Context failing on breakpoints with commands

[yoava333]

Description

When using commands in breakpoints with pwndbg the debugger will halt the execution of the debugee and will not respond to keyboard commands.
The same gdb script works without pwndbg (e.g. commenting out in ~/.gdbinit)

Steps to reproduce

wrote the following gdb script (gdb.cmds):

b malloc
commands
    printf "size = %d\n", $rdi
    c
end

run the following command:
gdb /usr/bin/python -x gdb.cmds

and type "r"

the output

user@ubuntu /tmp> gdb /usr/bin/python -x gdb.cmds
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Loaded 108 commands. Type pwndbg [filter] for a list.
Reading symbols from /usr/bin/python...(no debugging symbols found)...done.
Breakpoint 1 at 0x4168e0
pwndbg> r
Starting program: /usr/bin/python

Breakpoint 1, malloc (n=1425) at dl-minimal.c:94
94      dl-minimal.c: No such file or directory.
size = 1425



^C^C

BTW if you add the "r" commands to the end of gdb.cmds it works fine.

My setup

Ubuntu 16.04.02
pwndbg - dae257c
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1
python - 3.5.2 (default, Nov 17 2016, 17:05:23) [GCC 5.4.0 20160609]

[grant-h]

I am having this exact issue and it is preventing me from debugging function calls in the same way. This is a serious bug if any scripting for silent, print-only breakpoints is being done.

pwndbg stops responding to any input and the GDB process has to be killed.

[disconnect3d]

Hey,

Sorry for so long time with no response. Just for future: please add a minimal example that can be used to reproduce the bug as it would be nice to be able to reproduce and debug the issue (also my Python binary says "no malloc there").

Anyway, I have reproduced your case with simple C code:

#include <stdlib.h>

int main() {
    malloc(100);
}

First of all - the latest pwndbg does show much better result for it - it shows that an exception occured and that you can use set exceptions-verbose on to make it more verbose.

So after setting verbose exceptions, it turns out that we fail on trying to display context:

$ gdb ./a.out -x a.cmds
Loaded 108 commands. Type pwndbg [filter] for a list.
Reading symbols from ./a.out...(no debugging symbols found)...done.
Breakpoint 1 at 0x560
pwndbg> set exception-verbose on
Set whether to print a full stacktracefor exceptions raised in Pwndbg commands to True
pwndbg> r
Starting program: /home/dc/test/a.out 
size = 1425
Selected thread is running.
'context': Print out the current register, instruction, and stack context.

    Accepts subcommands 'reg', 'disasm', 'code', 'stack', 'backtrace', and 'args'.
Traceback (most recent call last):
  File "/home/dc/installed/pwndbg/pwndbg/commands/__init__.py", line 99, in __call__
    return self.function(*args, **kwargs)
  File "/home/dc/installed/pwndbg/pwndbg/commands/__init__.py", line 191, in _OnlyWhenRunning
    return function(*a, **kw)
  File "/home/dc/installed/pwndbg/pwndbg/commands/context.py", line 64, in context
    result.extend(func())
  File "/home/dc/installed/pwndbg/pwndbg/commands/context.py", line 77, in context_regs
    result.extend(get_regs())
  File "/home/dc/installed/pwndbg/pwndbg/commands/context.py", line 120, in get_regs
    desc = pwndbg.chain.format(value)
  File "/home/dc/installed/pwndbg/pwndbg/chain.py", line 82, in format
    chain = get(value, limit, offset, hard_stop, hard_end)
  File "/home/dc/installed/pwndbg/pwndbg/chain.py", line 48, in get
    address = int(pwndbg.memory.poi(pwndbg.typeinfo.ppvoid, address + offset))
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

If that is an issue, you can report it on https://github.com/pwndbg/pwndbg/issues
(Please don't forget to search if it hasn't been reported before)
PS: Pull requests are welcome
pwndbg> size = 1169
Selected thread is running.
Traceback (most recent call last):
  File "/home/dc/installed/pwndbg/pwndbg/commands/__init__.py", line 99, in __call__
    return self.function(*args, **kwargs)
  File "/home/dc/installed/pwndbg/pwndbg/commands/__init__.py", line 191, in _OnlyWhenRunning
    return function(*a, **kw)
  File "/home/dc/installed/pwndbg/pwndbg/commands/context.py", line 64, in context
    result.extend(func())
  File "/home/dc/installed/pwndbg/pwndbg/commands/context.py", line 234, in context_backtrace
    this_frame    = gdb.selected_frame()
gdb.error: No frame is currently selected.

The issue is here because we don't handle situation when gdb.selected_thread().is_stopped() returns False.

Such situation happens when command being attached to a breakpoint is being executed, and it seems that in such cases, our prompt_hook is launched.

pwndbg> help command
Set commands to be executed when the given breakpoints are hit.
Give a space-separated breakpoint list as argument after "commands".
A list element can be a breakpoint number (e.g. `5') or a range of numbers
(e.g. `5-7').
With no argument, the targeted breakpoint is the last one set.
The commands themselves follow starting on the next line.
Type a line containing "end" to indicate the end of them.
Give "silent" as the first line to make the breakpoint silent;
then no output is printed when it is hit, except what the commands print.

[disconnect3d]

The fix has been proposed in PR #324

If someone needs that ASAP - just cherry-pick the commit from my repo.

[disconnect3d]

Merged to both dev & stable.