[lldb] [gdb-remote] Support client fallback for servers without reg defs

Provide minimal register definition defaults for working with servers
that implement neither target.xml nor qRegisterInfo packets.  This is
useful e.g. when interacting with FreeBSD's kernel minimal gdbserver
that does not send target.xml but uses the same layout for its supported
register subset as GDB.

The prerequisite for this is the ability to determine the correct
architecture, e.g. from the target executable.

Differential Revision: https://reviews.llvm.org/D116896

Author: mgorny

lldb/source/Plugins/Process/gdb-remote/CMakeLists.txt

@@ -25,6 +25,7 @@ add_lldb_library(lldbPluginProcessGDBRemote PLUGIN
   GDBRemoteCommunicationServerLLGS.cpp
   GDBRemoteCommunicationServerPlatform.cpp
   GDBRemoteRegisterContext.cpp
+  GDBRemoteRegisterFallback.cpp
   ProcessGDBRemote.cpp
   ProcessGDBRemoteLog.cpp
   ThreadGDBRemote.cpp

lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterFallback.cpp

@@ -0,0 +1,86 @@
+//===-- GDBRemoteRegisterFallback.cpp -------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "GDBRemoteRegisterFallback.h"
+
+namespace lldb_private {
+namespace process_gdb_remote {
+
+#define REG(name, size)                                                        \
+  DynamicRegisterInfo::Register {                                              \
+    ConstString(#name), empty_alt_name, reg_set, size, LLDB_INVALID_INDEX32,   \
+        lldb::eEncodingUint, lldb::eFormatHex, LLDB_INVALID_REGNUM,            \
+        LLDB_INVALID_REGNUM, LLDB_INVALID_REGNUM, LLDB_INVALID_REGNUM, {}, {}  \
+  }
+#define R64(name) REG(name, 8)
+#define R32(name) REG(name, 4)
+
+static std::vector<DynamicRegisterInfo::Register> GetRegisters_aarch64() {
+  ConstString empty_alt_name;
+  ConstString reg_set{"general purpose registers"};
+
+  std::vector<DynamicRegisterInfo::Register> registers{
+      R64(x0),  R64(x1),  R64(x2),  R64(x3),  R64(x4),  R64(x5),   R64(x6),
+      R64(x7),  R64(x8),  R64(x9),  R64(x10), R64(x11), R64(x12),  R64(x13),
+      R64(x14), R64(x15), R64(x16), R64(x17), R64(x18), R64(x19),  R64(x20),
+      R64(x21), R64(x22), R64(x23), R64(x24), R64(x25), R64(x26),  R64(x27),
+      R64(x28), R64(x29), R64(x30), R64(sp),  R64(pc),  R32(cpsr),
+  };
+
+  return registers;
+}
+
+static std::vector<DynamicRegisterInfo::Register> GetRegisters_x86() {
+  ConstString empty_alt_name;
+  ConstString reg_set{"general purpose registers"};
+
+  std::vector<DynamicRegisterInfo::Register> registers{
+      R32(eax), R32(ecx), R32(edx), R32(ebx),    R32(esp), R32(ebp),
+      R32(esi), R32(edi), R32(eip), R32(eflags), R32(cs),  R32(ss),
+      R32(ds),  R32(es),  R32(fs),  R32(gs),
+  };
+
+  return registers;
+}
+
+static std::vector<DynamicRegisterInfo::Register> GetRegisters_x86_64() {
+  ConstString empty_alt_name;
+  ConstString reg_set{"general purpose registers"};
+
+  std::vector<DynamicRegisterInfo::Register> registers{
+      R64(rax), R64(rbx), R64(rcx), R64(rdx), R64(rsi), R64(rdi),
+      R64(rbp), R64(rsp), R64(r8),  R64(r9),  R64(r10), R64(r11),
+      R64(r12), R64(r13), R64(r14), R64(r15), R64(rip), R32(eflags),
+      R32(cs),  R32(ss),  R32(ds),  R32(es),  R32(fs),  R32(gs),
+  };
+
+  return registers;
+}
+
+#undef R32
+#undef R64
+#undef REG
+
+std::vector<DynamicRegisterInfo::Register>
+GetFallbackRegisters(const ArchSpec &arch_to_use) {
+  switch (arch_to_use.GetMachine()) {
+  case llvm::Triple::aarch64:
+    return GetRegisters_aarch64();
+  case llvm::Triple::x86:
+    return GetRegisters_x86();
+  case llvm::Triple::x86_64:
+    return GetRegisters_x86_64();
+  default:
+    break;
+  }
+
+  return {};
+}
+
+} // namespace process_gdb_remote
+} // namespace lldb_private

lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterFallback.h

@@ -0,0 +1,26 @@
+//===-- GDBRemoteRegisterFallback.h -----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_SOURCE_PLUGINS_PROCESS_GDB_REMOTE_GDBREMOTEREGISTERFALLBACK_H
+#define LLDB_SOURCE_PLUGINS_PROCESS_GDB_REMOTE_GDBREMOTEREGISTERFALLBACK_H
+
+#include <vector>
+
+#include "lldb/Target/DynamicRegisterInfo.h"
+#include "lldb/Utility/ArchSpec.h"
+
+namespace lldb_private {
+namespace process_gdb_remote {
+
+std::vector<DynamicRegisterInfo::Register>
+GetFallbackRegisters(const ArchSpec &arch_to_use);
+
+} // namespace process_gdb_remote
+} // namespace lldb_private
+
+#endif // LLDB_SOURCE_PLUGINS_PROCESS_GDB_REMOTE_GDBREMOTEREGISTERFALLBACK_H

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

@@ -72,6 +72,7 @@
 #include <thread>
 
 #include "GDBRemoteRegisterContext.h"
+#include "GDBRemoteRegisterFallback.h"
 #include "Plugins/Platform/MacOSX/PlatformRemoteiOS.h"
 #include "Plugins/Process/Utility/GDBRemoteSignals.h"
 #include "Plugins/Process/Utility/InferiorCallPOSIX.h"
@@ -394,6 +395,7 @@ void ProcessGDBRemote::BuildDynamicRegisterInfo(bool force) {
   //     2 - If the target definition doesn't have any of the info from the
   //     target.xml (registers) then proceed to read the target.xml.
   //     3 - Fall back on the qRegisterInfo packets.
+  //     4 - Use hardcoded defaults if available.
 
   FileSpec target_definition_fspec =
       GetGlobalPluginProperties().GetTargetDefinitionFile();
@@ -507,6 +509,9 @@ void ProcessGDBRemote::BuildDynamicRegisterInfo(bool force) {
     }
   }
 
+  if (registers.empty())
+    registers = GetFallbackRegisters(arch_to_use);
+
   AddRemoteRegisters(registers, arch_to_use);
 }