[lldb-dev] gdb-remote to android gdbserver

Abid, Hafiz Hafiz_Abid at mentor.com
Tue Aug 27 10:08:04 PDT 2013

Thanks for the patch. You choose a better place to decode the RLE. I am not sure about escape character though as gdbserver does not seem to use them for normal packets. The data byte is encoded in as 2-digit asci character. For example, this is a response to a g packet request.

putpkt ("$2a0*}0*)40e2f*"7f0*S20*hb0b6ddf7ff7f0*"020* 330*"2b0*}0*}0* 7f030*(f* 0*}0*}0*}0*}0*}0*c801f0* 3b0*}0*}0*}0*}0*}0*E#c8"); [noack mode]

and initial part of how gdb decoded it.

I will test the RLE part of your patch. If that is ok then I will merge it.


From: Virgile Bello [mailto:virgile.bello at gmail.com]
Sent: 27 August 2013 16:29
To: Abid, Hafiz
Cc: Sebastien Metrot; lldb-dev
Subject: Re: [lldb-dev] gdb-remote to android gdbserver


Sorry, since I also had a RLE patch pending, here it is in case it is useful (before it's too late).
Hope we can merge both of them together maybe?

This one also tries to handle escaped characters (#, $ and }), when receiving and sending.
Did it from http://sourceware.org/gdb/onlinedocs/gdb/Overview.html
Also, it converts everything where the packet copy was happening, to avoid extra copy and O(n) insertion.

Not sure if checksum is supposed to work on the escaped version or not. Need to check how GDB is working (spec doesn't say anything).

I didn't have a chance to test it again since I added character escaping, so I should probably do that again in case there is real interest in it.
Also couldn't polish it as I intended, but thought it would still be more useful as is before the other one get merged than after.


On Tue, Aug 27, 2013 at 6:51 PM, Abid, Hafiz <Hafiz_Abid at mentor.com<mailto:Hafiz_Abid at mentor.com>> wrote:
I have been working in the last few weeks on making LLDB working with gdbserver on Linux host. Some of the things that I noted missing are.

1.      Run length encoding as already mentioned. I have a patch for it that I will submit shortly.

2.      I had to hardcode the registers for x86_64 as gdbserver does not support qRegisterInfo.

3.      Make sure to use g/G packets for registers.

After that I had a working connection. But then I noticed the following 2 problem.
PC is not decremented by length of breakpoint instruction after target stops.
LLDB failing to recognize a breakpoint is for stepping and stopping before a source level step was really complete.

I am working on these 2 issues.


From: lldb-dev-bounces at cs.uiuc.edu<mailto:lldb-dev-bounces at cs.uiuc.edu> [mailto:lldb-dev-bounces at cs.uiuc.edu<mailto:lldb-dev-bounces at cs.uiuc.edu>] On Behalf Of Virgile Bello
Sent: 27 August 2013 02:34
To: Sebastien Metrot
Cc: lldb-dev
Subject: Re: [lldb-dev] gdb-remote to android gdbserver

Yes, I suppose a lldb-only change would be much more easier for now.

>From what I remember when I tested, hard-coding the ARM registers temporarily + the runtime-length encoding patch (that I will commit soon) was enough to to have a simple address breakpoint working.
Backtrace didn't, but I didn't bother investigate further.

On Tue, Aug 27, 2013 at 10:09 AM, Sebastien Metrot <meeloo at gmail.com<mailto:meeloo at gmail.com>> wrote:
If I understand correctly 1 and 3 implies modifying the android gdbserver while 2 could be done entirely in lldb? Having gdbserver changes accepted by the maintainers and integrated in future builds of android and/or the NDK looks like a hard path to walk.
Also I have looked at the gdbserver sources of the current android (found here https://android.googlesource.com/toolchain/gdb/ ) and a couple of greps seems to indicate that xmlRegisters is only supported on x86 although register definition xmls exists for many arm/linux variants.
In this light 2 seems like a good short term solution to force a register mapping on lldb by reading the xml from the gdb sources, until 1 is finally implemented in both projects and lldb is able to detect the register mapping.

Is that correct?


On 26 Aug 2013, at 19:34 , Greg Clayton <gclayton at apple.com<mailto:gclayton at apple.com>> wrote:

> The problem is we don't know the registers. GDB assumes that the GDB that is being used knows what registers are on the other side of the remote connection. New GDB binaries and GDBSERVER binaries are compiled for each target. LLDB has one binary for all systems. So LLDB expects to be able to query the GDBSERVER for the register information. LLDB has defined new packets to get this information. We have documented the packets we added:
> svn cat http://llvm.org/svn/llvm-project/lldb/trunk/docs/lldb-gdb-remote.txt
> I believe there is a way defined in the remote protocol to query for registers using the "qXfer" command with the "xmlRegisters" gdb feature. We haven't added support for this, but the "XML target description" format is missing information we need. The details on this are at:
> https://sourceware.org/gdb/onlinedocs/gdb/Target-Description-Format.html#Target-Description-Format
> It could also be easily extended to support giving us the information required.
> So, in order to get things going you will need to do one of the following:
> 1 - add support for the qRegisterInfo packet (easiest)
> 2 - add a new "settings set" setting for the "gdb-remote" plug-in that contains a path to a register description file. The file format should be something parseable like XML and you would need to write a parser for it and parse the contents by adding a method to the  GDBRemoteDynamicRegisterInfo class (harder)
> 3 - If the current gdbserver for android does support the "XML target description", we would need to modify the GDBRemoteCommunicationClient class to be able to retrieve the information and parse the XML. You would then need to modify the gdbserver for android to add extra data we need to the register info (compiler register numbers for each register if one exists, DWARF register numbers, generic register info) (hardest)
> I think option 1 or 2 would be the easiest.
> Greg
> On Aug 25, 2013, at 10:53 AM, Sebastien Metrot <meeloo at gmail.com<mailto:meeloo at gmail.com>> wrote:
>> Hi,
>> I have started to investigate connecting lldb to gdbserver running on an android device (arm). It connects correctly, I'm able to start the remote program and interrupt it but lldb doesn't permit me to access any data as it says the current frame is invalid:
>> mbp:NativeTest meeloo$ lldb ../../../ndk/samples/hello-gl2/libs/armeabi/libgl2jni.so
>> Current executable set to '../../../ndk/samples/hello-gl2/libs/armeabi/libgl2jni.so' (arm).
>> (lldb) gdb-remote
>> error: gdb-remote [<hostname>:]<portnum>
>> (lldb) gdb-remote<>
>> Process 12260 stopped
>> * thread #1: tid = 0x2fe4, , stop reason = signal SIGTRAP
>>    frame #0:
>> (lldb) frame variable
>> error: invalid frame
>> (lldb) continue
>> Process 12260 resuming
>> ---- Here I hit Ctrl+C
>> Process 12260 stopped
>> * thread #1: tid = 0x2fe4, , stop reason = signal SIGINT
>>    frame #0:
>> (lldb) continue
>> Process 12260 resuming
>> (lldb) process interrupt
>> Process 12260 stopped
>> * thread #1: tid = 0x2fe4, , stop reason = signal SIGINT
>>    frame #0:
>> (lldb) register read
>> error: invalid frame
>> (lldb) process status
>> Process 12260 stopped
>> * thread #1: tid = 0x2fe4, , stop reason = signal SIGINT
>>    frame #0:
>> Does anyone have already tried to do this kind of things? I'm trying to get it to work before moving to implementing android support in my program with the C++ API...
>> Thanks in advance,
>> S.
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>> lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu>
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