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Dr.Disrespect
Grandmaster Cheater
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Joined: 17 Feb 2016
Posts: 526
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 Posted: Tue Dec 18, 2018 2:48 am Post subject: (ASM) How to check if multiple pointers are valid? |
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I want to check if several pointers are valid before reading data from them(when I say invalid memory addresses, they are the ones shown as "?? ?? ??" in CE):
| Code: |
mov ecx,dword ptr[rcx+100]
mov ecx,dword ptr[rcx+200]
mov ecx,dword ptr[rcx+300]
mov ecx,dword ptr[rcx+400]
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How to check each of them in an efficient way?
I have read the related posts on this forum, there are two solutions according to them:
1. test ecx,ecx
2. isbadreadptr
The first solution doesn't work for me, because sometimes dword ptr[rcx+300] is invalid and crashes the game.
The second doesn't work for me either, it keeps crashing the game when calling "isbadreadptr" and I don't know why.
| Code: |
check1:
push rcx <--- I need the data in rcx for the second check
push 4
lea eax,dword ptr[rcx+100]
push eax
call isbadreadptr
add rsp,10
pop rcx
cmp eax,0
jne code
jmp check2
check2:
push rcx
push 4
lea eax,dword ptr[rcx+200]
push eax
call isbadreadptr <--------crashes here
add rsp,10
pop rcx
cmp eax,0
jne code
jmp check3
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Thanks in advance.
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Dark Byte
Site Admin
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Joined: 09 May 2003
Posts: 25779
Location: The netherlands
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| Posted: Tue Dec 18, 2018 3:45 am Post subject: |
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| Code: |
push rax
push rbx
push rcx
{$try}
mov eax,dword ptr[rcx+100]
mov ebx,dword ptr[rcx+200]
mov ecx,dword ptr[rcx+300]
//everything ok, do your stuff
jmp done
{$except}
//it failed somewhere
done:
pop rcx
pop rbx
pop rax
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ParkourPenguin
I post too much
 Reputation: 152
Joined: 06 Jul 2014
Posts: 4695
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| Posted: Tue Dec 18, 2018 9:34 am Post subject: |
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Use {$try} / {$except} as in DB's example.
Your call crashes because you're not using proper 64-bit calling conventions.
https://docs.microsoft.com/en-us/cpp/build/overview-of-x64-calling-conventions
- rcx, rdx, r8, and r9 are the first four parameters to functions (everything else goes in the stack)
- The stack must have 32 bytes of scratch space for the first four parameters
- The stack must be aligned on a 16-byte boundary at the call
There's some other stuff, but these three are your problem.
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Dr.Disrespect
Grandmaster Cheater
![]() Reputation: 3
Joined: 17 Feb 2016
Posts: 526
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| Posted: Tue Dec 18, 2018 9:47 am Post subject: |
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Thanks for the replies, I tried to use DB's method, but the game still crashes.
Here is my code:
| Code: |
newmem:
push rax
push rbx
push rsi
push rdi
xor rbx,rbx
{$try}
mov eax,dword ptr[rcx+100]
mov ebx,dword ptr[rax+200]
mov esi,dword ptr[rbx+300]
mov edi,dword ptr[esi+400]
mov eax,["game.exe"+5487912] <---this address is always valid
cmp eax,edi
jne code
{do something here, I commented this part out for testing purpose, so this part is not the problem}
{$except}
code:
pop rdi
pop rsi
pop rbx
pop rax
mov esi,00000010
jmp return
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Edit, I changed the code to this, and it still crashes, 
| Code: |
newmem:
push rax
push rbx
push rsi
push rdi
xor rbx,rbx
{$try}
mov eax,dword ptr[rcx+100]
mov ebx,dword ptr[rax+200]
mov esi,dword ptr[rbx+300]
mov edi,dword ptr[rsi+400]
mov eax,["game.exe"+5487912]
jmp code
{$except}
code:
pop rdi
pop rsi
pop rbx
pop rax
mov esi,00000010
jmp return
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Is there a way provided by CE to track the registers' status right before the crash happens? Such as the data in the registers before the crash, so I can see what went wrong.
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