Using Assembly Language in Linux.

Contents:

• Introduction
  
• Intel and AT&T Syntax
  
  • Prefixes
    
  • Direction of Operands
    
  • Memory Operands
    
  • Suffixes
    
• Syscalls
  
  • Syscalls with < 6 args
    
  • Syscalls with > 5 args
    
  • Socket syscalls
    
• Command Line Arguments
  
• GCC Inline ASM
  
• Compiling
  
• Further reference/Links
  
• Example Code.
  

This article will describe assembly language programming under Linux. Contained within the bounds of the article is a comparison between Intel and AT&T syntax asm, a guide to using syscalls and a introductory guide to using inline asm in gcc.

This article was written due to the lack of (good) info on this field of programming (inline asm section in particular), in which case i should remind thee that this is not a shellcode writing tutorial because there is no lack of info in this field.

Various parts of this text I have learnt about through experimentation and hence may be prone to error. Should you find any of these errors on my part, do not hesitate to notify me via email and enlighten me on the given issue.

There is only one prerequisite for reading this article, and thats obviously a basic knowledge of x86 assembly language and C.

Intel and AT&T syntax Assembly language are very different from each other in appearance, and this will lead to confusion when one first comes across AT&T syntax after having learnt Intel syntax first, or vice versa. So lets start with the basics.

In Intel syntax there are no register prefixes or immed prefixes. In AT&T however registers are prefixed with a '%' and immed's are prefixed with a '$'. Intel syntax hexadecimal or binary immed data are suffixed with 'h' and 'b' respectively. Also if the first hexadecimal digit is a letter then the value is prefixed by a '0'.

mov	eax,1
mov	ebx,0ffh
int	80h

movl	$1,%eax
movl	$0xff,%ebx
int 	$0x80

The direction of the operands in Intel syntax is opposite from that of AT&T syntax. In Intel syntax the first operand is the destination, and the second operand is the source whereas in AT&T syntax the first operand is the source and the second operand is the destination. The advantage of AT&T syntax in this situation is obvious. We read from left to right, we write from left to right, so this way is only natural.

instr	dest,source
mov	eax,[ecx]

instr 	source,dest
movl	(%ecx),%eax

Memory operands as seen above are different also. In Intel syntax the base register is enclosed in '[' and ']' whereas in AT&T syntax it is enclosed in '(' and ')'.

mov	eax,[ebx]
mov	eax,[ebx+3]

movl	(%ebx),%eax
movl	3(%ebx),%eax 

The AT&T form for instructions involving complex operations is very obscure compared to Intel syntax. The Intel syntax form of these is segreg:[base+index*scale+disp]. The AT&T syntax form is %segreg:disp(base,index,scale).

Index/scale/disp/segreg are all optional and can simply be left out. Scale, if not specified and index is specified, defaults to 1. Segreg depends on the instruction and whether the app is being run in real mode or pmode. In real mode it depends on the instruction whereas in pmode its unnecessary. Immediate data used should not '$' prefixed in AT&T when used for scale/disp.

instr 	foo,segreg:[base+index*scale+disp]
mov	eax,[ebx+20h]
add	eax,[ebx+ecx*2h
lea	eax,[ebx+ecx]
sub	eax,[ebx+ecx*4h-20h]

instr	%segreg:disp(base,index,scale),foo
movl	0x20(%ebx),%eax
addl	(%ebx,%ecx,0x2),%eax
leal	(%ebx,%ecx),%eax
subl	-0x20(%ebx,%ecx,0x4),%eax

As you can see, AT&T is very obscure. [base+index*scale+disp] makes more sense at a glance than disp(base,index,scale).

As you may have noticed, the AT&T syntax mnemonics have a suffix. The significance of this suffix is that of operand size. 'l' is for long, 'w' is for word, and 'b' is for byte. Intel syntax has similar directives for use with memory operands, i.e. byte ptr, word ptr, dword ptr. "dword" of course corresponding to "long". This is similar to type casting in C but it doesnt seem to be necessary since the size of registers used is the assumed datatype.

mov	al,bl
mov	ax,bx
mov	eax,ebx
mov	eax, dword ptr [ebx]

movb	%bl,%al
movw	%bx,%ax
movl	%ebx,%eax
movl	(%ebx),%eax

This section will outline the use of linux syscalls in assembly language. Syscalls consist of all the functions in the second section of the manual pages located in /usr/man/man2. They are also listed in: /usr/include/sys/syscall.h. A great list is at http://www.linuxassembly.org/syscall.html. These functions can be executed via the linux interrupt service: int $0x80.

For all syscalls, the syscall number goes in %eax. For syscalls that have less than six args, the args go in %ebx,%ecx,%edx,%esi,%edi in order. The return value of the syscall is stored in %eax.

The syscall number can be found in /usr/include/sys/syscall.h. The macros are defined as SYS_<syscall name> i.e. SYS_exit, SYS_close, etc.

According to the write(2) man page, write is declared as: ssize_t write(int fd, const void *buf, size_t count);

Hence fd goes in %ebx, buf goes in %ecx, count goes in %edx and SYS_write goes in %eax. This is followed by an int $0x80 which executes the syscall. The return value of the syscall is stored in %eax.

$ cat write.s
.include "defines.h"
.data
hello:
	.string "hello world\n"

.globl	main
main:
	movl	$SYS_write,%eax
	movl	$STDOUT,%ebx
	movl	$hello,%ecx
	movl	$12,%edx
	int	$0x80

	ret
$ 

The same process applies to syscalls which have less than five args. Just leave the un-used registers unchanged. Syscalls such as open or fcntl which have an optional extra arg will know what to use.

Syscalls whos number of args is greater than five still expect the syscall number to be in %eax, but the args are arranged in memory and the pointer to the first arg is stored in %ebx.

If you are using the stack, args must be pushed onto it backwards, i.e. from the last arg to the first arg. Then the stack pointer should be copied to %ebx. Otherwise copy args to an allocated area of memory and store the address of the first arg in %ebx.

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>

#define STDOUT	1

void main(void) {
	char file[]="mmap.s";
	char *mappedptr;
	int fd,filelen;

	fd=fopen(file, O_RDONLY);
	filelen=lseek(fd,0,SEEK_END);
	mappedptr=mmap(NULL,filelen,PROT_READ,MAP_SHARED,fd,0);
	write(STDOUT, mappedptr, filelen);
	munmap(mappedptr, filelen);
	close(fd);
}

$ cat mmap.s
.include "defines.h"

.data
file:
	.string "mmap.s"
fd:
	.long 	0
filelen:
	.long 	0
mappedptr:
	.long 	0

.globl main
main:
	push	%ebp
	movl	%esp,%ebp
	subl	$24,%esp

//	open($file, $O_RDONLY);

	movl	$fd,%ebx	// save fd
	movl	%eax,(%ebx)

//	lseek($fd,0,$SEEK_END);

	movl	$filelen,%ebx	// save file length
	movl	%eax,(%ebx)

	xorl	%edx,%edx

//	mmap(NULL,$filelen,PROT_READ,MAP_SHARED,$fd,0);
	movl	%edx,(%esp)
	movl	%eax,4(%esp)	// file length still in %eax
	movl	$PROT_READ,8(%esp)
	movl	$MAP_SHARED,12(%esp)
	movl	$fd,%ebx	// load file descriptor
	movl	(%ebx),%eax
	movl	%eax,16(%esp)
	movl	%edx,20(%esp)
	movl	$SYS_mmap,%eax
	movl	%esp,%ebx
	int	$0x80

	movl	$mappedptr,%ebx	// save ptr
	movl	%eax,(%ebx)
		
// 	write($stdout, $mappedptr, $filelen);
//	munmap($mappedptr, $filelen);
//	close($fd);
	
	movl	%ebp,%esp
	popl	%ebp

	ret
$

Socket syscalls make use of only one syscall number: SYS_socketcall which goes in %eax. The socket functions are identified via a subfunction numbers located in /usr/include/linux/net.h and are stored in %ebx. A pointer to the syscall args is stored in %ecx. Socket syscalls are also executed with int $0x80.

$ cat socket.s
.include "defines.h"

.globl	_start
_start:
	pushl	%ebp
	movl	%esp,%ebp
	sub	$12,%esp

//	socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
	movl	$AF_INET,(%esp)
	movl	$SOCK_STREAM,4(%esp)
	movl	$IPPROTO_TCP,8(%esp)

	movl	$SYS_socketcall,%eax
	movl	$SYS_socketcall_socket,%ebx
	movl	%esp,%ecx
	int	$0x80

	movl 	$SYS_exit,%eax
	xorl 	%ebx,%ebx
	int 	$0x80

	movl	%ebp,%esp
	popl	%ebp
	ret
$

Command line arguments in linux executables are arranged on the stack. argc comes first, followed by an array of pointers (**argv) to the strings on the command line followed by a NULL pointer. Next comes an array of pointers to the environment (**envp). These are very simply obtained in asm, and this is demonstrated in the example code (args.s).

This section on GCC inline asm will only cover the x86 applications. Operand constraints will differ on other processors. The location of the listing will be at the end of this article.

Basic inline assembly in gcc is very straightforward. In its basic form it looks like this:

	__asm__("movl	%esp,%eax");	// look familiar ?

	__asm__("
			movl	$1,%eax		// SYS_exit
			xor	%ebx,%ebx
			int	$0x80
	");

It is possible to use it more effectively by specifying the data that will be used as input, output for the asm as well as which registers will be modified. No particular input/output/modify field is compulsory. It is of the format:

	__asm__("<asm routine>" : output : input : modify);

The output and input fields must consist of an operand constraint string followed by a C expression enclosed in parentheses. The output operand constraints must be preceded by an '=' which indicates that it is an output. There may be multiple outputs, inputs, and modified registers. Each "entry" should be separated by commas (',') and there should be no more than 10 entries total. The operand constraint string may either contain the full register name, or an abbreviation.

	__asm__("test	%%eax,%%eax", : /* no output */ : "a"(foo));

	__asm__("test	%%eax,%%eax", : /* no output */ : "eax"(foo));

You can also use the keyword __volatile__ after __asm__: "You can prevent an `asm' instruction from being deleted, moved significantly, or combined, by writing the keyword `volatile' after the `asm'."

$ cat inline1.c
#include <stdio.h>

int main(void) {
	int foo=10,bar=15;
	
	__asm__ __volatile__ ("addl 	%%ebxx,%%eax" 
		: "=eax"(foo) 		// ouput
		: "eax"(foo), "ebx"(bar)// input
		: "eax"			// modify
	);
	printf("foo+bar=%d\n", foo);
	return 0;
}
$

You may have noticed that registers are now prefixed with "%%" rather than '%'. This is necessary when using the output/input/modify fields because register aliases based on the extra fields can also be used. I will discuss these shortly.

Instead of writing "eax" and forcing the use of a particular register such as "eax" or "ax" or "al", you can simply specify "a". The same goes for the other general purpose registers (as shown in the Abbrev table). This seems useless when within the actual code you are using specific registers and hence gcc provides you with register aliases. There is a max of 10 (%0-%9) which is also the reason why only 10 inputs/outputs are allowed.

$ cat inline2.c
int main(void) {
	long eax;
	short bx;
	char cl;

	__asm__("nop;nop;nop"); // to separate inline asm from the rest of
				// the code
	__volatile__ __asm__("
		test	%0,%0
		test	%1,%1
		test	%2,%2"
		: /* no outputs */
		: "a"((long)eax), "b"((short)bx), "c"((char)cl)
	);
	__asm__("nop;nop;nop");
	return 0; 
}
$ gcc -o inline2 inline2.c 
$ gdb ./inline2
GNU gdb 4.18
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnulibc1"...
(no debugging symbols found)...
(gdb) disassemble main
Dump of assembler code for function main: 
... start: inline asm ... 
0x8048427 : nop
0x8048428 : nop 
0x8048429 : nop 
0x804842a : mov 0xfffffffc(%ebp),%eax 
0x804842d : mov 0xfffffffa(%ebp),%bx
0x8048431 : mov 0xfffffff9(%ebp),%cl 
0x8048434 : test %eax,%eax 
0x8048436 : test %bx,%bx
0x8048439 : test %cl,%cl 
0x804843b : nop 
0x804843c : nop 
0x804843d : nop 
... end: inline asm ... 
End of assembler dump. 
$ 

As you can see, the code that was generated from the inline asm loads the values of the variables into the registers they were assigned to in the input field and then proceeds to carry out the actual code. The compiler auto detects operand size from the size of the variables and so the corresponding registers are represented by the aliases %0, %1 and %2. (Specifying the operand size in the mnemonic when using the register aliases may cause errors while compiling).

The aliases may also be used in the operand constraints. This does not allow you to specify more than 10 entries in the input/output fields. The only use for this i can think of is when you specify the operand constraint as "q" which allows the compiler to choose between a,b,c,d registers. When this register is modified we will not know which register has been chosen and consequently cannot specify it in the modify field. In which case you can simply specify "<number>".

$ cat inline3.c
#include <stdio.h>

int main(void) {
	long eax=1,ebx=2;

	__asm__ __volatile__ ("add %0,%2"
		: "=b"((long)ebx)
		: "a"((long)eax), "q"(ebx)
		: "2"
	);
	printf("ebx=%x\n", ebx);
	return 0;
}
$

Compiling assembly language programs is much like compiling normal C programs. If your program looks like Listing 1, then you would compile it like you would a C app. If you use _start instead of main, like in Listing 2 you would compile the app slightly differently:

$ cat write.s
.data
hw:
	.string "hello world\n"
.text
.globl main
main:
	movl	$SYS_write,%eax
	movl	$1,%ebx
	movl	$hw,%ecx
	movl	$12,%edx
	int	$0x80
	movl	$SYS_exit,%eax
	xorl	%ebx,%ebx
	int	$0x80
	ret
$ gcc -o write write.s
$ wc -c ./write
   4790 ./write
$ strip ./write
$ wc -c ./write
   2556 ./write

$ cat write.s
.data
hw:
	.string "hello world\n"
.text
.globl _start
_start:
	movl	$SYS_write,%eax
	movl	$1,%ebx
	movl	$hw,%ecx
	movl	$12,%edx
	int	$0x80
	movl	$SYS_exit,%eax
	xorl	%ebx,%ebx
	int	$0x80

$ gcc -c write.s
$ ld -s -o write write.o
$ wc -c ./write
    408 ./write

The -s switch is optional, it just creates a stripped ELF executable which is smaller than a non-stripped one. This method (Listing 2) also creates smaller executables, since the compiler isnt adding extra entry and exit routines as would normally be the case.

Further reference.