Annotated x86 Disassembly
The following section will walk you through a disassembly example.
Source Code
The following is the code for the function that will be analyzed.
HRESULT CUserView::CloseView(void)
{
if (m_fDestroyed) return S_OK;
BOOL fViewObjectChanged = FALSE;
ReleaseAndNull(&m_pdtgt);
if (m_psv) {
m_psb->EnableModelessSB(FALSE);
if(m_pws) m_pws->ViewReleased();
IShellView* psv;
HWND hwndCapture = GetCapture();
if (hwndCapture && hwndCapture == m_hwnd) {
SendMessage(m_hwnd, WM_CANCELMODE, 0, 0);
}
m_fHandsOff = TRUE;
m_fRecursing = TRUE;
NotifyClients(m_psv, NOTIFY_CLOSING);
m_fRecursing = FALSE;
m_psv->UIActivate(SVUIA_DEACTIVATE);
psv = m_psv;
m_psv = NULL;
ReleaseAndNull(&_pctView);
if (m_pvo) {
IAdviseSink *pSink;
if (SUCCEEDED(m_pvo->GetAdvise(NULL, NULL, &pSink)) && pSink) {
if (pSink == (IAdviseSink *)this)
m_pvo->SetAdvise(0, 0, NULL);
pSink->Release();
}
fViewObjectChanged = TRUE;
ReleaseAndNull(&m_pvo);
}
if (psv) {
psv->SaveViewState();
psv->DestroyViewWindow();
psv->Release();
}
m_hwndView = NULL;
m_fHandsOff = FALSE;
if (m_pcache) {
GlobalFree(m_pcache);
m_pcache = NULL;
}
m_psb->EnableModelessSB(TRUE);
CancelPendingActions();
}
ReleaseAndNull(&_psf);
if (fViewObjectChanged)
NotifyViewClients(DVASPECT_CONTENT, -1);
if (m_pszTitle) {
LocalFree(m_pszTitle);
m_pszTitle = NULL;
}
SetRect(&m_rcBounds, 0, 0, 0, 0);
return S_OK;
}
Assembly Code
This section contains the annotated disassembly example.
Functions which use the ebp register as a frame pointer start out as follows:
HRESULT CUserView::CloseView(void)
SAMPLE!CUserView__CloseView:
71517134 55 push ebp
71517135 8bec mov ebp,esp
This sets up the frame so the function can access its parameters as positive offsets from ebp, and local variables as negative offsets.
This is a method on a private COM interface, so the calling convention is __stdcall. This means that parameters are pushed right to left (in this case, there are none), the "this" pointer is pushed, and then the function is called. Thus, upon entry into the function, the stack looks like this:
[esp+0] = return address
[esp+4] = this
After the two preceding instructions, the parameters are accessible as:
[ebp+0] = previous ebp pushed on stack
[ebp+4] = return address
[ebp+8] = this
For a function that uses ebp as a frame pointer, the first pushed parameter is accessible at [ebp+8]; subsequent parameters are accessible at consecutive higher DWORD addresses.
71517137 51 push ecx
71517138 51 push ecx
This function requires only two local stack variables, so a sub esp, 8 instruction. The pushed values are then available as [ebp-4] and [ebp-8].
For a function that uses ebp as a frame pointer, stack local variables are accessible at negative offsets from the ebp register.
71517139 56 push esi
Now the compiler saves the registers that are required to be preserved across function calls. Actually, it saves them in bits and pieces, interleaved with the first line of actual code.
7151713a 8b7508 mov esi,[ebp+0x8] ; esi = this
7151713d 57 push edi ; save another registers
It so happens that CloseView is a method on ViewState, which is at offset 12 in the underlying object. Consequently, this is a pointer to a ViewState class, although when there is possible confusion with another base class, it will be more carefully specified as (ViewState*)this.
if (m_fDestroyed)
7151713e 33ff xor edi,edi ; edi = 0
XORing a register with itself is a standard way of zeroing it out.
71517140 39beac000000 cmp [esi+0xac],edi ; this->m_fDestroyed == 0?
71517146 7407 jz NotDestroyed (7151714f) ; jump if equal
The cmp instruction compares two values (by subtracting them). The jz instruction checks if the result is zero, indicating that the two compared values are equal.
The cmp instruction compares two values; a subsequent j instruction jumps based on the result of the comparison.
return S_OK;
71517148 33c0 xor eax,eax ; eax = 0 = S_OK
7151714a e972010000 jmp ReturnNoEBX (715172c1) ; return, do not pop EBX
The compiler delayed saving the EBX register until later in the function, so if the program is going to "early-out" on this test, then the exit path needs to be the one that does not restore EBX.
BOOL fViewObjectChanged = FALSE;
ReleaseAndNull(&m_pdtgt);
The execution of these two lines of code is interleaved, so pay attention.
NotDestroyed:
7151714f 8d86c0000000 lea eax,[esi+0xc0] ; eax = &m_pdtgt
The lea instruction computes the effect address of a memory access and stores it in the destination. The actual memory address is not dereferenced.
The lea instruction takes the address of a variable.
71517155 53 push ebx
You should save that EBX register before it is damaged.
71517156 8b1d10195071 mov ebx,[_imp__ReleaseAndNull]
Because you will be calling ReleaseAndNull frequently, it is a good idea to cache its address in EBX.
7151715c 50 push eax ; parameter to ReleaseAndNull
7151715d 897dfc mov [ebp-0x4],edi ; fViewObjectChanged = FALSE
71517160 ffd3 call ebx ; call ReleaseAndNull
if (m_psv) {
71517162 397e74 cmp [esi+0x74],edi ; this->m_psv == 0?
71517165 0f8411010000 je No_Psv (7151727c) ; jump if zero
Remember that you zeroed out the EDI register a while back and that EDI is a register preserved across function calls (so the call to ReleaseAndNull did not change it). Therefore, it still holds the value zero and you can use it to quickly test for zero.
m_psb->EnableModelessSB(FALSE);
7151716b 8b4638 mov eax,[esi+0x38] ; eax = this->m_psb
7151716e 57 push edi ; FALSE
7151716f 50 push eax ; "this" for callee
71517170 8b08 mov ecx,[eax] ; ecx = m_psb->lpVtbl
71517172 ff5124 call [ecx+0x24] ; __stdcall EnableModelessSB
The above pattern is a telltale sign of a COM method call.
COM method calls are pretty popular, so it is a good idea to learn to recognize them. In particular, you should be able to recognize the three IUnknown methods directly from their Vtable offsets: QueryInterface=0, AddRef=4, and Release=8.
if(m_pws) m_pws->ViewReleased();
71517175 8b8614010000 mov eax,[esi+0x114] ; eax = this->m_pws
7151717b 3bc7 cmp eax,edi ; eax == 0?
7151717d 7406 jz NoWS (71517185) ; if so, then jump
7151717f 8b08 mov ecx,[eax] ; ecx = m_pws->lpVtbl
71517181 50 push eax ; "this" for callee
71517182 ff510c call [ecx+0xc] ; __stdcall ViewReleased
NoWS:
HWND hwndCapture = GetCapture();
71517185 ff15e01a5071 call [_imp__GetCapture] ; call GetCapture
Indirect calls through globals is how function imports are implemented in Microsoft Win32. The loader fixes up the globals to point to the actual address of the target. This is a handy way to get your bearings when you are investigating a crashed machine. Look for the calls to imported functions and in the target. You will usually have the name of some imported function, which you can use to determine where you are in the source code.
if (hwndCapture && hwndCapture == m_hwnd) {
SendMessage(m_hwnd, WM_CANCELMODE, 0, 0);
}
7151718b 3bc7 cmp eax,edi ; hwndCapture == 0?
7151718d 7412 jz No_Capture (715171a1) ; jump if zero
The function return value is placed in the EAX register.
7151718f 8b4e44 mov ecx,[esi+0x44] ; ecx = this->m_hwnd
71517192 3bc1 cmp eax,ecx ; hwndCapture = ecx?
71517194 750b jnz No_Capture (715171a1) ; jump if not
71517196 57 push edi ; 0
71517197 57 push edi ; 0
71517198 6a1f push 0x1f ; WM_CANCELMODE
7151719a 51 push ecx ; hwndCapture
7151719b ff1518195071 call [_imp__SendMessageW] ; SendMessage
No_Capture:
m_fHandsOff = TRUE;
m_fRecursing = TRUE;
715171a1 66818e0c0100000180 or word ptr [esi+0x10c],0x8001 ; set both flags at once
NotifyClients(m_psv, NOTIFY_CLOSING);
715171aa 8b4e20 mov ecx,[esi+0x20] ; ecx = (CNotifySource*)this.vtbl
715171ad 6a04 push 0x4 ; NOTIFY_CLOSING
715171af 8d4620 lea eax,[esi+0x20] ; eax = (CNotifySource*)this
715171b2 ff7674 push [esi+0x74] ; m_psv
715171b5 50 push eax ; "this" for callee
715171b6 ff510c call [ecx+0xc] ; __stdcall NotifyClients
Notice how you had to change your "this" pointer when calling a method on a different base class from your own.
m_fRecursing = FALSE;
715171b9 80a60d0100007f and byte ptr [esi+0x10d],0x7f
m_psv->UIActivate(SVUIA_DEACTIVATE);
715171c0 8b4674 mov eax,[esi+0x74] ; eax = m_psv
715171c3 57 push edi ; SVUIA_DEACTIVATE = 0
715171c4 50 push eax ; "this" for callee
715171c5 8b08 mov ecx,[eax] ; ecx = vtbl
715171c7 ff511c call [ecx+0x1c] ; __stdcall UIActivate
psv = m_psv;
m_psv = NULL;
715171ca 8b4674 mov eax,[esi+0x74] ; eax = m_psv
715171cd 897e74 mov [esi+0x74],edi ; m_psv = NULL
715171d0 8945f8 mov [ebp-0x8],eax ; psv = eax
The first local variable is psv.
ReleaseAndNull(&_pctView);
715171d3 8d466c lea eax,[esi+0x6c] ; eax = &_pctView
715171d6 50 push eax ; parameter
715171d7 ffd3 call ebx ; call ReleaseAndNull
if (m_pvo) {
715171d9 8b86a8000000 mov eax,[esi+0xa8] ; eax = m_pvo
715171df 8dbea8000000 lea edi,[esi+0xa8] ; edi = &m_pvo
715171e5 85c0 test eax,eax ; eax == 0?
715171e7 7448 jz No_Pvo (71517231) ; jump if zero
Note that the compiler speculatively prepared the address of the m_pvo member, because you are going to use it frequently for a while. Thus, having the address handy will result in smaller code.
if (SUCCEEDED(m_pvo->GetAdvise(NULL, NULL, &pSink)) && pSink) {
715171e9 8b08 mov ecx,[eax] ; ecx = m_pvo->lpVtbl
715171eb 8d5508 lea edx,[ebp+0x8] ; edx = &pSink
715171ee 52 push edx ; parameter
715171ef 6a00 push 0x0 ; NULL
715171f1 6a00 push 0x0 ; NULL
715171f3 50 push eax ; "this" for callee
715171f4 ff5120 call [ecx+0x20] ; __stdcall GetAdvise
715171f7 85c0 test eax,eax ; test bits of eax
715171f9 7c2c jl No_Advise (71517227) ; jump if less than zero
715171fb 33c9 xor ecx,ecx ; ecx = 0
715171fd 394d08 cmp [ebp+0x8],ecx ; _pSink == ecx?
71517200 7425 jz No_Advise (71517227)
Notice that the compiler concluded that the incoming "this" parameter was not required (because it long ago stashed that into the ESI register). Thus, it reused the memory as the local variable pSink.
If the function uses an EBP frame, then incoming parameters arrive at positive offsets from EBP and local variables are placed at negative offsets. But, as in this case, the compiler is free to reuse that memory for any purpose.
If you are paying close attention, you will see that the compiler could have optimized this code a little better. It could have delayed the lea edi, [esi+0xa8] instruction until after the two push 0x0 instructions, replacing them with push edi. This would have saved 2 bytes.
if (pSink == (IAdviseSink *)this)
These next several lines are to compensate for the fact that in C++, (IAdviseSink *)NULL must still be NULL. So if your "this" is really "(ViewState*)NULL", then the result of the cast should be NULL and not the distance between IAdviseSink and IBrowserService.
71517202 8d46ec lea eax,[esi-0x14] ; eax = -(IAdviseSink*)this
71517205 8d5614 lea edx,[esi+0x14] ; edx = (IAdviseSink*)this
71517208 f7d8 neg eax ; eax = -eax (sets carry if != 0)
7151720a 1bc0 sbb eax,eax ; eax = eax - eax - carry
7151720c 23c2 and eax,edx ; eax = NULL or edx
Although the Pentium has a conditional move instruction, the base i386 architecture does not, so the compiler uses specific techniques to simulate a conditional move instruction without taking any jumps.
The general pattern for a conditional evaluation is the following:
neg r
sbb r, r
and r, (val1 - val2)
add r, val2
The neg r sets the carry flag if r is nonzero, because neg negates the value by subtracting from zero. And, subtracting from zero will generate a borrow (set the carry) if you subtract a nonzero value. It also damages the value in the r register, but that is acceptable because you are about to overwrite it anyway.
Next, the sbb r, r instruction subtracts a value from itself, which always results in zero. However, it also subtracts the carry (borrow) bit, so the net result is to set r to zero or -1, depending on whether the carry was clear or set, respectively.
Therefore, sbb r, r sets r to zero if the original value of r was zero, or to -1 if the original value was nonzero.
The third instruction performs a mask. Because the r register is zero or -1, "this" serves either to leave r zero or to change r from -1 to (val1 - val1), in that ANDing any value with -1 leaves the original value.
Therefore, the result of "and r, (val1 - val1)" is to set r to zero if the original value of r was zero, or to "(val1 - val2)" if the original value of r was nonzero.
Finally, you add val2 to r, resulting in val2 or (val1 - val2) + val2 = val1.
Thus, the ultimate result of this series of instructions is to set r to val2 if it was originally zero or to val1 if it was nonzero. This is the assembly equivalent of r = r ? val1 : val2.
In this particular instance, you can see that val2 = 0 and val1 = (IAdviseSink*)this. (Notice that the compiler elided the final add eax, 0 instruction because it has no effect.)
7151720e 394508 cmp [ebp+0x8],eax ; pSink == (IAdviseSink*)this?
71517211 750b jnz No_SetAdvise (7151721e) ; jump if not equal
Earlier in this section, you set EDI to the address of the m_pvo member. You are going to be using it now. You also zeroed out the ECX register earlier.
m_pvo->SetAdvise(0, 0, NULL);
71517213 8b07 mov eax,[edi] ; eax = m_pvo
71517215 51 push ecx ; NULL
71517216 51 push ecx ; 0
71517217 51 push ecx ; 0
71517218 8b10 mov edx,[eax] ; edx = m_pvo->lpVtbl
7151721a 50 push eax ; "this" for callee
7151721b ff521c call [edx+0x1c] ; __stdcall SetAdvise
No_SetAdvise:
pSink->Release();
7151721e 8b4508 mov eax,[ebp+0x8] ; eax = pSink
71517221 50 push eax ; "this" for callee
71517222 8b08 mov ecx,[eax] ; ecx = pSink->lpVtbl
71517224 ff5108 call [ecx+0x8] ; __stdcall Release
No_Advise:
All these COM method calls should look very familiar.
The evaluation of the next two statements is interleaved. Do not forget that EBX contains the address of ReleaseAndNull.
fViewObjectChanged = TRUE;
ReleaseAndNull(&m_pvo);
71517227 57 push edi ; &m_pvo
71517228 c745fc01000000 mov dword ptr [ebp-0x4],0x1 ; fViewObjectChanged = TRUE
7151722f ffd3 call ebx ; call ReleaseAndNull
No_Pvo:
if (psv) {
71517231 8b7df8 mov edi,[ebp-0x8] ; edi = psv
71517234 85ff test edi,edi ; edi == 0?
71517236 7412 jz No_Psv2 (7151724a) ; jump if zero
psv->SaveViewState();
71517238 8b07 mov eax,[edi] ; eax = psv->lpVtbl
7151723a 57 push edi ; "this" for callee
7151723b ff5034 call [eax+0x34] ; __stdcall SaveViewState
Here are more COM method calls.
psv->DestroyViewWindow();
7151723e 8b07 mov eax,[edi] ; eax = psv->lpVtbl
71517240 57 push edi ; "this" for callee
71517241 ff5028 call [eax+0x28] ; __stdcall DestroyViewWindow
psv->Release();
71517244 8b07 mov eax,[edi] ; eax = psv->lpVtbl
71517246 57 push edi ; "this" for callee
71517247 ff5008 call [eax+0x8] ; __stdcall Release
No_Psv2:
m_hwndView = NULL;
7151724a 83667c00 and dword ptr [esi+0x7c],0x0 ; m_hwndView = 0
ANDing a memory location with zero is the same as setting it to zero, because anything AND zero is zero. The compiler uses this form because, even though it is slower, it is much shorter than the equivalent mov instruction. (This code was optimized for size, not speed.)
m_fHandsOff = FALSE;
7151724e 83a60c010000fe and dword ptr [esi+0x10c],0xfe
if (m_pcache) {
71517255 8b4670 mov eax,[esi+0x70] ; eax = m_pcache
71517258 85c0 test eax,eax ; eax == 0?
7151725a 740b jz No_Cache (71517267) ; jump if zero
GlobalFree(m_pcache);
7151725c 50 push eax ; m_pcache
7151725d ff15b4135071 call [_imp__GlobalFree] ; call GlobalFree
m_pcache = NULL;
71517263 83667000 and dword ptr [esi+0x70],0x0 ; m_pcache = 0
No_Cache:
m_psb->EnableModelessSB(TRUE);
71517267 8b4638 mov eax,[esi+0x38] ; eax = this->m_psb
7151726a 6a01 push 0x1 ; TRUE
7151726c 50 push eax ; "this" for callee
7151726d 8b08 mov ecx,[eax] ; ecx = m_psb->lpVtbl
7151726f ff5124 call [ecx+0x24] ; __stdcall EnableModelessSB
CancelPendingActions();
In order to call CancelPendingActions, you have to move from (ViewState*)this to (CUserView*)this. Note also that CancelPendingActions uses the __thiscall calling convention instead of __stdcall. According to __thiscall, the "this" pointer is passed in the ECX register instead of being passed on the stack.
71517272 8d4eec lea ecx,[esi-0x14] ; ecx = (CUserView*)this
71517275 e832fbffff call CUserView::CancelPendingActions (71516dac) ; __thiscall
ReleaseAndNull(&_psf);
7151727a 33ff xor edi,edi ; edi = 0 (for later)
No_Psv:
7151727c 8d4678 lea eax,[esi+0x78] ; eax = &_psf
7151727f 50 push eax ; parameter
71517280 ffd3 call ebx ; call ReleaseAndNull
if (fViewObjectChanged)
71517282 397dfc cmp [ebp-0x4],edi ; fViewObjectChanged == 0?
71517285 740d jz NoNotifyViewClients (71517294) ; jump if zero
NotifyViewClients(DVASPECT_CONTENT, -1);
71517287 8b46ec mov eax,[esi-0x14] ; eax = ((CUserView*)this)->lpVtbl
7151728a 8d4eec lea ecx,[esi-0x14] ; ecx = (CUserView*)this
7151728d 6aff push 0xff ; -1
7151728f 6a01 push 0x1 ; DVASPECT_CONTENT = 1
71517291 ff5024 call [eax+0x24] ; __thiscall NotifyViewClients
NoNotifyViewClients:
if (m_pszTitle)
71517294 8b8680000000 mov eax,[esi+0x80] ; eax = m_pszTitle
7151729a 8d9e80000000 lea ebx,[esi+0x80] ; ebx = &m_pszTitle (for later)
715172a0 3bc7 cmp eax,edi ; eax == 0?
715172a2 7409 jz No_Title (715172ad) ; jump if zero
LocalFree(m_pszTitle);
715172a4 50 push eax ; m_pszTitle
715172a5 ff1538125071 call [_imp__LocalFree]
m_pszTitle = NULL;
Remember that EDI is still zero and EBX is still &m_pszTitle, because those registers are preserved by function calls.
715172ab 893b mov [ebx],edi ; m_pszTitle = 0
No_Title:
SetRect(&m_rcBounds, 0, 0, 0, 0);
715172ad 57 push edi ; 0
715172ae 57 push edi ; 0
715172af 57 push edi ; 0
715172b0 81c6fc000000 add esi,0xfc ; esi = &this->m_rcBounds
715172b6 57 push edi ; 0
715172b7 56 push esi ; &m_rcBounds
715172b8 ff15e41a5071 call [_imp__SetRect]
Notice that you do not need the value of "this" any more, so the compiler uses the add instruction to modify it in place instead of using up another register to hold the address. This is actually a performance win due to the Pentium u/v pipelining, because the v pipe can do arithmetic, but not address computations.
return S_OK;
715172be 33c0 xor eax,eax ; eax = S_OK
Finally, you restore the registers you are required to preserve, clean up the stack, and return to your caller, removing the incoming parameters.
715172c0 5b pop ebx ; restore
ReturnNoEBX:
715172c1 5f pop edi ; restore
715172c2 5e pop esi ; restore
715172c3 c9 leave ; restores EBP and ESP simultaneously
715172c4 c20400 ret 0x4 ; return and clear parameters
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