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Index > Main > Using 'virtual' to move the func's data into data section

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hellomachine



Joined: 18 May 2023
Posts: 22
Location: I don't even exist
hellomachine 18 May 2023, 12:31
Hello, I'm using FASM (not FASMg).
I know that it's possible to have a function with data at its end (or anywhere else near that function (data, for example a JMP table)).

For example,

Code:
func:
        ...
        jmp     QWORD [.jmp_table]
        ...
 .jmp_table:
        dq ...
        dq ...
        dq ...
    


But I want to move each function's data into the data section without
putting that data in a separate file and include it in that section (data).
So I searched, and I found virtual directive which seems going to fix my problem and this is my code:

Code:
format MS64 COFF

public func

virtual at 0x00
 data8::
end virtual

section '.text' code readable executable align 64

func:
        mov     rax, .func_data
        mov     rax, QWORD [rax]
        ret

 virtual data8
  .func_data dq 1024
 end virtual

section '.rdata' data readable align 64

 db 'BeginData'

 ; I expect data8 content to be placed here.
 virtual data8
  data8.size = $ - $$
 end virtual
 repeat data8.size
  load a byte from data8:%-1
  db a
 end repeat

 db 'EndData'
    



Everything is seems OK to me, but I got Segmentation fault error when I tried to run 'func' and the error is about this line :
Code:
mov     rax, QWORD [rax]    


What is the problem? I check the binary file and data was there too !!![/b]
Post 18 May 2023, 12:31
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20179
Location: In your JS exploiting you and your system
revolution 18 May 2023, 12:34
You have defined the base address of data8 as "virtual at 0x00", so the value in rax is null.

You can define a label before the repeat line and use that label for the table address.
Code:
some_label: ; <--- your data is here
repeat data8.size
; ...    
Post 18 May 2023, 12:34
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hellomachine



Joined: 18 May 2023
Posts: 22
Location: I don't even exist
hellomachine 18 May 2023, 12:44
revolution wrote:
You have defined the base address of data8 as "virtual at 0x00", so the value in rax is null.

You can define a label before the repeat line and use that label for the table address.
Code:
some_label: ; <--- your data is here
repeat data8.size
; ...    


Well I have too many functions and each function has its own jmp-table and in each function, I need a local name (for each function) to call that jmp-table so defining a name before the repeat is just like to not working with virtual and putting the jmp-tables in data section directly ...
Post 18 May 2023, 12:44
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20179
Location: In your JS exploiting you and your system
revolution 18 May 2023, 12:48
You can use the label as an offset.
Code:
mov     rax, .func_data + some_label
;...
mov     rax, .func2_data + some_label    
Post 18 May 2023, 12:48
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hellomachine



Joined: 18 May 2023
Posts: 22
Location: I don't even exist
hellomachine 18 May 2023, 12:57
revolution wrote:
You can use the label as an offset.
Code:
mov     rax, .func_data + some_label
;...
mov     rax, .func2_data + some_label    


Nice Solution, thank you. I'm going to use it, but isn't there any clean solution for this problem? For example, how about
Code:
'virtual at some_label'    
?
Post 18 May 2023, 12:57
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