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]

18 May 2023, 12:31

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20454
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
; ...

18 May 2023, 12:34

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 ...

18 May 2023, 12:44

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20454
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

18 May 2023, 12:48

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'

18 May 2023, 12:57

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