Advertise pointer authentication support to GDB

GDB uses the values of the provided pointer authentication mask
registers to mask return addresses from the stack trace. Because we
weren't providing these masks, the return addresses in the stack trace
were incorrect in programs built with -mbranch-protection=pac-ret, such
as most of Fedora. Fix it by advertising pointer authentication to GDB,
and providing the correct masks.

Fixes the following tests on PAC platforms:

fork_exec_info_thr
fork_exec_info_thr-no-syscallbuf
vdso_clock_gettime_stack
vdso_time_stack

Author: pcc

src/ExtraRegisters.cc

@@ -275,6 +275,21 @@ size_t ExtraRegisters::read_register(uint8_t* buf, GdbServerRegister regno,
     } else if (regno == DREG_FPCR) {
       reg_data = RegData(offsetof(ARM64Arch::user_fpsimd_state, fpcr),
                          sizeof(uint32_t));
+#ifdef __aarch64__
+    } else if (regno == DREG_PAUTH_DMASK || regno == DREG_PAUTH_CMASK) {
+      uint64_t ptr = 1ULL << 55;
+      // The XPAC instruction will copy bit 55 of the argument into the PAC mask
+      // bits, so ptr will be set to the mask plus bit 55.
+      if (regno == DREG_PAUTH_DMASK) {
+        __asm__ __volatile__(".arch armv8.3-a\nxpacd %0" : "+r"(ptr));
+      } else {
+        __asm__ __volatile__(".arch armv8.3-a\nxpaci %0" : "+r"(ptr));
+      }
+      uint64_t mask = ptr & ~(1ULL << 55);
+      *defined = true;
+      memcpy(buf, &mask, sizeof(mask));
+      return sizeof(mask);
+#endif
     } else {
       *defined = false;
       return 0;

src/GdbServerConnection.cc

@@ -18,11 +18,18 @@
 #include <poll.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/auxv.h>
 #include <sys/mman.h>
 #include <sys/socket.h>
 #include <sys/types.h>
 #include <unistd.h>
 
+#ifdef __aarch64__
+#ifndef HWCAP_PACA
+#define HWCAP_PACA (1 << 30)
+#endif
+#endif
+
 #include <iomanip>
 #include <sstream>
 #include <vector>
@@ -106,6 +113,11 @@ static uint32_t get_cpu_features(SupportedArch arch) {
     }
     case aarch64:
       cpu_features = GdbServerConnection::CPU_AARCH64;
+#ifdef __aarch64__
+      if (getauxval(AT_HWCAP) & HWCAP_PACA) {
+        cpu_features |= GdbServerConnection::CPU_PAUTH;
+      }
+#endif
       break;
     default:
       FATAL() << "Unknown architecture";

src/GdbServerConnection.h

@@ -752,7 +752,8 @@ class GdbServerConnection {
     CPU_AVX = 1 << 1,
     CPU_AARCH64 = 1 << 2,
     CPU_PKU = 1 << 3,
-    CPU_AVX512 = 1 << 4
+    CPU_AVX512 = 1 << 4,
+    CPU_PAUTH = 1 << 5
   };
 
   void set_cpu_features(SupportedArch arch);

src/GdbServerRegister.h

@@ -320,7 +320,11 @@ enum GdbServerRegister {
   DREG_FPSR,
   DREG_FPCR,
 
-  DREG_NUM_LINUX_AARCH64 = DREG_FPCR + 1,
+  // aarch64-pauth.xml
+  DREG_PAUTH_DMASK,
+  DREG_PAUTH_CMASK,
+
+  DREG_NUM_LINUX_AARCH64,
 };
 
 } // namespace rr

src/Registers.cc

@@ -180,6 +180,10 @@ RegisterInfo<rr::ARM64Arch>::Table RegisterInfo<rr::ARM64Arch>::registers = {
   // comparison errors. This seems to be a unused/undocumented bit in SPSR as per aarch64
   // documentation anyways so ignore it.
   RV_AARCH64_WITH_MASK(CPSR, pstate, 0xffffffffLL & ~AARCH64_DBG_SPSR_SS & ~AARCH64_DBG_SPSR_11, 4),
+  // PAuth masks. These aren't real registers, so we set the offset/size to 0
+  // and use custom handling in read_register().
+  RegisterInit(DREG_PAUTH_DMASK, RegisterValue("pauth_dmask", 0, 0)),
+  RegisterInit(DREG_PAUTH_CMASK, RegisterValue("pauth_cmask", 0, 0)),
 };
 
 #undef RV_X64

src/TargetDescription.cc

@@ -78,6 +78,9 @@ FeatureStream& operator<<(FeatureStream& stream, TargetFeature feature) {
     case TargetFeature::FPU:
       stream << "fpu.xml";
       break;
+    case TargetFeature::PAuth:
+      stream << "pauth.xml";
+      break;
   }
   stream << R"("/>)" << '\n';
   return stream;
@@ -103,6 +106,9 @@ TargetDescription::TargetDescription(rr::SupportedArch arch,
     case rr::aarch64:
       target_features.push_back(TargetFeature::Core);
       target_features.push_back(TargetFeature::FPU);
+      if (cpu_features & rr::GdbServerConnection::CPU_PAUTH) {
+        target_features.push_back(TargetFeature::PAuth);
+      }
       break;
   }
 

src/TargetDescription.h

@@ -20,6 +20,7 @@ enum class TargetFeature : uint32_t {
   AVX512,
   PKeys,
   FPU,
+  PAuth,
 };
 
 class TargetDescription {