I start gdb as following: gdb --args parentExecutable LotsOfArgsForParent
I also run: set follow-fork-mode child
parentExecutable forks at some point and executes a childExecutable with some arguments. I debug the child for a while. Then, I use the run command of gdb to restart the parentExecutable with the arguments given in the beginning. However, instead, the childExecutable restarts -- from scratch without any arguments. How can I make gdb start the parent with the arguments provided in the beginning?
There are actually two modes to pay attention to in this scenario. One mode is follow-fork-mode, which tells gdb what to do when the inferior forks. However, there is also follow-exec-mode, which tells gdb how to handle an exec call.
The default setting for follow-exec-mode is same, which tells gdb to reuse the current inferior for the exec'd process. In this situation, once the child process stops, run will re-run the child.
What you want, instead, is set follow-exec-mode new. In this mode, gdb will make a new inferior in response to the exec. Then, when you want to re-run the original executable, you can switch back to the first inferior (use info inferior to get a list and the inferior command to select one). Then run will re-run the original.
Another way to do all this is multi-inferior debugging, using set detach-on-fork off. However, in my experience, this mode is still a bit flaky. Once it works, though, I think it will be the preferred approach.
Related
I'd like to to debug a multiprocess C++ project with GDB, specifically I'd like to know if there is a way to achieve the following
Attach multiple processes to a single instance of GDB while letting all the processes run
Setting up a breakpoint in the source code of one of the processes stops all the attached processes
The ideal solution would be something similar to what is offered by the Visual Studio debugger as described here.
At the moment I'm able to attach multiple processes to a GDB instance but then only the current selected inferior is executed while the others are stopped and waiting for a continue command.
In order to be able to run inferiors in the background, one needs to issue this gdb command
set target-async on
after start up and before running anything. With this option in effect, one ca issue
continue&
(or just c&) and this will send the inferior to the background, giving an opportunity to switch to run another one.
Stopping all inferiors at once is a bit more difficult. There is no built-in command for that. Fortunately gdb is scriptable and it is possible to attach a script to a breakpoint. Once the breakpoint is hit, the commands are executed. Put inferior n and interrupt commands in the script for each inferior. It is probably more convenient to do that from a Python script, something like
(gdb) python
>inf = gdb.inferiors()
>for i in inf:
> gdb.execute("inferior %d" % i.num)
> gdb.execute("interrupt")
I am trying to do a stackoverflow for a course at university. The binary I am to exploit has a canary, however, there is a way to leak that canary to stdout. The canary of course consists of some random bytes so I can't just read them from the string that the program outputs to stdout.
For this reason I am using the python and pwntools like p.recv(timeout = 0.01).encode("hex").
(I'm using pwntools only because I don't know another way to read the output in hex format, if there is an easier way I can of course use something else)
This works more or less works as expected, I manage to write the memory area that is past the canary. However, I get a segfault, so I obviously have some problem with the stackoverflow I am causing. I need a way of debugging this, like seeing the stack after I provide the input that causes the stackoverflow.
And now without any further ado the actual question: Can I debug a process that I started with pwntools (like process("./myprog")) in GDB or some other program that can show me the content of the stack?
I already tried getting the pid in python and using gdb attach to attach to that pid, but that didn't work.
Note: The binary I am trying to exploit has the guid set. Don't know if that matters tho.
You can use the pwnlib.gdb to interface with gdb.
You can use the gdb.attach() function:
From the docs:
bash = process('bash')
# Attach the debugger
gdb.attach(bash, '''
set follow-fork-mode child
break execve
continue
''')
# Interact with the process
bash.sendline('whoami')
or you can use gdb.debug():
# Create a new process, and stop it at 'main'
io = gdb.debug('bash', '''
# Wait until we hit the main executable's entry point
break _start
continue
# Now set breakpoint on shared library routines
break malloc
break free
continue
''')
# Send a command to Bash
io.sendline("echo hello")
# Interact with the process
io.interactive()
The pwntools template contains code to get you started with debugging with gdb. You can create the pwntools template by running pwn template ./binary_name > template.py. Then you have to add the GDB arg when you run template.py to debug: ./template.py GDB.
If you get [ERROR] Could not find a terminal binary to use., you might need to set context.terminal before you use gdb.
If you're using tmux, the following will automatically open up a gdb debugging session in a new horizontally split window:
context.terminal = ["tmux", "splitw", "-h"]
And to split the screen with the new gdb session window vertically:
context.terminal = ["tmux", "splitw", "-v"]
(Note: I never got this part working, so idk if it'll work. Tell me if you get the gdb thing working).
(To use tmux, install tmux on your machine, and then just type tmux to start it. Then type python template.py GDB.
If none of the above works, then you can always just start your script, use ps aux, find the PID, and then use gdb -p PID to attach to the running process.
I have multiple different processes communicating over IPC and when debugging a single process using gdb, whenever a breakpoint is hit, I am trying to send a message to other processes. Is there a way to automatically invoke a function/piece of code (NotifyAll()) whenever a breakpoint is hit without manually running commands and invoking the function in the gdb console.
Basically, whenever a gdb debugger is attached to one of these processes, I want gdb to know that it should invoke NotifyAll() whenever a breakpoint (application-wide) is hit.
Yes, this can be done using the Python scripting capabilities in gdb.
In particular you want to add a listener to gdb.events.stop that checks for a breakpoint stop event, then calls your function. It's possible (I don't know offhand) that you'll have to defer the calling of the function by posting an event to the gdb event loop.
To make this work with the minimum of manual intervention, use the gdb script auto-loading feature to associate this Python script with your application. This will require users to trust the script (read about add-auto-load-safe-path), but that's all.
Note that doing things like this is potentially confusing to people trying to debug your application. For example, setting a breakpoint in the RPC code will cause problems unless your script takes extra care.
I'm currently debugging syslinux (a boot loader) through the gdb stub of qemu.
Recently, I wrote some gdb commands that (un)load the debug symbols everytime a module is dynamically (un)loaded. In order not to disrupt the execution, I ended the commands with continue.
break com32/lib/sys/module/elf_module.c:282
commands
silent
python
name = gdb.parse_and_eval("module->name").string()
addr = int(str(gdb.parse_and_eval("module->base_addr")), 0)
gdb.execute("load-syslinux-module %s 0x%08x" % (name, addr))
end
continue
end
However, when stepping through the code line by line, if the next or step command makes the execution hit the breakpoint, the breakpoints takes precedence, the commands are executed, including the continue. And the execution continue irrespectively of the line-by-line debugging I was doign. This also happen if I try to step over the function that has this breakpoint.
How can I keep (un)loading the debug symbols on the fly while not interfering with the debugging?
Is there an alternative to the continue command? Maybe using breakpoints isn't the right way? I'd take any solution.
This can't be done from the gdb CLI. However, it is easy to do from Python.
In Python the simplest way is to define one's own gdb.Breakpoint subclass, and define the stop method on it. This method can do the work you like, then return False to tell gdb to continue.
The stop facility was designed to avoid the problems with cont in commands. See the documentation for more details.
How do I stop a GDB execution without a breakpoint?
Just use a regular interrupt Ctrl-c will work just fine. GDB just forwards the SIGINT to the debugging process which then dies. GDB will catch the non-standard exit and break the process there, so you can still examine all the threads, their stacks and current values of variables. This works fine, though you would be better off using break points. The only time I find myself doing this is, if I think I've gotten into some sort of infinite loop.
GUI applications don't react to ^C and ^Break the way console applications do. Since these days most non-trivial projects tend to be GUI applications or libraries primarily used in GUI applications, you have two options:
Send SIGSTOP to the application from a separate terminal. This is cumbersome.
If you press ^C or ^Break on the GDB prompt, GDB will terminate but the application will remain running. You can then run GDB again to attach to it using the -p command-line switch. This loses debugger state.
In both cases, you might find this helpful: tasklist | grepProcessName| sed -e 's/ProcessName*\([0-9]*\).*/gdbModuleName-pid=\1/' > rungdb.sh You can modify this for use in shell scripts, makefiles or to send a signal instead of attaching GDB.
info threads will help you figure out which thread you want to look at. Then use threadThreadNumber to switch to it.
Start a shell, find the process ID using ps and send it SIGSTOP or SIGINT by using the kill command (e.g. kill -INT pid).
Just type BREAK without any arguments.
Break, when called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame
Ctrl + Z seems to work for me (but only in some cases - I'm not sure why).