[lldb-dev] starting a process in stop-at-entry-point mode on Linux
Todd Fiala
todd.fiala at gmail.com
Thu Jul 24 14:24:42 PDT 2014
I'm going to go ahead and step through it on MacOSX to make sure I'm not
mis-representing what's happening there. I may be seeing a path on the
Linux side that truly manifests differently than I laid out above - enough
of the behavior is conditional on predicates that are plugin specific that
MacOSX may be following a different branch than I thought.
In any event, this is all useful info - thanks!
-Todd
On Thu, Jul 24, 2014 at 2:12 PM, <jingham at apple.com> wrote:
> When you connect to debugserver from lldb, and it doesn't have a process
> running, you can already just tell it to "run" and it will start up the
> target process. That was actually the default in lldb for quite some time.
> I don't remember now why we switched to having lldb launch things and then
> attach debugserver to them, do you Greg?
>
> Jim
>
> > On Jul 24, 2014, at 2:00 PM, Todd Fiala <tfiala at google.com> wrote:
> >
> > Ah I see. Yeah that would work around the issues we have on Linux where
> different kernel versions differ on the handling of detaching from ptrace
> during a group stop (which is where I was originally going to start - have
> lldb fork/exec/ptrace, then detach, leaving process stopped, and hand off
> to llgs).
> >
> > So in that case:
> >
> > lldb can do the fork/exec, pass the inferior pid to llgs, wait for that
> to attach, lldb does the write to the pipe and close after this point, and
> then everything can move along merrily.
> >
> > All that said, this does sound way more complicated than just modifying
> the flow to have llgs just launch the exe in the first place like it
> already can.
> >
> > Greg - is there any reason it would be trouble for you guys if we
> modified the local debugging flow to have debugserver/llgs launch the
> inferior? Then this would just work in both places without having to jump
> through implementing the stop-on-launch bits on Linux and anywhere else
> that didn't have that startup mode?
> >
> > -Todd
> >
> >
> > On Thu, Jul 24, 2014 at 1:40 PM, Greg Clayton <gclayton at apple.com>
> wrote:
> > Richard is suggesting you:
> >
> > - open a pipe
> > - fork()
> > - child: close the write file descriptor and read from the read end
> (which blocks waiting for the parent)
> > - parent: close the read file descriptor
> > - parent: ptrace(PT_ATTACH, child_pid, ...)
> > - parent: write a byte to write end of the pipe and close write end
> > - child: read a byte from the read end and close the read end of pipe
> >
> >
> > > On Jul 24, 2014, at 1:28 PM, Todd Fiala <tfiala at google.com> wrote:
> > >
> > > 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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> > > lldb-dev at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> > >
> > >
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> > >
> > >
> > >
> > > --
> > > Todd Fiala | Software Engineer | tfiala at google.com |
> 650-943-3180
> > >
> > > _______________________________________________
> > > lldb-dev mailing list
> > > lldb-dev at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> >
> >
> >
> >
> > --
> > Todd Fiala | Software Engineer | tfiala at google.com |
> 650-943-3180
> >
> > _______________________________________________
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> > lldb-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>
>
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--
-Todd
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