[AArch64] Make gdbserver register set selection dynamic

The current register set selection mechanism for AArch64 is static, based
on a pre-populated array of register sets.

This means that we might potentially probe register sets that are not
available. This is OK if the kernel errors out during ptrace, but probing the
tag_ctl register, for example, does not result in a ptrace error if the kernel
supports the tagged address ABI but not MTE (PR 28355).

Making the register set selection dynamic, based on feature checks, solves
this and simplifies the code a bit. It allows us to list all of the register
sets only once, and pick and choose based on HWCAP/HWCAP2 or other properties.

I plan to backport this fix to GDB 11 as well.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28355

2 files changed, 110 insertions, 85 deletions

diff --git a/gdb/arch/aarch64.h b/gdb/arch/aarch64.h
index 0eb702c..95edb66 100644
--- a/gdb/arch/aarch64.h
+++ b/gdb/arch/aarch64.h
@@ -22,6 +22,15 @@
 
 #include "gdbsupport/tdesc.h"
 
+/* Holds information on what architectural features are available.  This is
+   used to select register sets.  */
+struct aarch64_features
+{
+  bool sve = false;
+  bool pauth = false;
+  bool mte = false;
+};
+
 /* Create the aarch64 target description.  A non zero VQ value indicates both
    the presence of SVE and the Vector Quotient - the number of 128bit chunks in
    an SVE Z register.  HAS_PAUTH_P indicates the presence of the PAUTH
diff --git a/gdbserver/linux-aarch64-low.cc b/gdbserver/linux-aarch64-low.cc
index daccfef..9a8cb41 100644
--- a/gdbserver/linux-aarch64-low.cc
+++ b/gdbserver/linux-aarch64-low.cc
@@ -196,16 +196,6 @@ is_64bit_tdesc (void)
   return register_size (regcache->tdesc, 0) == 8;
 }
 
-/* Return true if the regcache contains the number of SVE registers.  */
-
-static bool
-is_sve_tdesc (void)
-{
-  struct regcache *regcache = get_thread_regcache (current_thread, 0);
-
-  return tdesc_contains_feature (regcache->tdesc, "org.gnu.gdb.aarch64.sve");
-}
-
 static void
 aarch64_fill_gregset (struct regcache *regcache, void *buf)
 {
@@ -680,40 +670,6 @@ aarch64_target::low_new_fork (process_info *parent,
   *child->priv->arch_private = *parent->priv->arch_private;
 }
 
-/* Matches HWCAP_PACA in kernel header arch/arm64/include/uapi/asm/hwcap.h.  */
-#define AARCH64_HWCAP_PACA (1 << 30)
-
-/* Implementation of linux target ops method "low_arch_setup".  */
-
-void
-aarch64_target::low_arch_setup ()
-{
-  unsigned int machine;
-  int is_elf64;
-  int tid;
-
-  tid = lwpid_of (current_thread);
-
-  is_elf64 = linux_pid_exe_is_elf_64_file (tid, &machine);
-
-  if (is_elf64)
-    {
-      uint64_t vq = aarch64_sve_get_vq (tid);
-      unsigned long hwcap = linux_get_hwcap (8);
-      unsigned long hwcap2 = linux_get_hwcap2 (8);
-      bool pauth_p = hwcap & AARCH64_HWCAP_PACA;
-      /* MTE is AArch64-only.  */
-      bool mte_p = hwcap2 & HWCAP2_MTE;
-
-      current_process ()->tdesc
-	= aarch64_linux_read_description (vq, pauth_p, mte_p);
-    }
-  else
-    current_process ()->tdesc = aarch32_linux_read_description ();
-
-  aarch64_linux_get_debug_reg_capacity (lwpid_of (current_thread));
-}
-
 /* Wrapper for aarch64_sve_regs_copy_to_reg_buf.  */
 
 static void
@@ -730,21 +686,36 @@ aarch64_sve_regs_copy_from_regcache (struct regcache *regcache, void *buf)
   return aarch64_sve_regs_copy_from_reg_buf (regcache, buf);
 }
 
+/* Array containing all the possible register sets for AArch64/Linux.  During
+   architecture setup, these will be checked against the HWCAP/HWCAP2 bits for
+   validity and enabled/disabled accordingly.
+
+   Their sizes are set to 0 here, but they will be adjusted later depending
+   on whether each register set is available or not.  */
 static struct regset_info aarch64_regsets[] =
 {
+  /* GPR registers.  */
   { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_PRSTATUS,
-    sizeof (struct user_pt_regs), GENERAL_REGS,
+    0, GENERAL_REGS,
     aarch64_fill_gregset, aarch64_store_gregset },
+  /* Floating Point (FPU) registers.  */
   { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_FPREGSET,
-    sizeof (struct user_fpsimd_state), FP_REGS,
+    0, FP_REGS,
     aarch64_fill_fpregset, aarch64_store_fpregset
   },
+  /* Scalable Vector Extension (SVE) registers.  */
+  { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_ARM_SVE,
+    0, EXTENDED_REGS,
+    aarch64_sve_regs_copy_from_regcache, aarch64_sve_regs_copy_to_regcache
+  },
+  /* PAC registers.  */
   { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_ARM_PAC_MASK,
-    AARCH64_PAUTH_REGS_SIZE, OPTIONAL_REGS,
-    NULL, aarch64_store_pauthregset },
+    0, OPTIONAL_REGS,
+    nullptr, aarch64_store_pauthregset },
+  /* Tagged address control / MTE registers.  */
   { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_ARM_TAGGED_ADDR_CTRL,
-    AARCH64_LINUX_SIZEOF_MTE, OPTIONAL_REGS, aarch64_fill_mteregset,
-    aarch64_store_mteregset },
+    0, OPTIONAL_REGS,
+    aarch64_fill_mteregset, aarch64_store_mteregset },
   NULL_REGSET
 };
 
@@ -752,47 +723,95 @@ static struct regsets_info aarch64_regsets_info =
   {
     aarch64_regsets, /* regsets */
     0, /* num_regsets */
-    NULL, /* disabled_regsets */
+    nullptr, /* disabled_regsets */
   };
 
 static struct regs_info regs_info_aarch64 =
   {
-    NULL, /* regset_bitmap */
-    NULL, /* usrregs */
+    nullptr, /* regset_bitmap */
+    nullptr, /* usrregs */
     &aarch64_regsets_info,
   };
 
-static struct regset_info aarch64_sve_regsets[] =
+/* Given FEATURES, adjust the available register sets by setting their
+   sizes.  A size of 0 means the register set is disabled and won't be
+   used.  */
+
+static void
+aarch64_adjust_register_sets (const struct aarch64_features &features)
 {
-  { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_PRSTATUS,
-    sizeof (struct user_pt_regs), GENERAL_REGS,
-    aarch64_fill_gregset, aarch64_store_gregset },
-  { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_ARM_SVE,
-    SVE_PT_SIZE (AARCH64_MAX_SVE_VQ, SVE_PT_REGS_SVE), EXTENDED_REGS,
-    aarch64_sve_regs_copy_from_regcache, aarch64_sve_regs_copy_to_regcache
-  },
-  { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_ARM_PAC_MASK,
-    AARCH64_PAUTH_REGS_SIZE, OPTIONAL_REGS,
-    NULL, aarch64_store_pauthregset },
-  { PTRACE_GETREGSET, PTRACE_SETREGSET, NT_ARM_TAGGED_ADDR_CTRL,
-    AARCH64_LINUX_SIZEOF_MTE, OPTIONAL_REGS, aarch64_fill_mteregset,
-    aarch64_store_mteregset },
-  NULL_REGSET
-};
+  struct regset_info *regset;
 
-static struct regsets_info aarch64_sve_regsets_info =
-  {
-    aarch64_sve_regsets, /* regsets.  */
-    0, /* num_regsets.  */
-    NULL, /* disabled_regsets.  */
-  };
+  for (regset = aarch64_regsets; regset->size >= 0; regset++)
+    {
+      switch (regset->nt_type)
+	{
+	case NT_PRSTATUS:
+	  /* General purpose registers are always present.  */
+	  regset->size = sizeof (struct user_pt_regs);
+	  break;
+	case NT_FPREGSET:
+	  /* This is unavailable when SVE is present.  */
+	  if (!features.sve)
+	    regset->size = sizeof (struct user_fpsimd_state);
+	  break;
+	case NT_ARM_SVE:
+	  if (features.sve)
+	    regset->size = SVE_PT_SIZE (AARCH64_MAX_SVE_VQ, SVE_PT_REGS_SVE);
+	  break;
+	case NT_ARM_PAC_MASK:
+	  if (features.pauth)
+	    regset->size = AARCH64_PAUTH_REGS_SIZE;
+	  break;
+	case NT_ARM_TAGGED_ADDR_CTRL:
+	  if (features.mte)
+	    regset->size = AARCH64_LINUX_SIZEOF_MTE;
+	  break;
+	default:
+	  gdb_assert_not_reached ("Unknown register set found.");
+	}
+    }
+}
 
-static struct regs_info regs_info_aarch64_sve =
-  {
-    NULL, /* regset_bitmap.  */
-    NULL, /* usrregs.  */
-    &aarch64_sve_regsets_info,
-  };
+/* Matches HWCAP_PACA in kernel header arch/arm64/include/uapi/asm/hwcap.h.  */
+#define AARCH64_HWCAP_PACA (1 << 30)
+
+/* Implementation of linux target ops method "low_arch_setup".  */
+
+void
+aarch64_target::low_arch_setup ()
+{
+  unsigned int machine;
+  int is_elf64;
+  int tid;
+
+  tid = lwpid_of (current_thread);
+
+  is_elf64 = linux_pid_exe_is_elf_64_file (tid, &machine);
+
+  if (is_elf64)
+    {
+      struct aarch64_features features;
+
+      uint64_t vq = aarch64_sve_get_vq (tid);
+      features.sve = (vq > 0);
+      /* A-profile PAC is 64-bit only.  */
+      features.pauth = linux_get_hwcap (8) & AARCH64_HWCAP_PACA;
+      /* A-profile MTE is 64-bit only.  */
+      features.mte = linux_get_hwcap2 (8) & HWCAP2_MTE;
+
+      current_process ()->tdesc
+	= aarch64_linux_read_description (vq, features.pauth, features.mte);
+
+      /* Adjust the register sets we should use for this particular set of
+	 features.  */
+      aarch64_adjust_register_sets (features);
+    }
+  else
+    current_process ()->tdesc = aarch32_linux_read_description ();
+
+  aarch64_linux_get_debug_reg_capacity (lwpid_of (current_thread));
+}
 
 /* Implementation of linux target ops method "get_regs_info".  */
 
@@ -802,9 +821,7 @@ aarch64_target::get_regs_info ()
   if (!is_64bit_tdesc ())
     return &regs_info_aarch32;
 
-  if (is_sve_tdesc ())
-    return &regs_info_aarch64_sve;
-
+  /* AArch64 64-bit registers.  */
   return &regs_info_aarch64;
 }
 
@@ -3294,5 +3311,4 @@ initialize_low_arch (void)
   initialize_low_arch_aarch32 ();
 
   initialize_regsets_info (&aarch64_regsets_info);
-  initialize_regsets_info (&aarch64_sve_regsets_info);
 }