gdb, fork(), exec()

jason_m · 08-27-2010, 05:05 PM

I'm looking for a little help on using gdb to debug a program. The program is very similar to the one here.

The program creates a child process with fork(), which then executes a new file using execve(). The file that execve() loads up is compiled hand-written assembly compiled with "as -g".

I can get gdb to step into the child process after the call to fork() by issuing the command "set follow-fork-mode child". I've played with "set follow-exec-mode new", but that doesn't seem to be impacting what happens after I step past/into the execve() call.

The basic layout of what I am doing in gdb is as follows:

Code:

$ gdb parent_program

(gdb) break main
(gdb) run
(gdb) break assembly_program.s:9
No source file named assembly_program.s.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 2 (assembly_program.s:9) pending.
(gdb) set follow-fork-mode child
(gdb) set follow-exec-mode new

stepping into the execve() line

Code:

(gdb) s
process 6629 is executing new program: /home/jason/Development/asm/assembly_program

Program exited normally.

I read that re-issuing the "run" command might restart the program loaded by execve(), so issuing "run":

Code:

(gdb) run
Starting program: /home/jason/Development/asm/assembly_program

Program received signal SIGSEGV, Segmentation fault.
_start () at assembly_program.s:19
(gdb)

I have also tried setting the break point after the first program completes and prior to re-issuing the run command:

Code:

[New process 6638]
process 6638 is executing new program: /home/jason/Development/asm/assembly_program

Program exited normally.
(gdb) break assembly_program.s:9
Note: breakpoint -11 also set at pc 0x4000b0.
Breakpoint 2 at 0x4000b0: file assembly_program.s, line 9.
(gdb) run
Starting program: /home/jason/Development/asm/assembly_program

Program received signal SIGSEGV, Segmentation fault.
_start () at assembly_program.s:19
(gdb)

But either way, gdb blows right past my breakpoint. Any thoughts on how to debug this program?

vose · 08-28-2010, 11:03 AM

I have no idea whether this would help, but...

I would try to change my assembly code so that the first thing it does is (something like) go into an infinite loop (perhaps you want something nice in the loop, like sleep or yield) -- such that the loop would terminate if the value of a variable in main memory were different from what it was.

Next run the code, which eventually forks, etc., and eventually executes your modified assembly code and gets stuck in the infinite loop.

Fire up ddd and attach to the process that corresponds to the executing assembly (which is stuck in the infinite loop). Set a breakpoint after the loop. Set the value of the variable in main memory such that the loop will be exited. Continue (in ddd) and perhaps then -- fingers crossed -- your breakpoint will be encountered and not ignored.