[lldb-dev] Using file-defined registers on Android
Greg Clayton
gclayton at apple.com
Mon Dec 2 08:43:05 PST 2013
On Nov 26, 2013, at 3:57 PM, Todd Fiala <tfiala at google.com> wrote:
> Hi all,
>
> I'm attempting to follow the platform definition approach that Greg laid out when attempting to attach to a gdbserver running on an Android device. In particular, Android arm v7a devices (Nexus 10 and Nexus 7).
>
> I went ahead and created a python register definition. I generated the definition file based on referencing these:
>
> svn cat http://llvm.org/svn/llvm-project/lldb/trunk/examples/python/x86_64_linux_target_definition.py
> svn cat http://llvm.org/svn/llvm-project/lldb/trunk/examples/python/x86_64_target_definition.py
>
> and the output from using one of gdb's commands when gdb was attached to the gdbserver:
>
> (gdb) maint print raw-registers
>
>
> Now I'm attempting to do some debugging with lldb.
>
> I created an app, fired it up on the Android, and attempt to attach to the running process. Since I can debug this app fine remotely with gdb, I believe the basic pipe should be okay.
>
> Here's what I do on the lldb side. The Android app to be debugged is running at this point.
>
> lldb
>
> # set the platform file
> (lldb) settings set plugin.process.gdb-remote.target-definition-file /home/tfiala/work/arm-arch/armv7a_linux_target_definition.py
>
> # note I tried to use armv7-pc-linux, which said the file didn't match, and there
> # doesn't appear to be an armv7a-pc-linux. Should I be using something else here?
> (lldb) target create --arch arm-pc-linux libs/armeabi-v7a/libnative-activity.so
>
> # As above, only arm-pc-linux seemed to accept this file. The .so file
> # is an armv7a-built lib in this case and runs fine on Nexus 7 and 10 devices.
> (lldb) file --arch arm-pc-linux libs/armeabi-v7a/libnative-activity.so
>
> # Now ready for the connect: the adb redirector to communicate with
> # gdbserver is localhost:5039
> (lldb) gdb-remote 5039
>
>
> Here's what I get:
> (lldb) thread list
> Process 8176 stopped
> * thread #1: tid = 8176, , stop reason = signal SIGTRAP
> (lldb) bt
> * thread #1: tid = 8176, , stop reason = signal SIGTRAP
> * frame #0:
It looks like we didn't parse your register definition file correctly. Try a:
(lldb) read registers
I am guessing you will see no output. As already suggested, step through ProcessGDBRemote::ParsePythonTargetDefinition() and make sure this succeeds.
> The app itself is still running on the Android device - at least the main thread is. So the listing of it as stopped appears to be incorrect.
Anytime we attach to a GDB server and tell it to attach to a process, the reply to the "vAttach" packet is a stop reply packet which tells us the reason the process is stopped ("TXX" where XX is a signal (SIGTRAP in this case)), and also it tells us about the thread that is stopped and some expedited register values. After the attach packet, we are assuming your program must be stopped. The documentation seems to back this up:
https://sourceware.org/gdb/onlinedocs/gdb/Stop-Reply-Packets.html
So it sounds like the GDB server might not be doing the right thing here? We will need to look at the packet log to see what is going on.
> If I do "(lldb) exit", it will kill the main thread fwiw, but not nuke the process. I'm not particularly concerned with that piece yet as it might be related to the dual-heritage java/native aspect.
LLDB will send a "k" packet which tells the remote GDB server to kill the process. The GDB server needs to make sure the process and all its threads are killed. Sounds like a GDB server issue.
>
> I've got the architecture definition file indicating the triple it provides is arm-*-linux (at least, I think). I have no idea if the file is working since I haven't (yet) figured out how to get output from the loading process.
That might be the problem, try to match the architecture exactly to what it is for now. I don't believe I made the wildcard matching work yet.
>
> I'm attaching my architecture definition file and the maintenance dump in case anybody sees something obviously wrong.
>
> Some questions:
>
> * Am I running the right commands in the right order to connect to a gdbserver where I'm specifying the register information explicitly? Are the target and file commands needed with the architecture file?
You only need to run "target create" _or_ the "file" command. "file" is an alias to "target create", so you have to execute one of these commands. You should be able to specify "armv7-pc-linux". I am guessing the "remote-linux" platform is not liking this? It would be worth stepping through the code to see why "armv7-pc-linux" us being rejected and by whom.
>
> * Why is LLDB telling me the armv7a object files are not valid armv7 files?
We don't currently have "armv7a" in our architecture list. Does LLVM/clang understand "armv7a"? I would try using "armv7-pc-linux". If "armv7a" is recognized by LLVM/Clang, feel free to add it by modifying the code in ArchSpec.cpp. There are a few tables you will need to edit:
g_core_definitions on ArchSpec.cpp:50
g_elf_arch_entries on ArchSpec.cpp:228
The main problem we currently have with ELF, is ELF file tell us "ARM" and that is it. Is there a note or anything else inside the ELF file that can help us figure out the exact ARM variant contained in an ELF executable? If so we need to modify ObjectFileELF:
bool
ObjectFileELF::GetArchitecture (ArchSpec &arch)
{
if (!ParseHeader())
return false;
arch.SetArchitecture (eArchTypeELF, m_header.e_machine, LLDB_INVALID_CPUTYPE);
arch.GetTriple().setOSName (Host::GetOSString().GetCString());
arch.GetTriple().setVendorName(Host::GetVendorString().GetCString());
return true;
}
Because just "ARM" really isn't enough. The only thing we have to go on with ELF is the e_machine from the ELF header. If there isn't a way to detect the correct arch variant for ELF is, we will need to add an LLDB setting that can be used to set substitute the correct value in when parsing ARM files and we would use it to change all "ARM" generic architectures to the result of the setting and this would need to be done in ObjectFileELF::GetArchitecture(...). It would be better if there is some data in the ARM ELF files in the object file itself that we can parse though, so we should pursue this angle first.
>
> * Is the "pc" part of the arm-pc-linux part right, wrong, or a don't care for my scenario?
Not sure. The triple should match what LLVM/Clang thinks the standard triple should be for ARM on linux.
>
> * Is it the mere fact that I'm attaching remotely good enough for lldb to be using the architecture definition specified with "settings set plugin.process.gdb-remote.target-definition-file ...", or is it keying off of some of the meta data it has (like me specifying the "target create" and "file --arch" commands)?
It might currently be checking the target arch and trying to match it up to the arch you have in your target definition file. I would remove the wildcard and have it match exactly what you type for now.
>
> * How do I debug python loaded via lldb or get feedback from the lldb python support (e.g. if there's a syntax error or something else goofy) when running lldb?
You really can't debug the python right now as far as I know. print statements are my current choice when things go wrong.
For the target definition file, set a breakpoint in ProcessGDBRemote.cpp in ProcessGDBRemote::ParsePythonTargetDefinition(), which currently is line 334:
bool
ProcessGDBRemote::ParsePythonTargetDefinition(const FileSpec &target_definition_fspec)
{
#ifndef LLDB_DISABLE_PYTHON
ScriptInterpreter *interpreter = GetTarget().GetDebugger().GetCommandInterpreter().GetScriptInterpreter();
Error error;
lldb::ScriptInterpreterObjectSP module_object_sp (interpreter->LoadPluginModule(target_definition_fspec, error));
if (module_object_sp)
{
lldb::ScriptInterpreterObjectSP target_definition_sp (interpreter->GetDynamicSettings(module_object_sp,
&GetTarget(),
"gdb-server-target-definition",
error));
PythonDictionary target_dict(target_definition_sp);
if (target_dict)
{
You will want to see that "module_object_sp" is valid and also that "target_definition_sp" and eventually "target_dict" test true. Let me know if they don't.
You might want to verify that you can do some rudimentary python first:
% lldb
(lldb) script 2+3
>
> I assume I have something really basic wrong at this point since the arch definition file specified seems to make no difference on the output vs. what I see when I attach with lldb without specifying the architecture file.
I am guessing that the arch file is not getting loaded due to the architecture having a wildcard? Let me know what you find on that end.
>
> Thanks for any suggestions and for helping fill in my understanding!
>
> Sincerely,
> Todd Fiala
> <armv7a_linux_target_definition.py><gdb-reg-output-armv7a.out>_______________________________________________
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