Re: Need help doing a jmp rather than a call

[Sofiane Akermoun <akersof@xxxxxxxxx>]

Yes it is easy.
I am just asking because you started it in C, and mixing C and
assembly is not really a good idea.
But anyway, in  func3 you have to write the function prologue of func3
himself and func2 that will delete 2 stack frames, and of course you
have to prepare the stack for func4 before jumping into the code
location of func4 by passing the argument needed.

1) Delete Stack frame func3:
leave
//else use the classical frame erasing (intel syntax)
mov esp, ebp
pop ebp

2) Delete stack frame of func2:
leave
//else use the classical frame erasing (intel syntax)
mov esp, ebp
pop ebp

Now you have your stack frame exactly as when you called func2.
And you have to prepare a manage a new  stack frame usable by func4


3)Pass argument of func4 to the stack
The best is to disassemble func4 and to check how it get its parameters.
As you know function code is compiled and fixed addresses into the
stack will be used
Usually parameters for functions are pushed or moved into the stack by
the caller, and the function get it back from esp or ebp register as a
reference + an offset.
Actually esp address contains the right return address, and the next
upper addresses contains the parameters passed to func2.
Unless func4 has the same parameter as func2 and func4 get its
parameters as func2 does then you can now jump directly to func4.
Else you have to disassemble disassemble func4 and check where to copy
the params into this stack frame in the uper addresses above esp.
This depends on your compiler options/optimisation/alignement of course.

Sofiane Akermoun






2013/11/9 Blake McBride <blake@xxxxxxxxxxx>:
> Thanks for your input.  The system I am refering to (Dynace OO extension to
> C) already does what you describe by default.  It has support for an
> alternate "assembly language" mechinism that operates faster becasue of the
> redused number of contexts to pop and returns to execute.  This mechinism is
> a core component of some very large systems that perform this operation tens
> to hundreds of thousands of times per second.  So a small gain is a large
> gain.
>
> Although I used assambly language many years ago, that world keeps changing.
> These days my only use of assembly is this one piece of code.  I believe
> this is a trivial task for someone who uses it regularly, so I really
> appreciate the help.
>
> Incidently, in addition to being able to solve this problem in C only,
> Dynace implements threads (the kind where multiple C functions can share the
> same OS thread), and a mark-and-sweep garbage collector all in C too.  So
> there is a lot that can be done without assembler.  (Note, in addition to
> the above, Dynace also supports native/true threads and other GCs like the
> Boehm-Demers-Weiser collector).
>
> Thanks.
>
> Blake McBride
>
>
>
>
> On 2013-11-09 00:19:34 -0600, Sofiane Akermoun said:
>
>> Do you really need assembly to perform this?
>>
>> Why func2 doesn't return a pointer on func4 to the caller func1?
>> This way func2 do this job, and return to func1 a pointer on a
>> function to call func4.
>> A very little, simple and naive simple:
>>
>> #include <stdio.h>
>>
>> void func4(char *str)
>> {
>>     printf("%s", str);
>> }
>>
>> void (*func2(void))(char *)
>> {
>>     //we can imagine here some work on func2 that will decide what
>> function  to return
>>     return func4;
>> }
>>
>> void func1(char *str)
>> {
>>    //Here func2 return func4 pointer and
>>    //it is used ny func1 to call func4 function with "str" argument
>>    func2()(str);
>> }
>>
>> int main(int argc, char *argv[])
>> {
>>     func1("Hello world!");
>>     return 0;
>> }
>>
>> It is what you need?
>>
>> kind regards,
>> Sofiane Akermoun
>>
>> 2013/11/9 Blake McBride <blake@xxxxxxxxxxx>:
>>>
>>> I am trying to get a simple piece of x64 assembly working on 64 bit linux
>>> and a Mac.  I am using GCC.
>>>
>>> Let's say I have 4 (C language) functions.  Function fun1 calls fun2,
>>> fun2
>>> calls fun3, and fun3 calls fun4.  I need fun 4 to operate and run as if
>>> it
>>> was called directly from fun1.  So, fun4 should see the arguments passed
>>> to
>>> fun2, and when fun4 returns it should return diretly to fun1 as if it was
>>> called by fun1.
>>>
>>> Basically, this is an OO language that uses fun2 to calculate what fun4
>>> is.
>>> fun3 is used to manipulate the stack so that when fun4 starts up it
>>> thinks
>>> it was called directly from fun1.  fin1, fun2, and fun4 are plain C code.
>>> fun3 performs the magic.  I have been doing this easily on many different
>>> 32
>>> bit machines for years but I haven't been able to get it going on 64 bit
>>> machines.
>>>
>>>
>>> Typically fun3 would:
>>>
>>> 1. create a new stack frame
>>> 2.  call fun3
>>> 3.  pop local call frame
>>> 4.  return
>>>
>>> What I need fun3 to do is:
>>>
>>> 1.  either pop its stack frame or don't create one
>>> 2.  pop the stack frame from fun2
>>> 3.  jump to fun4
>>>
>>> Then fun4 will start executing as if it was called from fun1.  I have
>>> sample
>>> (errant) C code for the entire process.  It will all work fine if fun3
>>> (_jumpToMethod) is rewritten in assembler.
>>>
>>> (What I did in the past was compile the C code for _jumpToMethod into
>>> assembly code, modify the code, and then use that assembly code.)
>>>
>>> Here is the C code for fun1, fun2, and fun3:
>>>
>>> #include <stdio.h>
>>>
>>> char    *obj = "Some object pointer";
>>>
>>> char    *GenObj = "Some Generic Object Pointer";
>>>
>>> typedef int     (*ofun)();
>>>
>>>
>>> int     Method(char *self, int a, int b, int c)  /*  fun4  */
>>> {
>>> printf("Method reached with args %s %d %d %d\n", self, a, b, c);
>>> return a + b + c;
>>> }
>>>
>>> ofun    FindMethod(char *obj, char *gen)
>>> {
>>> return Method;
>>> }
>>>
>>> GenericFunction(char *self, ...)  /*  fun2  */
>>> {
>>> _jumpToMethod( FindMethod(self, GenObj) );
>>> }
>>>
>>> main(void)  /* fun1  */
>>> {
>>> printf("Method is at %lx\n", (long unsigned int) &Method);
>>> /* both calls to Method should look alike to Method  */
>>> int     r = Method(obj, 1, 2, 3);
>>> printf("Value returned from GenericFunction = %d\n", r);
>>> r = GenericFunction(obj, 1, 2, 3);
>>> printf("Value returned from GenericFunction = %d\n", r);
>>> return 0;
>>> }
>>>
>>>
>>> -----------------------------------------------------------------
>>>
>>> Here is the C code for _jumpToMethod (fun3)
>>>
>>>
>>> void    _jumpToMethod(void (*function) (/* ??? */))
>>> {
>>>
>>> /*      pop_this_stack_frame;   */
>>>
>>> /*      pop previous (generics) stack frame  */
>>>
>>> (*function)();  /*  must be changed to jump instruction   */
>>> }
>>>
>>>
>>>
>>> I think this would be easy for someone who knows this assembly language.
>>> I
>>> have spent about 5 hurs on it and I am lost as I can be.  Your help is
>>> greatly appreciated!
>>>
>>> Thanks!
>>>
>>> Blake McBride
>>>
>>>
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>>
>>
>>
>>
>> --
>> Sofiane AKERMOUN
>> akersof@xxxxxxxxx
>
>
>
>
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> To unsubscribe from this list: send the line "unsubscribe linux-assembly" in
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-- 
Sofiane AKERMOUN
akersof@xxxxxxxxx
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