How to debug a NASM build program that requires user input?

Question

I walk through the studio step by step , and I am in chapter 8, where it shows an example of a build program that receives a file from the user this way:

SECTION .bss ; Section containing uninitialized data BUFFLEN equ 1024 ; Length of buffer Buff: resb BUFFLEN ; Text buffer itself 

it reads the text of the file in Buff and displays the version of this text in ALL CAPS in another file.

I want to run this program in debug mode in order to go through it in order to analyze what happens with all the registers.

I am running this on ubuntu using INSIGHT.

I am a complete newbie. I know how to use Insight to navigate, but the way the user should run this program:

 myProgram > outputfile.txt < inputfile.txt 

How do I imitate this in the debugger?

here is the full source:

 ; Executable name : uppercaser2 ; Version : 1.0 ; Created date : 3/25/2009 ; Last update : 3/25/2009 ; Author : Jeff Duntemann ; Description : A simple program in assembly for Linux, using NASM 2.05, ; demonstrating simple text file I/O (through redirection) for reading an ; input file to a buffer in blocks, forcing lowercase characters to ; uppercase, and writing the modified buffer to an output file. ; ; Run it this way: ; uppercaser2 > (output file) < (input file) ; ; Build using these commands: ; nasm -f elf -g -F stabs uppercaser2.asm ; ld -o uppercaser2 uppercaser2.o ; SECTION .bss ; Section containing uninitialized data BUFFLEN equ 1024 ; Length of buffer Buff: resb BUFFLEN ; Text buffer itself SECTION .data ; Section containing initialised data SECTION .text ; Section containing code global _start ; Linker needs this to find the entry point! _start: nop ; This no-op keeps gdb happy... ; Read a buffer full of text from stdin: read: mov eax,3 ; Specify sys_read call mov ebx,0 ; Specify File Descriptor 0: Standard Input mov ecx,Buff ; Pass offset of the buffer to read to mov edx,BUFFLEN ; Pass number of bytes to read at one pass int 80h ; Call sys_read to fill the buffer mov esi,eax ; Copy sys_read return value for safekeeping cmp eax,0 ; If eax=0, sys_read reached EOF on stdin je Done ; Jump If Equal (to 0, from compare) ; Set up the registers for the process buffer step: mov ecx,esi ; Place the number of bytes read into ecx mov ebp,Buff ; Place address of buffer into ebp dec ebp ; Adjust count to offset ; Go through the buffer and convert lowercase to uppercase characters: Scan: cmp byte [ebp+ecx],61h ; Test input char against lowercase 'a' jb Next ; If below 'a' in ASCII, not lowercase cmp byte [ebp+ecx],7Ah ; Test input char against lowercase 'z' ja Next ; If above 'z' in ASCII, not lowercase ; At this point, we have a lowercase char sub byte [ebp+ecx],20h ; Subtract 20h to give uppercase... Next: dec ecx ; Decrement counter jnz Scan ; If characters remain, loop back ; Write the buffer full of processed text to stdout: Write: mov eax,4 ; Specify sys_write call mov ebx,1 ; Specify File Descriptor 1: Standard output mov ecx,Buff ; Pass offset of the buffer mov edx,esi ; Pass the # of bytes of data in the buffer int 80h ; Make kernel call jmp read ; Loop back and load another buffer full ; All done! Let end this party: Done: mov eax,1 ; Code for Exit Syscall mov ebx,0 ; Return a code of zero int 80H ; Make kernel call 

Answer 1

It looks like you want to attach a process to GDB. You can try this.

 shell$ gdb ./uppercaser2 gdb> list gdb> break read gdb> run > ouput.txt < input.txt gdb> x/5i $eip 

At the command line, you start GDB and attach uppercaser2 to GDB. Debugging symbols should load, and you can check using the list to display your source. Make a breakpoint in the right place by line number or function name. use gdb to run the program with your input and output files. From here you can analyze the registers and do it step by step using the GDB commands.

Answer 2

In the start command, you set user input as follows:

 shell$ gdb ./executable gdb> break main gdb> run user input