[lldb-dev] Fwd: GDB Remote target descriptions

[jw4242@gmail.com via lldb-dev]

Never got a response to this, did I miss it somehow?  If not, please see
below.

---------- Forwarded message ----------
From: jw4242 at gmail.com <jw4242 at gmail.com>
Date: Fri, Dec 2, 2016 at 9:21 PM
Subject: GDB Remote target descriptions
To: lldb-dev at lists.llvm.org


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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