[lldb-dev] GDB Remote target descriptions

[jw4242@gmail.com via lldb-dev]

Hi all,

I've been adding support for the qXfer:features:read:target.xml message for
our tools at $WORK and have run into a couple hiccups to puzzle over.
First off, the request message as defined at
https://sourceware.org/gdb/onlinedocs/gdb/General-Query-Packets.html#qXfer%20target%20description%20read
is in the form qXfer:features:read:annex:offset,size.  When talking to GDB
it works to respond with a 'l' or 'm' message smaller than size, then GDB
proceeds to request the next chunk, and so on until the debug server says
there is no more data.  I believe there is a bug in lldb's handling of this
message in that if the response is shorter than 'size' while also being
incomplete, lldb requests the next chunk with a giant offset.  This patch
fixes the behavior:

Index: source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp
===================================================================
--- source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp
(revision 288181)
+++ source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp
(working copy)
@@ -3342,7 +3342,7 @@
     case ('m'):
       if (str.length() > 1)
         output << &str[1];
-      offset += size;
+      offset += str.length() - 1;
       break;

     // unknown chunk

Secondly, it appears that lldb only processes a single <feature> element
per "file" when discovering the register descriptions, the workaround seems
to be to put each individual feature in an <xi:include> file or
conglomerate everything under a single feature in target.xml, but that has
an unfortunate side effect of not working right when talking to GDB.  This
one I didn't dig too far into that code but it looks like
ProcessGDBRemote::GetGDBServerRegisterInfo only extracts a single <feature>
node (the last one) from the target description.

In any case, these are both able to be worked around in my environment
without too much work but it would be nice to get at least that first item
fixed.
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