Programming Forums > Application Development > Assembly

para · Dec 14th, 2005, 9:14 PM

Ok this is mostly C but my question is really based around ASM techniques so I decided it would be best to post it here.

I'm working on a small project that will overwrite parts of the machine code in the current process space. My goal is to always redirect the function a() to function b(). Not a very practical application but it's something to learn from

Anyhow, here's the general code:

(Toggle Plain Text)

byte patch[] = { 0xe9, 0x00, 0x00, 0x00, 0x00 };

void a() {
	printf("a() called\n");
}

void b() {
	printf("b() called\n");
}

int main() {
	void* aptr = a;
	printf("a: %p\n", a);
	memcpy(patch+1, &aptr, 4);

	printf("patching..\n");
	memcpy(a, patch, sizeof(patch));
	printf("patched\n");

	printf("calling a()...\n");
	a();

	return 0;
}

I cut out the part of the code that makes the memory space writable, so assume the memcpy() is legal. The array is a 5 byte jmp function I compiled and grabbed out of a disassembly. 0xe9 is the jmp, then 4 0x00's which is updated in the first memcpy() in main(). So everything works until I try to call a() at the bottom of main(), where it crashes. Is this pointer I copied into the function code valid? Why is it crashing?

nnxion · Dec 15th, 2005, 6:03 AM

Variable 'a' is not declared so: void* aptr = a; will not point to somewhere undefined.
In your case, memcpy(a, patch, sizeof(patch)); makes it crash.

para · Dec 15th, 2005, 6:09 AM

Quote:

Originally Posted by nnxion

Variable 'a' is not declared so: void* aptr = a; will not point to somewhere undefined.
In your case, memcpy(a, patch, sizeof(patch)); makes it crash.

Actually I think that's C99; (void*)&a would work too, it's a pointer to the a function.

The code ends up being in the array:
0xe9 0x04 0x03 0x02 0x01
assuming the address for the function a is 0x01020304 (little endian machine).

lectricpharaoh · Dec 15th, 2005, 6:34 AM

Quote:

Originally Posted by para

(Toggle Plain Text)

byte patch[] = { 0xe9, 0x00, 0x00, 0x00, 0x00 };
 
void a() {
	printf("a() called\n");
}
 
void b() {
	printf("b() called\n");
}
 
int main() {
	void* aptr = a;
	printf("a: %p\n", a);
	memcpy(patch+1, &aptr, 4);
 
	printf("patching..\n");
	memcpy(a, patch, sizeof(patch));
	printf("patched\n");
 
	printf("calling a()...\n");
	a();
 
	return 0;
}

I think the two lines in red might be part of the problem. First you copy the address of a into elements 1 through 4 of the patch array, so it contains 0E9h followed by the address- fine. Then, in the second memcpy(), you copy five bytes from patch to a four-byte, constant value. Assuming you have indeed made the function's offset address value writable in the code segment, you're still trying to copy five bytes to a four-byte value (oops). Second, I'm not sure this would work even aside from that. The function's address is a constant value, and will likely be resolved at link time, with all references to the function being replaced by a constant value. You replace one instance, but not all the others. Get what I mean?

Why not try something like using double indirection to get at the memory pointed at by the function's offset, then changing the data there? Replace the function body with a RET instruction, or something. See if that works.

[edit] No need for double indirection, my bad. I need sleep. [/edit]

lectricpharaoh · Dec 15th, 2005, 6:53 AM

I tried this code, but I got an access violation for trying to write to the memory. I dunno how to make it writable in Windows (using DevC++/MinGW, so it might be similar to how you do it with gcc under Linux, if you care to share).

(Toggle Plain Text)

#include <stdio.h>
#include <stdlib.h>
 
void manlyMan(void);
void neuter(void (*soonToBeEunuch)(void));
 
void manlyMan(void)
{
  puts("I am a manly man!");
}
 
 
void neuter(void (*soonToBeEunuch)(void))
{
  char *target = (char *)soonToBeEunuch;
  *target = 0xC3; // plain old RET (near)
}
 
 
int main(void)
{
  puts("---");
  manlyMan();
  puts("---");
  neuter(manlyMan);
  manlyMan();
  puts("---");
  system("pause");
  return 0;
}

splyxx · Dec 15th, 2005, 7:52 AM

> Then, in the second memcpy(), you copy five bytes from patch to a four-byte,
> constant value. Assuming you have indeed made the function's offset address value
> writable in the code segment, you're still trying to copy five bytes to a four-byte value
> (oops). [...]

What makes you think it is four bytes? He's changing the machine code of a() itself, not the symbol "a".

"&a" is the same as "a" - the address of some code. And he is patching that code.

void a() {
printf("a() called\n");

Assuming push ebp, mov ebp, esp, push string, call printf, add esp, 4, pop ebp, we have at least six instructions occupying (much) more than four bytes.

> byte patch[] = { 0xe9, 0x00, 0x00, 0x00, 0x00 };

You should be using some form of "call" instruction because a jump will not save the return address. Also, my opcode reference says 0xe9 is a relative jump, but you need an indirect one. The code you use should be the same as the code generated for

void foo() { stuff }
...
void (*p)() = foo;
p();

para · Dec 15th, 2005, 11:12 AM

Quote:

Originally Posted by splyxx

> byte patch[] = { 0xe9, 0x00, 0x00, 0x00, 0x00 };

You should be using some form of "call" instruction because a jump will not save the return address. Also, my opcode reference says 0xe9 is a relative jump, but you need an indirect one. The code you use should be the same as the code generated for

void foo() { stuff }
...
void (*p)() = foo;
p();

My goal is similar to parts of the "Detours" library (GPL'ed so others can use it since Detours is "research only").
http://research.microsoft.com/sn/detours/

Anyhow, I don't want to overwrite the opcodes with a call because I don't want control to return to that function. The function that is jumped to by the function a() should be of the same type so and parameter accessing/returning will (hopefully) be compatible.

Quote:

Originally Posted by splyxx

Also, my opcode reference says 0xe9 is a relative jump

This certainly makes sense to me since I'm on a 64-bit machine (program compiled in 32-bit). Looking at the disassembly if I have two consecutively jmp opcodes to the same label, I get something like:

(Toggle Plain Text)

08048468 <a>:
 8048468:       e9 1a 00 00 00          jmp    8048487 <b>
 804846d:       e9 15 00 00 00          jmp    8048487 <b>

Evidently the address is relative so I must be using the wrong opcode.
Is there a chart somewhere on available opcodes and their binary representation? Google turns up very little, and I got this code from a objdump disassembly :\

nnxion · Dec 15th, 2005, 12:07 PM

Quote:

Originally Posted by para

My goal is similar to parts of the "Detours" library (GPL'ed so others can use it since Detours is "research only").
http://research.microsoft.com/sn/detours/

Hehe had to check it out, MS to GPL something, but indeed they didn't. It's a Microsoft Research Shared Source license agreement, which is kind of the same I think.

splyxx · Dec 15th, 2005, 2:08 PM

> Is there a chart somewhere on available opcodes and their binary representation?

I sort of totally love OPCODES.HLP

http://www.google.de/search?hl=de&q=opcodes.hlp&spell=1

I use winhelp in Wine on Linux to access this file. Had to get a friend to send me a dll and I think some other Windows file to make it work though

NASM has a ton of good docs as well, including an x86 reference:

http://nasm.sourceforge.net/doc/html/nasmdocb.html

(BTW, this will be my last post in this forum, but you can PM me if you run into more problems regarding this interesting project.)

para · Dec 15th, 2005, 2:29 PM

Quote:

Originally Posted by splyxx

> Is there a chart somewhere on available opcodes and their binary representation?

I sort of totally love OPCODES.HLP

http://www.google.de/search?hl=de&q=opcodes.hlp&spell=1

I use winhelp in Wine on Linux to access this file. Had to get a friend to send me a dll and I think some other Windows file to make it work though

NASM has a ton of good docs as well, including an x86 reference:

http://nasm.sourceforge.net/doc/html/nasmdocb.html

(BTW, this will be my last post in this forum, but you can PM me if you run into more problems regarding this interesting project.)

Excellent, thanks a lot! I was in need of additional resources, so this is just perfect

As a follow up I was able to solve the issue with using a relative address like you said before. Copied the 4 byte offset to the target function which was calculated with (dest - (source + 5)).

Invoking the source() function now "redirects" to the dest() function as desired. Thanks everyone for your help.

Dec 14th, 2005, 9:14 PM	#1
para Programmer Join Date: Dec 2005 Posts: 65 Rep Power: 3	C/asm jmp calculation Ok this is mostly C but my question is really based around ASM techniques so I decided it would be best to post it here. I'm working on a small project that will overwrite parts of the machine code in the current process space. My goal is to always redirect the function a() to function b(). Not a very practical application but it's something to learn from Anyhow, here's the general code: (Toggle Plain Text) byte patch[] = { 0xe9, 0x00, 0x00, 0x00, 0x00 }; void a() { printf("a() called\n"); } void b() { printf("b() called\n"); } int main() { void* aptr = a; printf("a: %p\n", a); memcpy(patch+1, &aptr, 4); printf("patching..\n"); memcpy(a, patch, sizeof(patch)); printf("patched\n"); printf("calling a()...\n"); a(); return 0; } byte patch[] = { 0xe9, 0x00, 0x00, 0x00, 0x00 }; void a() { printf("a() called\n"); } void b() { printf("b() called\n"); } int main() { void* aptr = a; printf("a: %p\n", a); memcpy(patch+1, &aptr, 4); printf("patching..\n"); memcpy(a, patch, sizeof(patch)); printf("patched\n"); printf("calling a()...\n"); a(); return 0; } I cut out the part of the code that makes the memory space writable, so assume the memcpy() is legal. The array is a 5 byte jmp function I compiled and grabbed out of a disassembly. 0xe9 is the jmp, then 4 0x00's which is updated in the first memcpy() in main(). So everything works until I try to call a() at the bottom of main(), where it crashes. Is this pointer I copied into the function code valid? Why is it crashing?

Dec 15th, 2005, 6:03 AM	#2
nnxion Programming Guru Join Date: Jun 2005 Location: elemental plane Posts: 1,429 Rep Power: 5	Variable 'a' is not declared so: void* aptr = a; will not point to somewhere undefined. In your case, memcpy(a, patch, sizeof(patch)); makes it crash. __________________ "Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for." -- Socrates

Dec 15th, 2005, 6:53 AM	#5
lectricpharaoh Caffeinated Neural Net Join Date: Jun 2005 Location: Dry west coast of Canada Posts: 1,031 Rep Power: 5	I tried this code, but I got an access violation for trying to write to the memory. I dunno how to make it writable in Windows (using DevC++/MinGW, so it might be similar to how you do it with gcc under Linux, if you care to share). (Toggle Plain Text) #include <stdio.h> #include <stdlib.h> void manlyMan(void); void neuter(void (soonToBeEunuch)(void)); void manlyMan(void) { puts("I am a manly man!"); } void neuter(void (soonToBeEunuch)(void)) { char target = (char )soonToBeEunuch; target = 0xC3; // plain old RET (near) } int main(void) { puts("---"); manlyMan(); puts("---"); neuter(manlyMan); manlyMan(); puts("---"); system("pause"); return 0; } #include <stdio.h> #include <stdlib.h> void manlyMan(void); void neuter(void (soonToBeEunuch)(void)); void manlyMan(void) { puts("I am a manly man!"); } void neuter(void (soonToBeEunuch)(void)) { char target = (char )soonToBeEunuch; target = 0xC3; // plain old RET (near) } int main(void) { puts("---"); manlyMan(); puts("---"); neuter(manlyMan); manlyMan(); puts("---"); system("pause"); return 0; } __________________ And once again, Probability proves itself willing to sneak into a back alley and service Drama as would a copper-piece harlot. - Vaarsuvius, Order of the Stick Last edited by lectricpharaoh; Dec 15th, 2005 at 7:05 AM.

Dec 15th, 2005, 7:52 AM	#6
splyxx Newbie Join Date: Nov 2005 Posts: 4 Rep Power: 0	> Then, in the second memcpy(), you copy five bytes from patch to a four-byte, > constant value. Assuming you have indeed made the function's offset address value > writable in the code segment, you're still trying to copy five bytes to a four-byte value > (oops). [...] What makes you think it is four bytes? He's changing the machine code of a() itself, not the symbol "a". "&a" is the same as "a" - the address of some code. And he is patching that code. void a() { printf("a() called\n"); Assuming push ebp, mov ebp, esp, push string, call printf, add esp, 4, pop ebp, we have at least six instructions occupying (much) more than four bytes. > byte patch[] = { 0xe9, 0x00, 0x00, 0x00, 0x00 }; You should be using some form of "call" instruction because a jump will not save the return address. Also, my opcode reference says 0xe9 is a relative jump, but you need an indirect one. The code you use should be the same as the code generated for void foo() { stuff } ... void (p)() = foo; p();* Last edited by splyxx; Dec 15th, 2005 at 8:05 AM.

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Dec 15th, 2005, 2:08 PM	#9
splyxx Newbie Join Date: Nov 2005 Posts: 4 Rep Power: 0	> Is there a chart somewhere on available opcodes and their binary representation? I sort of totally love OPCODES.HLP http://www.google.de/search?hl=de&q=opcodes.hlp&spell=1 I use winhelp in Wine on Linux to access this file. Had to get a friend to send me a dll and I think some other Windows file to make it work though NASM has a ton of good docs as well, including an x86 reference: http://nasm.sourceforge.net/doc/html/nasmdocb.html (BTW, this will be my last post in this forum, but you can PM me if you run into more problems regarding this interesting project.)

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Programming Forums > Application Development > Assembly

C/asm jmp calculation