[lldb-dev] starting a process in stop-at-entry-point mode on Linux

[Todd Fiala]

Hi Richard,

Can you say more about this part:
> is to simply use a pipe between the parent and child to indicate when
they're both ready.

I think I assumed too much context with Greg -

The exact scenario we're talking about is:

1. Start up a process, going to be debugged, needs to be stopped at
very-very start point (i.e. c/c++ library entry point).  Debugger will pick
it up from here.

2. Apple's flow for this when debugging over debugserver on a local process
is to start the exe up from lldb, in a mode that starts the process but
keeps it in (essentially) a group-stop state at that first entry point.
 Then they have debugserver attach to that process.

I could try to rewire that scenario for Linux/* (which I had originally
considered and may eventually get back to that point) to have llgs
configured and then have llgs launch the process, which would work right
now.  However, at the moment I'd like to see if I can not monkey around too
much with the flow of starting a local llgs-debugged exe and just follow
MacOSX's flow here.

Does your idea still fit into that context?

-Todd



On Thu, Jul 24, 2014 at 12:46 PM, Richard Mitton <richard at codersnotes.com>
wrote:

> A better way, rather than messing with signals, is to simply use a pipe
> between the parent and child to indicate when they're both ready.
>
> Richard Mitton
> richard at codersnotes.com
>
>
> On 07/24/2014 11:28 AM, Greg Clayton wrote:
>
>> On Jul 23, 2014, at 4:44 PM, Todd R. Fiala <todd.fiala at gmail.com> wrote:
>>>
>>> Hey Greg,
>>>
>>> I’m looking into implementing process start-up so I can just follow your
>>> flow of launching the exe, then connecting via llgs with reverse connect.
>>>  I’d like to take a shot at getting Linux processes to start up in a
>>> “stopped at initial entry point” behavior rather than doing something
>>> different than MacOSX at this point.
>>>
>>> Linux PTRACE doesn’t provide this out of the box.  There is an
>>> alternative that could work but is not reliable across Linux kernel
>>> versions: detaching from a PTRACE’d exe during a group-stop will leave it
>>> stopped.  I think the initial startup signal I get with PTRACE may yield a
>>> group stop.  If it’s not, I can immediately turn around and issue a stop,
>>> deliver that and get the real group stop.  (Not 100% sure I could do that
>>> last part with guaranteed no-execution semantics at the entry point
>>> location).  Unfortunately, the detach is not reliable everywhere I need
>>> this to run to keep the process in a stopped state at that point for
>>> handoff to llgs.
>>> What I could do instead, is fork, and have the child process self-send a
>>> SIGSTOP before doing the exec.  And, on Linux (and maybe FreeBSD), when
>>> llgs attaches, it just needs to know that it has to wait for one exec
>>> signal before the process really starts. (I’m not sure if there is a shell
>>> mode for debugging with Linux - if there is, then we need to exec through
>>> the shell script too, I think — I don’t remember seeing that on the linux
>>> code path so it likely is buggy and/or unsupported at the moment).  In any
>>> event, if I do this, I’m pretty sure I can guarantee that I can start a
>>> process in debug-ready mode with the caveat that there is an exec that has
>>> to be silently ignored when llgs attaches.
>>>
>>> How does that sound?  Thoughts?
>>>
>> The idea sounds good, as you can just do:
>>
>> kill(getpid(), SIGSTOP)
>>
>> but you can run into problems with the foreground app in a terminal not
>> being able to SIGSTOP itself and you might get some SIGTTOU or SIGTTIN
>> signals.
>>
>> But do try it out. The ProcessLaunchInfo have:
>>
>>          uint32_t
>>          GetResumeCount () const
>>          {
>>              return m_resume_count;
>>          }
>>
>>          void
>>          SetResumeCount (uint32_t c)
>>          {
>>              m_resume_count = c;
>>          }
>>
>>
>> So you just need to set this to 1 and it will continue past 1 exec
>> without trying to read the dyld info for any execs before m_resume_count
>> gets to zero.
>>
>> Greg
>>
>>
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-- 
Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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