To debug C program on assembly level

[Nicholas Mc Guire]

On Fri, 10 Jul 2015, Andrew Haley wrote:

> On 07/10/2015 06:03 PM, xparmenides wrote:
> > 1. To debug at assembly level, the steps to compile is so complicated 
> > that is somewhat impractical. Is there any way make the process easily?
> 
> Yes.  Compile your program C in the normal way, then:
>
if you compile your c files with
 gcc -S --verbose-asm file.c

you will get much more readable assembly output than if you use the
normal disasseble output within gcc.

e.g:
<snip>
devkmsg_poll:
        pushq   %rbp    #
        movl    $40, %eax       #, D.38788
        movq    %rsp, %rbp      #,
        pushq   %rbx    #
        subq    $8, %rsp        #,
        movq    208(%rdi), %rbx # file_3(D)->private_data, user
        testq   %rbx, %rbx      # user
        je      .L27    #,
        testq   %rsi, %rsi      # wait
        je      .L28    #,
        movq    (%rsi), %rax    # wait_7(D)->_qproc, D.41321
        testq   %rax, %rax      # D.41321
<snip>
telling you what is being put in what register and what intermediate
values gcc added in (like D.41321 here)

furhter there are methods to interleave C and assembly code - atleast in
the linux kernel build system  make path/file.lst  will generate
the interleaved assembly/C files

e.g.
<snip>
static ssize_t msg_print_ext_body(char *buf, size_t size,
                                  char *dict, size_t dict_len,
                                  char *text, size_t text_len)
{
ffffffff81f40f55:       41 81 fc 00 00 02 00    cmp    $0x20000,%r12d
        char *p = buf, *e = buf + size;
        size_t i;

        /* escape non-printable characters */
        for (i = 0; i < text_len; i++) {
ffffffff81f40f5c:       76 44                   jbe    ffffffff81f40fa2 <setup_log_buf+0x94>
ffffffff81f40f5e:       be 00 10 00 00          mov    $0x1000,%esi

        return scnprintf(buf, size, "%u,%llu,%llu,%c;",
                       (msg->facility << 3) | msg->level, seq, ts_usec, cont);
}
<snip>

see the Linux kernel scripts/makelst for more.

notably when debugging optimized code this output makes it much easier to
read the disassembler output in gdb.

thx!
hofrat