[lldb-dev] LLDB and gdbserver

Greg Clayton gclayton at apple.com
Fri Oct 11 09:46:03 PDT 2013

The patch looks good. The only thing I worry about is someone setting this adjust PC value to something like -1, and then forgetting about it. Then they attach to a program with a different architecture and the PC gets adjusted. I am wondering if this setting should be a dictionary that takes target triple regular expresssions as the key and the value is the adjustment amount? This would stop random PC adjustments from happening on different architectures.


On Oct 4, 2013, at 9:44 AM, Abid, Hafiz <Hafiz_Abid at mentor.com> wrote:

> Hi Greg,
> I am attaching a patch that implements the 2nd part as discussed below i.e. to adjust the pc after hitting breakpoint. The response of qHostInfo packet is checked for ' adjusts_breakpoint_pc' key. If found, it gives the value by which the pc should be adjusted. In its absence, a new setting ' plugin.process.gdb-remote .adjust-breakpoint-pc' can be used to provide this information. The default of this settings is 0. If a user is connecting to a stub that needs the debugger to adjust the pc (e.g. gdbserver) then user can adjust the value of this settings accordingly.
> I tested the new setting and it is working fine. As I don't have a stub that sends qHostInfo with this new key, so that code is un-tested. OK to commit?
> Regards,
> Abid
>> -----Original Message-----
>> From: Greg Clayton [mailto:gclayton at apple.com]
>> Sent: 05 September 2013 19:26
>> To: Abid, Hafiz
>> Cc: lldb-dev at cs.uiuc.edu
>> Subject: Re: [lldb-dev] LLDB and gdbserver
>> On Sep 5, 2013, at 9:16 AM, Abid, Hafiz <Hafiz_Abid at mentor.com> wrote:
>>> I was able to make the LLDB work with gdbserver. Running, stopping,
>> source stepping were working ok. I needed to patch 2 areas to make it work.
>> I would like comments on those changes before I can get them ready for
>> submission.
>>> If dynamic register info is not available then GDBRemoteRegisterContext is
>> relying on hardcoded registers for ARM. I have to added similar hard-coded
>> registers for x86_64. Would it make any sense if we keep
>> GDBRemoteRegisterContext for reading/writing the register packets only.
>> The task of translating those packets should be left to some higher level
>> classes. Perhaps something like GDBRemoteRegisterContext_arm,
>> GDBRemoteRegisterContext_x86_64 etc. Or for the time being, hardcoding is
>> considered ok.
>> The only hard coded registers should be for ARM as this was needed for
>> legacy iOS support. Any new registers should use a new plugin setting that
>> supplies an XML file that describes the registers. The setting should be
>> something like:
>> (lldb) settings set plugin.process.gdb-remote.register-definition-file
>> /path/to/registers.xml
>> The XML register description should supply the same information as the
>> qRegisterInfo packet for all registers. Then the
>> GDBRemoteDynamicRegisterInfo class will need to be able to set itself up
>> using an XML file. Another way to do this would be to supply a python file.
>> We have Python bridging objects available where you could easily make a
>> new python callback where a python dictionary could be returned. Python
>> might also be more useful because you could create classes that know the
>> common register numbers for certain architectures and ABIs...
>> So the flow would be:
>> - debugging starts with debugserver
>> - we stop for the first time and "qRegisterInfo0" packet is sent, and the
>> unsupported response of "$#00" is returned.
>> - we grab the value of the "plugin.process.gdb-remote.register-definition-
>> file" setting, and if it is set, we parse that file
>> - else fallback to hard coded registers.
>> So I would avoid adding anymore hard coded registers to LLDB otherwise we
>> will end up with a mess in LLDB with all the different gdb servers that we can
>> attach to.
>>> It seems that debugserver decrements the pc after stopping on breakpoint.
>> To find the stop reason, code in ProcessGDBRemote::SetThreadStopInfo()
>> checks for breakpoint on pc. But gdbserver does not decrement the pc in this
>> case. I had to duplicate the above check for (pc - 1) and then decrement the
>> pc accordingly. I was wondering if there is some good way to distinguish if I
>> am connected to debugserver or gdbserver. Can I make use of some of the
>> new packets added by LLDB or perhaps add some option in the gdb-remote
>> command?
>> I would modify the qHostInfo to return a new key/value pair like:
>> "adjusts_breakpoint_pc:1;" or "adjusts_breakpoint_pc:0;" so we know for
>> certain architectures if we need to manually adjust the PC. Then we use a
>> LazyBool instance variable in GDBRemoteCommunicationClient to detect this
>> setting via the qHostInfo packet. If the "adjusts_breakpoint_pc" key/value
>> isn't specified in the qHostInfo packet, or if the qHostInfo packet isn't
>> supported, we should fall back to a setting:
>> (lldb) settings set plugin.process.gdb-remote.adjust-breakpoint-pc true
>> (lldb) settings set plugin.process.gdb-remote.adjust-breakpoint-pc false
>> So with all settings when using GDB server, we try to detect things
>> dynamically (registers and other settings like the adjust breakpoint PC), and
>> if that fails, we fall back to manual settings.
>> Greg
> <adjust_pc.patch>

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