GDB complains No Source Available

Question

I'm running on Ubuntu 12.10 64bit.

I am trying to debug a simple assembly program in GDB. However GDB's gui mode (-tui) seems unable to find the source code of my assembly file. I've rebuilt the project in the currently directory and searched google to no avail, please help me out here.

My commands:

nasm -f elf64 -g -F dwarf hello.asm

gcc -g hello.o -o hello

gdb -tui hello

Debug information seems to be loaded, I can set a breakpoint at main() but the top half the screen still says '[ No Source Available ]'.

Here is hello.asm if you're interested:

;  hello.asm  a first program for nasm for Linux, Intel, gcc
;
; assemble: nasm -f elf -l hello.lst  hello.asm
; link:     gcc -o hello  hello.o
; run:          hello 
; output is:    Hello World 

    SECTION .data       ; data section
msg:    db "Hello World",10 ; the string to print, 10=cr
len:    equ $-msg       ; "$" means "here"
                ; len is a value, not an address

    SECTION .text       ; code section
        global main     ; make label available to linker 
main:               ; standard  gcc  entry point

    mov edx,len     ; arg3, length of string to print
    mov ecx,msg     ; arg2, pointer to string
    mov ebx,1       ; arg1, where to write, screen
    mov eax,4       ; write command to int 80 hex
    int 0x80        ; interrupt 80 hex, call kernel

    mov ebx,0       ; exit code, 0=normal
    mov eax,1       ; exit command to kernel
    int 0x80        ; interrupt 80 hex, call kernel

Answer 1

This statement is false.

The assembler does produce line number information (note the -g -F dwarf) bits.

On the other hand he assembles what is obviously 32-bit code as 64 bits, which may or may not work.

Now if there are bugs in NASM's debugging output we need to know that.

A couple of quick experiments shows that addr2line (but not gdb!) does decode NASM-generated line number information correctly using stabs but not using dwarf, so there is probably something wrong in the way NASM generates DWARF... but also something odd with gdb.

GNU addr2line version 2.22.52.0.1-10.fc17 20120131, GNU gdb (GDB) Fedora (7.4.50.20120120-52.fc17)).

Answer 2

The problem in this case is that the assembler isn't producing line-number information for the debugger. So although the source is there (if you do "list" in gdb, it shows a listing of the source file - at least when I follow your steps, it does), but the debugger needs line-number information from the file to know what line corresponds to what address. It can't do that with the information given.

As far as I can find, there isn't a way to get NASM to issue the .loc directive that is used by as when using gcc for example. But as isn't able to take your source file without generating a gazillion errors [even with -msyntax=intel -mmnemonic=intel -- you would think that should work].

So unless someone more clever can come up with a way to generate the .loc entries which gives the debugger line number information, I'm not entirely sure how we can answer your question in a way that you'll be happy with.