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Calling x64 functions assembly.

2 posts in this topic

Hello everybody,
I'm writing a scripting virtual machine from which I want to directly call C/C++ functions.
To do this I have a function: void x64_InvokeStatic(NativeTypes* args, NativeTypes retType, char** stack, void* func);
- func -> Function pointer.
- stack -> A pointer to the VM's stack.
- retType -> The return type.
- args -> The argument types.
This function is written in x64 assembly.
NativeTypes is defined as:
enum class NativeTypes : char
NT_Void = 1,
NT_Int8 = 2,
NT_UInt8 = 2,
NT_Int16 = 3,
NT_UInt16 = 3,
NT_Int32 = 4,
NT_UInt32 = 4,
NT_Object = 1,
NT_Int64 = 5,
NT_UInt64 = 5
The args array is 0-terminated (example, a single int32 value: NativeTypes args[2] = { NativeTypes::NT_Int32, (NativeTypes)0}; )
The values of the NativeTypes values group the size and type (eg. NT_Int32 and NT_UInt32 have the same size and do not require seperated handling).
Working on x64 Windows, I know the arguments are passed:
rcx - first argument
rdx - second argument
r8 - third argument
r9 - fourth argument
stack - rest -> all 64 bit values.
All integer values are returned in rax.
My problem is with the values pushed on the (x64) stack, they are not passed correctly.
I suspect this has to do with my own setup of the (x64) stack but after several days of trying things I still haven't found a solution.
Does anyone have an idea about what I'm doing wrong (and possibly how to fix it)?
Thanks a lot in advance.

My x64 assembly:


; Help function to get values from the VM's stack, puts them in rax.
get_arg proc
; snipped away, does not modify the (x64) stack.
get_arg endp

; void x64_InvokeStatic(NativeTypes* args, NativeTypes retType, char** stack, void* func);
x64_InvokeStatic proc
; Reserve stack space
push rbp
sub rsp, 72

; Store values
mov [rsp+8], rsp
mov [rsp+16], r15
mov [rsp+24], r14
mov [rsp+32], r13
mov [rsp+40], r12
mov [rsp+48], r9
mov [rsp+56], r8
mov [rsp+64], rdx
mov [rsp+72], rcx
mov rbp, rsp

; Load argument array.
xor r12, r12
mov r12, [rsp+72]

; First argument  ;
mov r13b, [r12]
cmp r13b, 0
je call_func

; Load VM stack
mov r14, [rsp+56]
mov r15, [r14]

call get_arg
mov rcx, rax

; Second argument  ;
add r12, 1
mov r13b, [r12]
cmp r13b, 0
je call_func

call get_arg
mov rdx, rax

; Third argument  ;
add r12, 1
mov r13b, [r12]
cmp r13b, 0
je call_func

call get_arg
mov r8, rax

; Fourth argument  ;
add r12, 1
mov r13b, [r12]
cmp r13b, 0
je call_func

call get_arg
mov r9, rax

; Stack arguments  ;
add r12, 1
mov r13b, [r12]
cmp r13b, 0
je call_func

call get_arg
push rax
jmp push_stack

; Call function
mov rax, [rbp+48]
call rax

; Discard all pushed variables.
mov r8, rbp
mov rsp, r8

; Load return type, if void -> done.
mov r12, [rsp+64]
cmp r12, 1
je done

; Load stackpointerpointer en stackpointer.
mov r13, [rsp+56]
mov r14, [r13]

; Determine what type to return.
cmp r12, 2
je ret_int8
cmp r12, 3
je ret_int16
cmp r12, 4
je ret_int32
cmp r12, 5
je ret_int64

; No known returntype, return for now.
jmp done

; Process various return type sizes.
; - Snipped, does not modify the x64 stack -

; done, restore stack and then return.
mov r15, [rsp+16]
mov r14, [rsp+24]
mov r13, [rsp+32]
mov r12, [rsp+40]
add rsp, 72
pop rbp
x64_InvokeStatic endp




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Did you try using a debugger? You can look at the content of the stack and every register at each step of the execution. That's what I would do.

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can't make much sense of the ASM code, but it appears you may be thrashing the saved RBP, FWIW.



are you taking into account that there is a 32-byte shadow space on the stack?, IOW (on call / post-call, *1):

1st arg -> RCX -> [RSP+0] / [RBP+16]

2nd arg -> RDX -> [RSP+8] / [RBP+24]

3rd arg -> R8 -> [RSP+16] / [RBP+32]

4th arg -> R9 -> [RSP+24] / [RBP+40]

5th arg -> [RSP+32] / [RBP+48]

6th arg -> [RSP+40] / [RBP+56]



*1: basically, assuming following a typical "push rbp; mov rbp, rsp" entry sequence.


also keep in mind that the ABI requires keeping the stack aligned on a 16-byte boundary during calls, where it appears as if the code in question may not keep proper alignment, and I suspect may also invert the arguments (IOW: using push, where the first thing pushed will have the highest address, and as such will be the *last* argument).


better is probably to reserve stack-space for the arguments, load the first 4 into registers, position a register to point at where the 5th argument would go, and walk the address upwards (assuming I understand the logic correctly).

Edited by cr88192

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