]{
kind (240): CONSTANT_QName
multiname_kind_QNAME{
ns (240): 1
name (241): 12 ("EventDispatcher")
}
}
multiname_info[10]{
kind (243): CONSTANT_QName
multiname_kind_QNAME{
ns (243): 3
name (244): 13 ("DisplayObject")
}
}
multiname_info[11]{
kind (246): CONSTANT_QName
multiname_kind_QNAME{
ns (246): 3
name (247): 14 ("InteractiveObject")
}
}
multiname_info[12]{
kind (249): CONSTANT_QName
multiname_kind_QNAME{
ns (249): 3
name (250): 15 ("DisplayObjectContainer")
}
}
multiname_info[13]{
kind (252): CONSTANT_QName
multiname_kind_QNAME{
ns (252): 3
name (253): 16 ("Sprite")
}
}
}
method_count (254): 4
method_info[0]{
param_count (255): 0
return_type (256): 0
name (257): 0
flags (258): 0
NEED_ARGUMENTS (0x01): false
NEED_ACTIVATION (0x02): false
NEED_REST (0x04): false
HAS_OPTIONAL (0x08): false
SET_DXNS (0x40): false
HAS_PARAM_NAMES (0x80): false
}
method_info[1]{
param_count (259): 0
return_type (260): 0
name (261): 0
flags (262): 0
NEED_ARGUMENTS (0x01): false
NEED_ACTIVATION (0x02): false
NEED_REST (0x04): false
HAS_OPTIONAL (0x08): false
SET_DXNS (0x40): false
HAS_PARAM_NAMES (0x80): false
}
method_info[2]{
param_count (263): 1
return_type (264): 0
param_type[0] (265): 1
name (266): 0
flags (267): 0
NEED_ARGUMENTS (0x01): false
NEED_ACTIVATION (0x02): false
NEED_REST (0x04): false
HAS_OPTIONAL (0x08): false
SET_DXNS (0x40): false
HAS_PARAM_NAMES (0x80): false
}
method_info[3]{
param_count (268): 0
return_type (269): 0
name (270): 0
flags (271): 0
NEED_ARGUMENTS (0x01): false
NEED_ACTIVATION (0x02): false
NEED_REST (0x04): false
HAS_OPTIONAL (0x08): false
SET_DXNS (0x40): false
HAS_PARAM_NAMES (0x80): false
}
metadata_count (272): 0
class_count (273): 1
instance_info[0]{
name (274): 2 (RedCircle)
super_name (275): 3 (MovieClip)
flags (276): 9
CONSTANT_ClassSealed (0x01): true
CONSTANT_ClassFinal (0x02): false
CONSTANT_ClassInterface (0x04): false
CONSTANT_ClassProtectedNs (0x08): true
protectedNs (277): 4
intrf_count (278): 0
iinit (279): 1
trait_count (280): 1
traits_info[0]{
name (281): 4 (moveit)
kind (282): Trait_Method
ATTR_Final (0x1): false
ATTR_Override (0x2): false
ATTR_Metadata (0x4): false
trait_method{
disp_id (283): 0
method (284): 2
}
}
}
class_info[0]{
cinit (285): 0
trait_count (286): 0
}
script_count (287): 1
init (288): 3
trait_count (289): 1
traits_info[0]{
name (290): 2 (RedCircle)
kind (291): Trait_Class
ATTR_Metadata (0x4): false
trait_class{
slot_id (292): 1
classi (293): 0
}
}
method_body_count (294): 4
method_body_info[0]{
method (295): 0
max_stack (296): 1
local_count (297): 1
init_scope_depth (298): 9
max_scope_depth (299): 10
code_length (300): 3
208 0xD0 (301) getlocal_0
48 0x30 (302) pushscope
71 0x47 (303) returnvoid
exception_count (304): 0
trait_count (305): 0
}
method_body_info[1]{
method (306): 1
max_stack (307): 3
local_count (308): 1
init_scope_depth (309): 10
max_scope_depth (310): 11
code_length (311): 17
208 0xD0 (312) getlocal_0
48 0x30 (313) pushscope
208 0xD0 (314) getlocal_0
73 0x49 (315) constructsuper
arg_count: 0
208 0xD0 (317) getlocal_0
96 0x60 (318) getlex
index: 1 (Event)
102 0x66 (320) getproperty
index: 5 (ENTER_FRAME)
208 0xD0 (322) getlocal_0
102 0x66 (323) getproperty
index: 4 (moveit)
79 0x4F (325) callpropvoid
index: 6 (addEventListener)
arg_count: 2
71 0x47 (328) returnvoid
exception_count (329): 0
trait_count (330): 0
}
method_body_info[2]{
method (331): 2
max_stack (332): 3
local_count (333): 2
init_scope_depth (334): 10
max_scope_depth (335): 11
code_length (336): 10
208 0xD0 (337) getlocal_0
48 0x30 (338) pushscope
208 0xD0 (339) getlocal_0
208 0xD0 (340) getlocal_0
102 0x66 (341) getproperty
index: 7
147 0x93 (343) decrement
97 0x61 (344) setproperty
index: 7
71 0x47 (346) returnvoid
exception_count (347): 0
trait_count (348): 0
}
method_body_info[3]{
method (349): 3
max_stack (350): 2
local_count (351): 1
init_scope_depth (352): 1
max_scope_depth (353): 9
code_length (354): 39
208 0xD0 (355) getlocal_0
48 0x30 (356) pushscope
101 0x65 (357) getscopeobject
index: 0
96 0x60 (359) getlex
index: 8
48 0x30 (361) pushscope
96 0x60 (362) getlex
index: 9
48 0x30 (364) pushscope
96 0x60 (365) getlex
index: 10
48 0x30 (367) pushscope
96 0x60 (368) getlex
index: 11
48 0x30 (370) pushscope
96 0x60 (371) getlex
index: 12
48 0x30 (373) pushscope
96 0x60 (374) getlex
index: 13
48 0x30 (376) pushscope
96 0x60 (377) getlex
index: 3
48 0x30 (379) pushscope
96 0x60 (380) getlex
index: 3
88 0x58 (382) newclass
index: 0
29 0x1D (384) popscope
29 0x1D (385) popscope
29 0x1D (386) popscope
29 0x1D (387) popscope
29 0x1D (388) popscope
29 0x1D (389) popscope
29 0x1D (390) popscope
104 0x68 (391) initproperty
index: 2
71 0x47 (393) returnvoid
exception_count (394): 0
trait_count (395): 0
}
}
What you're interested in here is instance_info[0]. This is the definition of a run-time instance of a class, which would be RedCircle here. Instances have an array of Traits of various types. RedCircle has one trait (moveit) of kind Trait_Method which means the trait has a reference to a method (2).
So if you skip to method_body_info[1] (the constructor of RedCircle) you can see at byte 323 that getProperty is called with an index of 4.
102 0x66 (323) getproperty
index: 4 (moveit)
Which is a reference to the multiname constant pool.
multiname_info[4]{
kind (225): CONSTANT_QName
multiname_kind_QNAME{
ns (225): 5
name (226): 7 ("moveit")
}
}
When it comes to calling the method it looks up the name index in the traits for the instance.
traits_info[0]{
name (281): 4 (moveit)
kind (282): Trait_Method
ATTR_Final (0x1): false
ATTR_Override (0x2): false
ATTR_Metadata (0x4): false
trait_method{
disp_id (283): 0
method (284): 2
}
}
Then calls the relevant method.
method_info[2]{
param_count (263): 1
return_type (264): 0
param_type[0] (265): 1
name (266): 0
flags (267): 0
NEED_ARGUMENTS (0x01): false
NEED_ACTIVATION (0x02): false
NEED_REST (0x04): false
HAS_OPTIONAL (0x08): false
SET_DXNS (0x40): false
HAS_PARAM_NAMES (0x80): false
}
method_body_info[2]{
method (331): 2
max_stack (332): 3
local_count (333): 2
init_scope_depth (334): 10
max_scope_depth (335): 11
code_length (336): 10
208 0xD0 (337) getlocal_0
48 0x30 (338) pushscope
208 0xD0 (339) getlocal_0
208 0xD0 (340) getlocal_0
102 0x66 (341) getproperty
index: 7 (x)
147 0x93 (343) decrement
97 0x61 (344) setproperty
index: 7 (x)
71 0x47 (346) returnvoid
exception_count (347): 0
trait_count (348): 0
}
A somewhat simplified answer, but I hope it clears up a few issues.
回答2:
i'm exploring as3swf and as3abc libs exactly at this current moment) and getlex is mentioned twice in as3abc lib:
package com.codeazur.as3abc.factories
public function create(code:int):AbstractOperation
switch (code) {//in real life this switch block is really huge
case Opcodes.GetLex: return new MultinameOperation(code);
}
and:
package com.codeazur.as3abc.data.bytecode
public class Opcodes
public static const GetLex:uint = 0x60;
_opNames[ GetLex ] = "GetLex";
so as i understand it's just a keyword inside AVM2. and more closely about your question: i think here:
getlex(flash.events.Event);
getproperty(ENTER_FRAME);
getlex(internal .moveit);
callpropvoid(addEventListener, 2);
we can see a kind of adding an event listener (or maybe i'm just crazy) and one more thing to mention: your former moveit function is the only one that accepts Event as a parameter so it's not hard to call exactly it. And by the way: how did you get inside AVM2? maybe the function is so internal that it's name is visible only if you are an entity in this class ;)?
and here's a link to some avm2 bytecode from adobe
回答3:
Turns out that the method names are multinames stored in the class traits; a problem compounded by the fact that I was looking for the trait names in the string table, not the multiname table. Oops.
Seems though, that the method name field is redundant in the ABC file.
来源：https://stackoverflow.com/questions/4089291/how-are-methods-found-in-avm2-bytecode