Accept g packet responses that don't supply all registers

[jasonmolenda]

Summary

This was encountered while debugging a cortex m4 board with a Segger J-Link v 6.54. It only supports g/G for reading/writing registers, and it supports an xml register description. The register description includes the GPRs, the exception registers, and the floating point registers. But g only provides the general purpose registers.

lldb accepts a g response that is too large for the expected register context size, but it treats a too-small payload as an error. This change accepts a too-small payload, marking registers not included in the payload as being unavailable for retrieval (so "register read msp" will return the error that the register is unavailable).

The J-Link will accept a G packet (set registers) for the entire register context, so I didn't try to change the write registers path to recognize a register buffer where all the final registers are unavailable, and truncate the payload it sends.

I wrote a small gdb_remote_client test for this.

[jasonmolenda]

It might be worth adding an extra conditional to check for a buffer of 0 bytes returned. I haven't checked if ReadAllRegisters() would return success if a 0-length register context was received.

[labath]

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

215

Should this be something like reginfo->byte_offset+reg_info->byte_size < ... ?

[jasonmolenda]

Updated GDBRemoteRegisterContext::ReadRegisterBytes to only mark registers as valid if the full contents for the register are included in the g packet. Updated gdbclientutils.py to strip off the thread specifier from the G packet, if it's present. Change the test file to check that the G packet either includes the full register context or just the general purpose registers.

[labath]

looks good to me

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

215–220

maybe just m_reg_valid[i] = reginfo->byte_offset + reginfo->byte_size <= buffer_sp->GetByteSize()

[jasonmolenda]

Thanks for looking this over Pavel.

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

215

Yeah, that was my first thought too, then I thought, SURELY we'll have the correct # of bytes for complete registers, even if it's less registers than expected. But that's maybe not a great assumption. I'll change it.

215–220

I think I like the more verbose form, but I don't feel strongly about it. It takes up more screen real estate but I think it's easier to understand at a glance - purely a personal opinion. I'm sure they compile to the same code.

lldb/packages/Python/lldbsuite/test/functionalities/gdb_remote_client/TestJLink6Armv7RegisterDefinition.py

+from __future__ import print_function
+import lldb
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.decorators import *
+from gdbclientutils import *
+
+class TestJLink6Armv7RegisterDefinition(GDBRemoteTestBase):
+
+    @skipIfXmlSupportMissing
+    @skipIfRemote
+    def test(self):
+        """
+        Test lldb's parsing of SEGGER J-Link v6.54 register
+        definition for a Cortex M-4 dev board, and the fact
+        that the J-Link only supports g/G for reading/writing
+        register AND the J-Link v6.54 doesn't provide anything
+        but the general purpose registers."""
+        class MyResponder(MockGDBServerResponder):
+
+            # Initial r1 bytes, in little-endian order
+            r1_bytes = "01000000"
+
+            def qXferRead(self, obj, annex, offset, length):
+                if annex == "target.xml":
+                    return """<?xml version="1.0"?>
+<!-- Copyright (C) 2008 Free Software Foundation, Inc.
+
+     Copying and distribution of this file, with or without modification,
+     are permitted in any medium without royalty provided the copyright
+     notice and this notice are preserved.  -->
+
+<!DOCTYPE feature SYSTEM "gdb-target.dtd">
+<target version="1.0">
+  <architecture>arm</architecture>
+  <feature name="org.gnu.gdb.arm.m-profile">
+    <reg name="r0" bitsize="32" regnum="0" type="uint32" group="general"/>
+    <reg name="r1" bitsize="32" regnum="1" type="uint32" group="general"/>
+    <reg name="r2" bitsize="32" regnum="2" type="uint32" group="general"/>
+    <reg name="r3" bitsize="32" regnum="3" type="uint32" group="general"/>
+    <reg name="r4" bitsize="32" regnum="4" type="uint32" group="general"/>
+    <reg name="r5" bitsize="32" regnum="5" type="uint32" group="general"/>
+    <reg name="r6" bitsize="32" regnum="6" type="uint32" group="general"/>
+    <reg name="r7" bitsize="32" regnum="7" type="uint32" group="general"/>
+    <reg name="r8" bitsize="32" regnum="8" type="uint32" group="general"/>
+    <reg name="r9" bitsize="32" regnum="9" type="uint32" group="general"/>
+    <reg name="r10" bitsize="32" regnum="10" type="uint32" group="general"/>
+    <reg name="r11" bitsize="32" regnum="11" type="uint32" group="general"/>
+    <reg name="r12" bitsize="32" regnum="12" type="uint32" group="general"/>
+    <reg name="sp" bitsize="32" regnum="13" type="data_ptr" group="general"/>
+    <reg name="lr" bitsize="32" regnum="14" type="uint32" group="general"/>
+    <reg name="pc" bitsize="32" regnum="15" type="code_ptr" group="general"/>
+    <reg name="xpsr" bitsize="32" regnum="25" type="uint32" group="general"/>
+  </feature>
+  <feature name="org.gnu.gdb.arm.m-system">
+    <reg name="msp" bitsize="32" regnum="26" type="uint32" group="general"/>
+    <reg name="psp" bitsize="32" regnum="27" type="uint32" group="general"/>
+    <reg name="primask" bitsize="32" regnum="28" type="uint32" group="general"/>
+    <reg name="basepri" bitsize="32" regnum="29" type="uint32" group="general"/>
+    <reg name="faultmask" bitsize="32" regnum="30" type="uint32" group="general"/>
+    <reg name="control" bitsize="32" regnum="31" type="uint32" group="general"/>
+  </feature>
+  <feature name="org.gnu.gdb.arm.m-float">
+    <reg name="fpscr" bitsize="32" regnum="32" type="uint32" group="float"/>
+    <reg name="s0" bitsize="32" regnum="33" type="float" group="float"/>
+    <reg name="s1" bitsize="32" regnum="34" type="float" group="float"/>
+    <reg name="s2" bitsize="32" regnum="35" type="float" group="float"/>
+    <reg name="s3" bitsize="32" regnum="36" type="float" group="float"/>
+    <reg name="s4" bitsize="32" regnum="37" type="float" group="float"/>
+    <reg name="s5" bitsize="32" regnum="38" type="float" group="float"/>
+    <reg name="s6" bitsize="32" regnum="39" type="float" group="float"/>
+    <reg name="s7" bitsize="32" regnum="40" type="float" group="float"/>
+    <reg name="s8" bitsize="32" regnum="41" type="float" group="float"/>
+    <reg name="s9" bitsize="32" regnum="42" type="float" group="float"/>
+    <reg name="s10" bitsize="32" regnum="43" type="float" group="float"/>
+    <reg name="s11" bitsize="32" regnum="44" type="float" group="float"/>
+    <reg name="s12" bitsize="32" regnum="45" type="float" group="float"/>
+    <reg name="s13" bitsize="32" regnum="46" type="float" group="float"/>
+    <reg name="s14" bitsize="32" regnum="47" type="float" group="float"/>
+    <reg name="s15" bitsize="32" regnum="48" type="float" group="float"/>
+    <reg name="s16" bitsize="32" regnum="49" type="float" group="float"/>
+    <reg name="s17" bitsize="32" regnum="50" type="float" group="float"/>
+    <reg name="s18" bitsize="32" regnum="51" type="float" group="float"/>
+    <reg name="s19" bitsize="32" regnum="52" type="float" group="float"/>
+    <reg name="s20" bitsize="32" regnum="53" type="float" group="float"/>
+    <reg name="s21" bitsize="32" regnum="54" type="float" group="float"/>
+    <reg name="s22" bitsize="32" regnum="55" type="float" group="float"/>
+    <reg name="s23" bitsize="32" regnum="56" type="float" group="float"/>
+    <reg name="s24" bitsize="32" regnum="57" type="float" group="float"/>
+    <reg name="s25" bitsize="32" regnum="58" type="float" group="float"/>
+    <reg name="s26" bitsize="32" regnum="59" type="float" group="float"/>
+    <reg name="s27" bitsize="32" regnum="60" type="float" group="float"/>
+    <reg name="s28" bitsize="32" regnum="61" type="float" group="float"/>
+    <reg name="s29" bitsize="32" regnum="62" type="float" group="float"/>
+    <reg name="s30" bitsize="32" regnum="63" type="float" group="float"/>
+    <reg name="s31" bitsize="32" regnum="64" type="float" group="float"/>
+    <reg name="d0" bitsize="64" regnum="65" type="ieee_double" group="float"/>
+    <reg name="d1" bitsize="64" regnum="66" type="ieee_double" group="float"/>
+    <reg name="d2" bitsize="64" regnum="67" type="ieee_double" group="float"/>
+    <reg name="d3" bitsize="64" regnum="68" type="ieee_double" group="float"/>
+    <reg name="d4" bitsize="64" regnum="69" type="ieee_double" group="float"/>
+    <reg name="d5" bitsize="64" regnum="70" type="ieee_double" group="float"/>
+    <reg name="d6" bitsize="64" regnum="71" type="ieee_double" group="float"/>
+    <reg name="d7" bitsize="64" regnum="72" type="ieee_double" group="float"/>
+    <reg name="d8" bitsize="64" regnum="73" type="ieee_double" group="float"/>
+    <reg name="d9" bitsize="64" regnum="74" type="ieee_double" group="float"/>
+    <reg name="d10" bitsize="64" regnum="75" type="ieee_double" group="float"/>
+    <reg name="d11" bitsize="64" regnum="76" type="ieee_double" group="float"/>
+    <reg name="d12" bitsize="64" regnum="77" type="ieee_double" group="float"/>
+    <reg name="d13" bitsize="64" regnum="78" type="ieee_double" group="float"/>
+    <reg name="d14" bitsize="64" regnum="79" type="ieee_double" group="float"/>
+    <reg name="d15" bitsize="64" regnum="80" type="ieee_double" group="float"/>
+  </feature>
+</target>""", False
+                else:
+                    return None, False
+
+            def readRegister(self, regnum):
+                return "E01"
+
+            ## readRegisters only provides reg values up through xpsr (0x61000000)
+            ## it doesn't send up any of the exception registers or floating point
+            ## registers that the above register xml describes.
+            def readRegisters(self):
+                return "00000000" + self.r1_bytes + "010000000100000001000000000000008c080020a872012000000000a0790120000000008065012041ad0008a0720120692a00089e26000800000061"
+
+            ## the J-Link accepts a register write packet with just the GPRs
+            ## defined.
+            def writeRegisters(self, registers_hex):
+                if registers_hex.startswith("0000000044332211010000000100000001000000000000008c080020a872012000000000a0790120000000008065012041ad0008a0720120692a00089e26000800000061"):
+                    self.r1_bytes = "44332211"
+                    return "OK"
+                else:
+                    return "E07"
+
+            def haltReason(self):
+                return "S05"
+
+            def qfThreadInfo(self):
+                return "mdead"
+            
+            def qC(self):
+                return ""
+
+            def qSupported(self, client_supported):
+                return "PacketSize=4000;qXfer:memory-map:read-;QStartNoAckMode+;hwbreak+;qXfer:features:read+"
+
+            def QThreadSuffixSupported(self):
+                return "OK"
+
+            def QListThreadsInStopReply(self):
+                return "OK"
+
+        self.server.responder = MyResponder()
+        if self.TraceOn():
+            self.runCmd("log enable gdb-remote packets")
+            self.addTearDownHook(
+                    lambda: self.runCmd("log disable gdb-remote packets"))
+
+        self.dbg.SetDefaultArchitecture("armv7em")
+        target = self.dbg.CreateTargetWithFileAndArch(None, None)
+
+        process = self.connect(target)
+
+        if self.TraceOn():
+            interp = self.dbg.GetCommandInterpreter()
+            result = lldb.SBCommandReturnObject()
+            interp.HandleCommand("target list", result)
+            print(result.GetOutput())
+
+        r1_valobj = process.GetThreadAtIndex(0).GetFrameAtIndex(0).FindRegister("r1")
+        self.assertEqual(r1_valobj.GetValueAsUnsigned(), 1)
+
+        pc_valobj = process.GetThreadAtIndex(0).GetFrameAtIndex(0).FindRegister("pc")
+        self.assertEqual(pc_valobj.GetValueAsUnsigned(), 0x0800269e)
+
+        xpsr_valobj = process.GetThreadAtIndex(0).GetFrameAtIndex(0).FindRegister("xpsr")
+        self.assertEqual(xpsr_valobj.GetValueAsUnsigned(), 0x61000000)
+
+        msp_valobj = process.GetThreadAtIndex(0).GetFrameAtIndex(0).FindRegister("msp")
+        err = msp_valobj.GetError()
+        self.assertTrue(err.Fail(), "lldb cannot fetch the msp register from this stub")
+
+        val = b'\x11\x22\x33\x44'
+        error = lldb.SBError()
+        data = lldb.SBData()
+        data.SetData(error, val, lldb.eByteOrderBig, 4)
+        self.assertEqual(r1_valobj.SetData(data, error), True)
+
+        r1_valobj = process.GetThreadAtIndex(0).GetFrameAtIndex(0).FindRegister("r1")
+        self.assertEqual(r1_valobj.GetValueAsUnsigned(), 0x11223344)
+

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

 const RegisterSet *GDBRemoteRegisterContext::GetRegisterSet(size_t reg_set) {
   return m_reg_info.GetRegisterSet(reg_set);
 }
 
 bool GDBRemoteRegisterContext::ReadRegister(const RegisterInfo *reg_info,
                                             RegisterValue &value) {
   // Read the register
   if (ReadRegisterBytes(reg_info, m_reg_data)) {
+    const uint32_t reg = reg_info->kinds[eRegisterKindLLDB];
+    if (m_reg_valid[reg] == false)
+      return false;
     const bool partial_data_ok = false;
     Status error(value.SetValueFromData(
         reg_info, m_reg_data, reg_info->byte_offset, partial_data_ok));
     return error.Success();
   }
   return false;
 }
 
if (m_read_all_at_once) {
               gdb_comm.ReadAllRegisters(m_thread.GetProtocolID())) {
         memcpy(const_cast<uint8_t *>(m_reg_data.GetDataStart()),
                buffer_sp->GetBytes(),
                std::min(buffer_sp->GetByteSize(), m_reg_data.GetByteSize()));
         if (buffer_sp->GetByteSize() >= m_reg_data.GetByteSize()) {
           SetAllRegisterValid(true);
           return true;
         } else {
-          Log *log(ProcessGDBRemoteLog::GetLogIfAnyCategoryIsSet(GDBR_LOG_THREAD |
-                                                                GDBR_LOG_PACKETS));
-          LLDB_LOGF(
-              log,
-              "error: GDBRemoteRegisterContext::ReadRegisterBytes tried "
-              "to read the "
-              "entire register context at once, expected at least %" PRId64
-              " bytes "
-              "but only got %" PRId64 " bytes.",
-              m_reg_data.GetByteSize(), buffer_sp->GetByteSize());
-        }
+          const int regcount = m_reg_info.GetNumRegisters();
+          for (int i = 0; i < regcount; i++) {
+            struct RegisterInfo *reginfo = m_reg_info.GetRegisterInfoAtIndex(i);
+            if (reginfo->byte_offset < buffer_sp->GetByteSize()) {
+              m_reg_valid[i] = true;
+            } else {
+              m_reg_valid[i] = false;
+            }
+          }
+          return true;
+        } 
       }
       return false;
     }
     if (reg_info->value_regs) {
       // Process this composite register request by delegating to the
       // constituent primordial registers.
 
       // Index of the primordial register.