hoang
Uploaded by
16 SLIDES
321 VUES
160LIKES

Practical Session 5

DESCRIPTION

Practical Session 5. GNU Linker. Links object files together Used as the last step in the compilation We will use ld to link together compiled assembly without using c. Usage: nasm –f elf myprog.s –o myprog.o ld -m elf_i386 myprog.o –o myprog.bin. Command Line Arguments in NASM.

1 / 16

Télécharger la présentation

Practical Session 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Practical Session 5

  2. GNU Linker • Links object files together • Used as the last step in the compilation • We will use ld to link together compiled assembly without using c. • Usage: • nasm –f elf myprog.s –o myprog.o • ld -m elf_i386 myprog.o –o myprog.bin

  3. Command Line Arguments in NASM

  4. Command-line arguments In Linux, we receive command-line arguments on the stack as execution starts:- The first argument; number of arguments (i.e. argc) - The rest of the arguments; each one is a pointer to an argument string. (i.e. argv[0], argv[1] argv[2])

  5. Function calls in NASM

  6. Caller side • The caller pushes the function's parameters on the stack, one after another, in reverse order. • The caller executes a CALL instruction to pass control to the callee. This CALL is either near or far depending on the memory model. • The address of the next instruction is pushed onto the stack as the return address.

  7. Function _start Stack structure dword EBP argc dword Ret add (offset) argv[0] argv[1] Param #1 Param #2 argv[2] … …

  8. Function call example STR: DB ‘Printing two ints: %d, %d’,10,0 … MOV EAX, 15 PUSH EAX ; 3rd parameter PUSH DWORD 20 ; 2nd parameter PUSH DWORD STR ; 1st parameter CALL PRINTF ADD ESP, 12 ; 3 dwords = 12 bytes

  9. Stack contents ; how to get parameters PUSH EBP MOV EBP, ESP MOV EAX, dword [EBP+8] ; 1st parameter MOV EBX, dword [EBP+12] ; 2nd parameter MOV ECX, dword [EBP+16] ; 3rd parameter … … dword EBP Ret add (offset) dword dword STR dword 20 15 dword

  10. section .rodata error_string: db "Not enough arguments!",10,0 section .bss input_string_ptr: resd 1 ; Will contain the pointer to the input string output_string: resb 256 ; Will contain the actual string after copy section .text global _start _start: pop ebx ; Contains the number of arguments (Including argv[0]) cmpebx, 2 jlprint_error pop ebx ; First argument: Name of the program pop ebx ; The first argument - Our argument movdword [input_string_ptr], ebx push dword [input_string_ptr] push dwordoutput_string callcpy_string ; Copy the string to our own buffer add esp, 8 Print String

  11. Print String push dwordoutput_string callinsert_linefeed ; Add a linefeed character (not present in the input) add esp, 4 push dwordoutput_string callmy_print ; Print the output string add esp, 4 mov ebx,0 ; Exit with return code of 0 (no error) proc_exit: mov eax,1 ; The system call for exit (sys_exit) int 80h print_error: ; This is not a function! Just a jmp destination! push error_string callmy_print add esp, 4 movebx, 1 ; Exit with return code of 1 (error) jmpproc_exit

  12. ;;;;; This function calculates the length of a string ;;;;; Parameter 1: A pointer to a string ;;;;; Return Value: Integer calc_str_len: push ebp movebp, esp push ebx push ecx movebx, dword [ebp+8] ; The input in ebx moveax, 0 ; The result will be in eax len_next_char: movcl, byte [ebx] cmpcl, 0 je len_next_char_end inc eax inc ebx jmplen_next_char len_next_char_end: pop ecx pop ebx movesp, ebp pop ebp ret calc_str_len

  13. ;;;;; This function prints an input string. Note: input string must end with 0! ;;;;; Parameter 1: A pointer to a string ;;;;; Return Value: VOID my_print: push ebp movebp, esp pushad movecx, dword [ebp+8] push ecx callcalc_str_len ; Calculate the length of the input string add esp, 4 movedx, eax moveax, 4 movebx, 1 int 80h popad movesp, ebp pop ebp ret my_print

  14. ;;;;; This function will copy a string from an input memory buffer to another ;;;;; Parameter 1: Destination Buffer (Address) ;;;;; Parameter 2: Source Buffer (Address) ;;;;; Return Value: VOID cpy_string: push ebp movebp, esp pushad movebx, dword [ebp+8] ; Pointer to Destination movecx, dword [ebp+12] ; Pointer to Source mov dl, byte[ecx] cpy_next_char: mov byte [ebx], dl cmp dl, 0 je cpy_next_char_end inc ebx inc ecx mov dl, byte[ecx] jmpcpy_next_char cpy_next_char_end: popad movesp, ebp pop ebp ret cpy_string

  15. ;;;;; This function will add a linefeed character to a string in a memory buffer ;;;;; Parameter 1: Address of memory buffer containing the string ;;;;; Return Value: VOID insert_linefeed: push ebp movebp, esp pushad movebx, dword [ebp+8] push ebx callcalc_str_len add esp,4 add ebx, eax ; Go to the end of the string mov byte [ebx], 10 ; Add linefeed inc ebx mov byte [ebx], 0 ; Add null terminal popad movesp, ebp pop ebp ret insert_linefeed

  16. ld vs. gcc main _start This is just like C’s main(int argc, char* argv[]) argc Ret address argv[0] argc &{argv[0],arg[1],…} argv[1] argv[2] … …

More Related