Fix floating point register reads x86_64 linux on targets with no AVX support

[labath]

Summary

On for 64-bit targets, the correct register set to read the fxsave are is
NT_PRFPREG (only 32-bit targets need NT_PRXFPREG, presumably for historic
reasons). Reference:
https://github.com/torvalds/linux/blob/v4.8/arch/x86/kernel/ptrace.c#L1261.

[valentinagiusti]

lgtm, thanks!

[tberghammer]

LGTM

source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.cpp

246–247 ↗

(On Diff #79388)

(nit): Please revert

lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.cpp

 #include "lldb/Core/Error.h"
 #include "lldb/Core/Log.h"
 #include "lldb/Core/RegisterValue.h"
 #include "lldb/Host/HostInfo.h"
 
 #include "Plugins/Process/Utility/RegisterContextLinux_i386.h"
 #include "Plugins/Process/Utility/RegisterContextLinux_x86_64.h"
 
+#include <linux/elf.h>
+
 using namespace lldb_private;
 using namespace lldb_private::process_linux;
 
 // ----------------------------------------------------------------------------
 // Private namespace.
 // ----------------------------------------------------------------------------
 
 namespace {

 // Support ptrace extensions even when compiled without required kernel support
 #ifndef NT_X86_XSTATE
 #define NT_X86_XSTATE 0x202
 #endif
 #ifndef NT_PRXFPREG
 #define NT_PRXFPREG 0x46e62b7f
 #endif
 
+// On x86_64 NT_PRFPREG is used to access the FXSAVE area. On i386, we need to
+// use NT_PRXFPREG.
+static inline unsigned int fxsr_regset(const ArchSpec &arch) {
+  return arch.GetAddressByteSize() == 8 ? NT_PRFPREG : NT_PRXFPREG;
+}
+
 // ----------------------------------------------------------------------------
 // Required MPX define.
 // ----------------------------------------------------------------------------
 
 // Support MPX extensions also if compiled with compiler without MPX support.
 #ifndef bit_MPX
 #define bit_MPX 0x4000
 #endif

 bool NativeRegisterContextLinux_x86_64::IsFPR(uint32_t reg_index) const {
   return (m_reg_info.first_fpr <= reg_index &&
           reg_index <= m_reg_info.last_fpr);
 }
 
 Error NativeRegisterContextLinux_x86_64::WriteFPR() {
   switch (m_xstate_type) {
   case XStateType::FXSAVE:
-      return WriteRegisterSet(&m_iovec, sizeof(m_fpr.xstate.xsave),
-                              NT_PRXFPREG);
+    return WriteRegisterSet(
+        &m_iovec, sizeof(m_fpr.xstate.xsave),
+        fxsr_regset(GetRegisterInfoInterface().GetTargetArchitecture()));
   case XStateType::XSAVE:
     return WriteRegisterSet(&m_iovec, sizeof(m_fpr.xstate.xsave),
                             NT_X86_XSTATE);
   default:
     return Error("Unrecognized FPR type.");
   }
 }
 
Error NativeRegisterContextLinux_x86_64::ReadFPR() {
   if (m_xstate_type != XStateType::FXSAVE) {
     error =
         ReadRegisterSet(&m_iovec, sizeof(m_fpr.xstate.xsave), NT_X86_XSTATE);
     if (!error.Fail()) {
       m_xstate_type = XStateType::XSAVE;
       return error;
     }
   }
-  error = ReadRegisterSet(&m_iovec, sizeof(m_fpr.xstate.xsave), NT_PRXFPREG);
+  error = ReadRegisterSet(
+      &m_iovec, sizeof(m_fpr.xstate.xsave),
+      fxsr_regset(GetRegisterInfoInterface().GetTargetArchitecture()));
   if (!error.Fail()) {
     m_xstate_type = XStateType::FXSAVE;
     return error;
   }
   return Error("Unrecognized FPR type.");
 }
 
 bool NativeRegisterContextLinux_x86_64::IsMPX(uint32_t reg_index) const {