[lldb-dev] LLDB Process Attach Failed When Waiting

Tue Jun 5 19:51:21 PDT 2018

Thank you that was a huge help.  I'm making some progress now.

Though I wonder, is there any documentation of the GDB packet format? The reason I ask is that I'm trying to figure out to get the process name from vAttach.

I've looked at how debugserver does it, namely the method GetProcessNameFrom_vAttach [1], and I cannot seem to find the equivalent method in StringExtractorGDBRemote or StringExtractor (though I'll admit that it might be obvious and I'm just missing it). If that's the case I imagine I'll have to implement it and I would like to at least understand the packet format to do that.

[1] https://github.com/apple/swift-lldb/blob/a8c149f75a8cba674bead048cd9c80ddc8166a8a/tools/debugserver/source/RNBRemote.cpp#L3585

On Tue, Jun 5, 2018, at 9:03 PM, Jim Ingham wrote:
> Except for Windows and FreeBSD, lldb uses a server program to do the 
> actual debugging - either debugserver on Darwin or lldb-server 
> elsewhere.  The interface to these servers (and to the in-process 
> debugging in Windows & FreeBSD) is abstracted being Process Plugins, 
> which the generic code uses.  Target::Attach is in generic code, it 
> won't know anything about the actual method used to attach, wait for 
> attach, whatever.  That will be dispatched to the currently active 
> ProcessPlugin.
> 
> ProcessGDBRemote is the plugin that is used on Linux & Darwin.  It is 
> the code that actually talks to debugserver or lldb-rpc-server from 
> within lldb.  And that's the code in lldb that sends the vAttachWait 
> gdb-remote protocol packet to instruct the above-mentioned servers to 
> implement attach-wait.  That request works on Darwin because debugserver 
> handles the vAttachWait packet, but doesn't work on Linux because lldb-
> rpc-server doesn't know currently respond to the vAttachWait packet.  So 
> all you should need to do is teach lldb-server to handle the vAttachWait 
> packet.
> 
> Jim
>   
> 
> > On Jun 5, 2018, at 5:44 PM, Ryan Lovelett via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> > 
> > I think I might be a little lost. I built a lldb in debug mode and I am running lldb in an lldb debugger (very meta).
> > 
> > Earlier in the thread you said "we need a better error message when vAttachWait returns unsupported" I have found where the error message, e.g., "error: attach failed: lost connection" is constructed. The "attach failed: " comes from here [1] and the "lost connection" comes from here [2].
> > 
> > What do you mean by "vAttachWait"? Am I missing something obvious?
> > 
> > It seems like you are expecting lldb-server to be the place where the fix will be implemented. Though in the debugger I'm seeing the method Target::Attach() [3] as the place where the connection attempt is made and fails.
> > 
> > [1] https://github.com/apple/swift-lldb/blob/a8c149f75a8cba674bead048cd9c80ddc8166a8a/source/Commands/CommandObjectProcess.cpp#L518
> > [2] https://github.com/apple/swift-lldb/blob/a8c149f75a8cba674bead048cd9c80ddc8166a8a/source/Target/Target.cpp#L3444-L3445
> > [3] https://github.com/apple/swift-lldb/blob/a8c149f75a8cba674bead048cd9c80ddc8166a8a/source/Target/Target.cpp#L3374
> > 
> > On Mon, Jun 4, 2018, at 7:04 PM, Greg Clayton wrote:
> >> I will be too busy this week to get to this, so please do have a stab at it.
> >> 
> >> Basically the flow that debug server does is:
> >> 1 - get a list of all processes whose basename matches and remember those pids so we don't try to attach to them since we are waiting for a new process to show up
> >> 2 - poll the OS for the process list and wait for a new pid to show up with the basename and attach to the first one that matches whose pid isn't i the list from step #1
> >> 
> >> On MacOSX we don't have a way to be notified of new processes are spawned, not sure on other unix variants. If it is possible, we would want change to:
> >> 1 - sign up to be notified about new processes
> >> 2 - as each process gets launched, check for a match and attach as quickly as possible
> >> 
> >> Hope this helps and I look forward to seeing your patch!
> >> 
> >> Greg
> >> 
> >>> On Jun 4, 2018, at 5:56 AM, Ryan Lovelett <ryan at lovelett.me> wrote:
> >>> 
> >>> Greg,
> >>> 
> >>> Is there anything I can do to help you implement or test this feature? Obviously I'm willing to roll-up my sleeves and work on this myself too if you've become more busy than you expected. That happens to us all and I completely understand.
> >>> 
> >>> Not being able to debug in this manner is blocking me from adding Linux support to a Swift program. As you might imagine, I'm a little antsy to get past this so I can start writing some code. 
> >>> 
> >>> On Thu, May 31, 2018, at 5:08 PM, Greg Clayton wrote:
> >>>> 
> >>>> 
> >>>>> On May 31, 2018, at 12:40 PM, Ryan Lovelett <ryan at lovelett.me> wrote:
> >>>>> 
> >>>>> Well at least there is a little good news. I understood way more of those logs than I thought I did.
> >>>>> 
> >>>>>> So two issues here:
> >>>>>> 1 - we need a better error message when vAttachWait returns unsupported
> >>>>>> 2 - fix lldb-server to support vAttachWait
> >>>>> 
> >>>>> A few questions.
> >>>>> 
> >>>>> What is the protocol here? Can/should I report the issues at https://bugs.llvm.org/ ? Or is that something that a project member needs to do? Assuming that I can, would this be two seperate issues or one "large" issue?
> >>>>> 
> >>>>> Barring that this is not something someone else might implement better/faster than me. Considering I have zero LLDB development experience is this something that I could tackle?
> >>>> 
> >>>> I didn't realize this functionality was missing in lldb-server. I can take a stab at implementing it and see what I can do. Stay tuned.
> >>>> 
> >>>>> 
> >>>>> On Thu, May 31, 2018, at 3:02 PM, Greg Clayton wrote:
> >>>>>> lldb-server claims it doesn't support it:
> >>>>>> 
> >>>>>> lldb             <  48> send packet: $vAttachWait;6c616e677365727665722d7377696674#d6
> >>>>>> lldb             <   4> read packet: $#00
> >>>>>> 
> >>>>>> 
> >>>>>> Returning the empty packet ("$#00") means it is not supported, but we really should give a better error message than this. 
> >>>>>> 
> >>>>>> It isn't that hard to implement. It would be easy to copy the code that is in debugserver in DNB.cpp in the DNBProcessAttachWait function. Most of it is probably posix style stuff. 
> >>>>>> 
> >>>>>> So two issues here:
> >>>>>> 1 - we need a better error message when vAttachWait returns unsupported
> >>>>>> 2 - fix lldb-server to support vAttachWait
> >>>>>> 
> >>>>>> Greg
> >>>>>> 
> >>>>>> 
> >>>>>>> On May 31, 2018, at 11:04 AM, Ryan Lovelett <ryan at lovelett.me> wrote:
> >>>>>>> 
> >>>>>>>> It might be a lldb-server issue?
> >>>>>>> 
> >>>>>>> How would one go about determining this?
> >>>>>>> 
> >>>>>>> (lldb) log enable -f /tmp/packates.txt gdb-remote packets
> >>>>>>> (lldb) process attach --name "<name>" --waitfor
> >>>>>>> error: attach failed: lost connection
> >>>>>>> 
> >>>>>>> Attached the log output. Maybe it'll make more sense to you than me.
> >>>>>>> 
> >>>>>>> That escalated quickly. So I've read through that log. I found something that says $qVAttachOrWaitSupported#38 with the following response being $#00. Perhaps I'm reading too much but is it that it's reporting that it is not supported?
> >>>>>>> 
> >>>>>>> Does lldb-server need to be compiled in a certain way to add support for attach wait?
> >>>>>>> 
> >>>>>>> On Thu, May 31, 2018, at 1:49 PM, Greg Clayton wrote:
> >>>>>>>> Try enabling logging with:
> >>>>>>>> 
> >>>>>>>> (lldb) log enable -f /tmp/packets.txt gdb-remote packets
> >>>>>>>> 
> >>>>>>>> And the try the attach. It might be a lldb-server issue?
> >>>>>>>> 
> >>>>>>>>> On May 31, 2018, at 9:02 AM, Ryan Lovelett via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> >>>>>>>>> 
> >>>>>>>>> I am attempting to use: process attach --name "<name>" --waitfor
> >>>>>>>>> 
> >>>>>>>>> The problem is that it immediately returns error: attach failed: lost connection.
> >>>>>>>>> 
> >>>>>>>>> Is this something I can troubleshoot further? Or is this not supported on Linux?
> >>>>>>>>> 
> >>>>>>>>> -- 
> >>>>>>>>> Ryan Lovelett
> >>>>>>>>> _______________________________________________
> >>>>>>>>> lldb-dev mailing list
> >>>>>>>>> lldb-dev at lists.llvm.org
> >>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >>>>>>> <packates.txt>
> > 
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