Launching the program through 'gdb-remote' process plugin

Subprojects LLDB
#1

Hi,

I am connecting lldb with my simulator through ‘gdb-remote’ process plugin. After extablishing the connection, I want to issue the ‘process launch’ command in lldb which should send the arguments packet ‘A’ to simulator asking it to launch the program.

I am facing the problem while issuing the ‘process launch’ command in lldb. When I give the ‘process launch’ command, lldb hangs and there is no communication with simulator. When I debugged with gdb, I got the backtrace as follows:

#2 0xb6703a6f in lldb_private::Condition::Wait (this=0x8077dac, mutex=…, abstime=0x0, timed_out=0xbfffaa9f) at /home/chandra/llvm-3.1/tools/lldb/source/Host/common/Condition.cpp:92
#3 0xb662768d in lldb_private::Predicate::WaitForValueEqualTo (this=0x8077d90, value=true, abstime=0x0, timed_out=0xbfffaa9f) at /home/chandra/llvm-3.1/tools/lldb/source/Core/…/…/include/lldb/Host/Predicate.h:317
#4 0xb662b03f in lldb_private::Listener::WaitForEventsInternal (this=0x8077d3c, timeout=0x0, broadcaster=0xadb0179c, broadcaster_names=0x0, num_broadcaster_names=0, event_type_mask=3, event_sp=…) at /home/chandra/llvm-3.1/tools/lldb/source/Core/Listener.cpp:409
#5 0xb662b1f7 in lldb_private::Listener::WaitForEventForBroadcasterWithType (this=0x8077d3c, timeout=0x0, broadcaster=0xadb0179c, event_type_mask=3, event_sp=…) at /home/chandra/llvm-3.1/tools/lldb/source/Core/Listener.cpp:440
#6 0xb69421ec in lldb_private::Process::WaitForStateChangedEvents (this=0xadb01788, timeout=0x0, event_sp=…) at /home/chandra/llvm-3.1/tools/lldb/source/Target/Process.cpp:1069
#7 0xb6941f2c in lldb_private::Process::WaitForProcessToStop (this=0xadb01788, timeout=0x0) at /home/chandra/llvm-3.1/tools/lldb/source/Target/Process.cpp:977
#8 0xb6587d98 in CommandObjectProcessLaunch::DoExecute (this=0x80813d0, launch_args=…, result=…) at /home/chandra/llvm-3.1/tools/lldb/source/Commands/CommandObjectProcess.cpp:220
#9 0xb672c999 in lldb_private::CommandObjectParsed::Execute (this=0x80813d0, args_string=0xb4d667bc “”, result=…) at /home/chandra/llvm-3.1/tools/lldb/source/Interpreter/CommandObject.cpp:856
#10 0xb67215d3 in lldb_private::CommandInterpreter::HandleCommand (this=0x8077fc8, command_line=0x883a614 “process launch”, lazy_add_to_history=lldb_private::eLazyBoolYes, result=…,
override_context=0x0, repeat_on_empty_command=true, no_context_switching=false) at /home/chandra/llvm-3.1/tools/lldb/source/Interpreter/CommandInterpreter.cpp:1571
#11 0xb64e0ced in lldb::SBCommandInterpreter::HandleCommand (this=0xbfffc2cc, command_line=0x883a614 “process launch”, result=…, add_to_history=true) at /home/chandra/llvm-3.1/tools/lldb/source/API/SBCommandInterpreter.cpp:97
#12 0x0805a103 in Driver::HandleIOEvent (this=0xbfffec7c, event=…) at /home/chandra/llvm-3.1/tools/lldb/tools/driver/Driver.cpp:996
#13 0x0805b516 in Driver::MainLoop (this=0xbfffec7c) at /home/chandra/llvm-3.1/tools/lldb/tools/driver/Driver.cpp:1430
#14 0x0805bbf2 in main (argc=4, argv=0xbfffede4, envp=0xbfffedf8) at /home/chandra/llvm-3.1/tools/lldb/tools/driver/Driver.cpp:1562

Though I am analyzing on this, Can someone help me in finding out what goes wrong while trying to launch a program in simulator through ‘gdb-remote’ process plugin?

Following is the communication between lldb and simulator for the “process connect --plugin gdb-remote connect://localhost:51000” command.

Send Packet: QStartNoAckMode: 15 chars
Read Packet:
Send Packet: QThreadSuffixSupported: 22 chars
Read Packet:
Send Packet: QListThreadsInStopReply: 23 chars
Read Packet: E01
Send Packet: qHostInfo: 9 chars
Read Packet: cputype:201;cpusubtype:-2;ostype:unknown;vendor:unknown;endian:little;ptrsize:4;
Send Packet: vCont?: 6 chars
Read Packet: vCont[;c;s]
Send Packet: qVAttachOrWaitSupported: 23 chars
Read Packet:
Send Packet: qC: 2 chars
Read Packet: QC0

LLDB sets the status as connected after this communication.
(snip start)
if (pid == LLDB_INVALID_PROCESS_ID)
{
// We don’t have a valid process ID, so note that we are connected
// and could now request to launch or attach, or get remote process
// listings…
SetPrivateState (eStateConnected);
}
(snip end)
function: ProcessGDBRemote::DoConnectRemote ()

Thanks,
Chandra Kumar R.

#2

The "process launch" command should send the 'A' packet, and it is waiting for a stop reply response from the 'A' packet. Try enabling the packet logging in LLDB:

(lldb) log enable gdb-remote packets
(lldb) process connect ...
(lldb) run

My guess is that the 'A' packet isn't responding with a stop reply packet.

#3

Thanks for the reply.

I enabled the packet logging in LLDB.

(lldb) log enable gdb-remote packets
(lldb) process connect --plugin gdb-remote connect://localhost:51000
< 1> send packet: +
history[1] tid=0xb1d819d0 < 1> send packet: +
< 26> send packet: $QThreadSuffixSupported#e4
< 1> read packet: +
< 4> read packet: $#00
< 1> send packet: +
< 27> send packet: $QListThreadsInStopReply#21
< 1> read packet: +
< 7> read packet: $E01#a6
< 1> send packet: +
< 10> send packet: $vCont?#49
< 1> read packet: +
< 15> read packet: $vCont[;c;s]#0e
< 1> send packet: +
< 27> send packet: $qVAttachOrWaitSupported#38
< 1> read packet: +

< 6> send packet: $qC#b4
< 1> read packet: +
< 7> read packet: $QC0#c4
< 1> send packet: +
(lldb) run

After this lldb just hangs. lldb doesn’t send any packet after the ‘run’ command. I debugged by inserting a breakpoint in lldb’s code which sends the ‘A’ packet. And that breakpoint was not hit.

#4

You probably didn't give LLDB a file with "target create". If you look at the "ProcessGDBRemote::DoLaunch (...)" function, it gets the object file from the target's executable module to ensure there is something to launch:

    ObjectFile * object_file = exe_module->GetObjectFile();
    if (object_file)
    {

        ... send 'A' packet

    }

It really doesn't make sense to run a program that has no executable and has no arguments. If you have a simulator that is debugging something already, then you must reply with a non-zero PID to the "qC" packet. Below your packets were:

< 6> send packet: $qC#b4
< 1> read packet: +
< 7> read packet: $QC0#c4
< 1> send packet: +

This is telling us, we are connected, but aren't debugging anything yet. There is a bug in LLDB right now if this is the case where we aren't propagating an error back from Process::Launch() correctly stating the 'A' packet can't be sent without a valid executable.

IS this what is going on? If so, just give lldb the executable file:

(lldb) target create --arch <ARCH> /path/to/local/copy/of/sim/exe/a.out
(lldb) process connect --plugin gdb-remote connect://localhost:51000

#5

IS this what is going on? If so, just give lldb the executable file:

No. I initiated lldb with the arguments “–arch vliw-unknown-unknown workplace/app.elf”. But I haven’t mentioned in my previous mail.

$lldb
(lldb) log enable gdb-remote packets
(lldb) target create --arch vliw-unknown-unknown workplace/app.elf
Current executable set to ‘workplace/app.elf’ (vliw).
(lldb) process connect --plugin gdb-remote connect://localhost:51000
< 1> send packet: +
history[1] tid=0xb1cbe9d0 < 1> send packet: +
< 19> send packet: $QStartNoAckMode#b0
< 1> read packet: +
< 4> read packet: $#00
< 1> send packet: +
< 26> send packet: $QThreadSuffixSupported#e4
< 1> read packet: +
< 4> read packet: $#00
< 1> send packet: +
< 27> send packet: $QListThreadsInStopReply#21
< 1> read packet: +
< 4> read packet: $#00
< 1> send packet: +
< 13> send packet: $qHostInfo#9b
< 1> read packet: +
< 84> read packet: $cputype:201;cpusubtype:-2;ostype:unknown;vendor:unknown;endian:little;ptrsize:4;#90
< 1> send packet: +
< 10> send packet: $vCont?#49
< 1> read packet: +
< 15> read packet: $vCont[;c;s]#0e
< 1> send packet: +
< 27> send packet: $qVAttachOrWaitSupported#38
< 1> read packet: +
< 4> read packet: $#00
< 1> send packet: +
< 6> send packet: $qC#b4
< 1> read packet: +

< 7> read packet: $QC0#c4
< 1> send packet: +
(lldb) process launch

I debugged lldb to check the “ProcessGDBRemote::DoLaunch()” method. But the execution didn’t reach this function for the above said communication.

Actually after the “process connect” the state is set to “eStateConnected”. So the “process launch” should call the “DoLaunch()” method of “gdb-remote” process plugin. But lldb hangs on the last line of the following code snip.

(snip start)
if (error.Success())
{
const char *archname = exe_module->GetArchitecture().GetArchitectureName();

//result.AppendMessageWithFormat (“Process %llu launched: ‘%s’ (%s)\n”, process->GetID(), filename, archname);
result.SetDidChangeProcessState (true);
if (m_options.launch_info.GetFlags().Test(eLaunchFlagStopAtEntry) == false)
{
result.SetStatus (eReturnStatusSuccessContinuingNoResult);
StateType state = process->WaitForProcessToStop (NULL);
(snip end)
file: lldb/source/Commands/CommandObjectProcess.cpp
method: CommandObjectProcessLaunch::DoExecute()

#6

Its good to know you had a file specified.

Now you will need to start higher up and step down through the launch starting in lldb_private::Process::Launch() and see where things go wrong.

Greg