• sprucely <timberdig gmail.com> Oct 23 2009
• sprucely <timberdig gmail.com> Oct 24 2009

sprucely <timberdig gmail.com> writes:

Okay, between gdb and obj2asm I've been able to figure out that the address
placed in EAX in the following instructions...

		mov EAX, jumpTo;
		jmp EAX;

is actually the address of the initialization function for jumpTo... 

byte* jumpTo = &jumpHere;

which is named _D9conductor6jumpToPg. Execution then ends up segfaulting on a
(bad) instruction in function _moduleinfo_array.

So I'm not sure if the wrong address is being stored in jumpTo or if I'm simply
not correctly dereferencing it. Ultimately I need to know the offset from
function pointers to the point at which the prolog has completed and execution
of the main function body begins. I'm attempting to discover the offset by
using a dummy function containing a "jumpHere:" label and subtracting the
function address from the label address. Is there a better way to do this?

Thanks for the suggestions I've received. I'm learning a lot!


sprucely Wrote:

 This works with g++ and inline ATT assembly, but I have had no such luck in D.
I have many simple functions that need to be executed sequentially and have
identical stack frames. To avoid the overhead of setting up and tearing down
the stack frames I want to jmp from the body of one function to the body of the
next. A simplified example...
 
 extern(C) byte jumpHere;
 
 byte* jumpTo = &jumpHere;
 
 void f1()
 {
 	asm
 	{
 		//jmp dword ptr jumpTo;
 		mov EAX, jumpTo;
 		jmp EAX;
 		//jmp [EAX]
 	}
 }
 
 void f2()
 {
 	asm{jumpHere:;}
 }
 
 No matter what I try I get a segfault. My assembly skills are very limited.
I'm not using the naked keyword yet, because I want to get a proof-of-concept
working first. Anyone see anything wrong with this? Any suggestions?

sprucely <timberdig gmail.com> writes:

I've been able to determine that the trick, extern(C) byte jumpHere, is not
providing the correct address. Since all my functions will have an identical
stack frame, it will be easy enough to just hard code the proper offset.

sprucely Wrote:

 Okay, between gdb and obj2asm I've been able to figure out that the address
placed in EAX in the following instructions...
 
 		mov EAX, jumpTo;
 		jmp EAX;
 
 is actually the address of the initialization function for jumpTo... 
 
 byte* jumpTo = &jumpHere;
 
 which is named _D9conductor6jumpToPg. Execution then ends up segfaulting on a
(bad) instruction in function _moduleinfo_array.
 
 So I'm not sure if the wrong address is being stored in jumpTo or if I'm
simply not correctly dereferencing it. Ultimately I need to know the offset
from function pointers to the point at which the prolog has completed and
execution of the main function body begins. I'm attempting to discover the
offset by using a dummy function containing a "jumpHere:" label and subtracting
the function address from the label address. Is there a better way to do this?
 
 Thanks for the suggestions I've received. I'm learning a lot!
 
 
 sprucely Wrote:
 
 This works with g++ and inline ATT assembly, but I have had no such luck in D.
I have many simple functions that need to be executed sequentially and have
identical stack frames. To avoid the overhead of setting up and tearing down
the stack frames I want to jmp from the body of one function to the body of the
next. A simplified example...
 
 extern(C) byte jumpHere;
 
 byte* jumpTo = &jumpHere;
 
 void f1()
 {
 	asm
 	{
 		//jmp dword ptr jumpTo;
 		mov EAX, jumpTo;
 		jmp EAX;
 		//jmp [EAX]
 	}
 }
 
 void f2()
 {
 	asm{jumpHere:;}
 }
 
 No matter what I try I get a segfault. My assembly skills are very limited.
I'm not using the naked keyword yet, because I want to get a proof-of-concept
working first. Anyone see anything wrong with this? Any suggestions?