[lldb-dev] [RFC] Improving protocol-level compatibility between LLDB and GDB

[Michał Górny via lldb-dev]

On Mon, 2021-04-19 at 16:29 -0700, Greg Clayton wrote:
> > I think the first blocker towards this project are existing
> > implementation bugs in LLDB. For example, the vFile implementation is
> > documented as using incorrect data encoding and open flags. This is not
> > something that can be trivially fixed without breaking compatibility
> > between different versions of LLDB.
> 
> We should just fix this bug in LLDB in both LLDB's logic and lldb-server IMHO. We typically distribute both "lldb" and "lldb-server" together so this shouldn't be a huge problem.

Hmm, I've focused on this because I recall hearing that OSX users
sometimes run new client against system server... but now I realized
this isn't relevant to LLGS ;-).  Still, I'm happy to do things
the right way if people feel like it's needed, or the easy way if it's
not.

> The other main issue LLDB has when using other GDB servers is the dynamic register information is not enough for debuggers to live on unless there is some hard coded support in the debugger that can help fill in register numberings. The GDB server has its own numbers, and that is great, but in order to truly be dynamic, we need to know the compiler register number (such as the reg numbers used for .eh_frame) and the DWARF register numbers for debug info that uses registers numbers (these are usually the same as the compiler register numbers, but they do sometimes differ (like x86)). LLDB also likes to know "generic" register numbers like which register it the PC (RIP for x86_64, EIP for x86, etc), SP, FP and a few more. lldb-server has extensions for this so that the dynamic register info it emits is enough for LLDB. We have added extra key/value pairs to the XML that is retrieved via "target.xml" so that it can be complete. See the function in lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp:
> 
> bool ParseRegisters(XMLNode feature_node, GdbServerTargetInfo &target_info,
>                     GDBRemoteDynamicRegisterInfo &dyn_reg_info, ABISP abi_sp,
>                     uint32_t &reg_num_remote, uint32_t &reg_num_local);
> 
> There are many keys we added: "encoding", "format", "gcc_regnum", "ehframe_regnum", "dwarf_regnum", "generic", "value_regnums", "invalidate_regnums", "dynamic_size_dwarf_expr_bytes"
> 

Yes, this is probably going to be the hardest part.  While working
on plugins, I've found LLDB register implementation very hard to figure
out, especially that the plugins seem to be a mix of new, old and older
solutions to the same problem.

We will probably need more ground-level design changes too.  IIRC lldb
sends YMM registers as a whole (i.e. with duplication with XMM
registers) while GDB sends them split like in XSAVE.  I'm not yet sure
how to handle this best -- if we don't want to push the extra complexity
on plugins, it might make sense to decouple the packet format from
the data passed to plugins.

-- 
Best regards,
Michał Górny