[PATCH v5 2/5] gdb: Improve SIGSEGV diagnostics for POE faults

[<srinath.parvathaneni@arm.com>]

From: Srinath Parvathaneni <srinath.parvathaneni@arm.com>

When a SIGSEGV is caused by a Permission Overlay violation, include
additional information in GDB's error message to help identify the
failing address along with the responsible protection key causing
this fault.

Example:
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault
Protection Key Violation while accessing address 0x0000fffff7ff3000 with Protection Key = 1.
0x0000aaaaaaaa0a3c in main () at poe_sigsegv.c:26

Approved-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
---
 gdb/Makefile.in              |  1 +
 gdb/aarch64-linux-tdep.c     | 21 ++++++++++++++++++---
 gdb/arch/aarch64-poe-linux.h | 29 +++++++++++++++++++++++++++++
 3 files changed, 48 insertions(+), 3 deletions(-)
 create mode 100644 gdb/arch/aarch64-poe-linux.h

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 6909123e863..cc04f0de68e 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -1297,6 +1297,7 @@ HFILES_NO_SRCDIR = \
 	arch/aarch64-mte.h \
 	arch/aarch64-mte-linux.h \
 	arch/aarch64-pauth-linux.h \
+	arch/aarch64-poe-linux.h \
 	arch/aarch64-scalable-linux.h \
 	arch/amd64.h \
 	arch/amd64-linux-tdesc.h \
diff --git a/gdb/aarch64-linux-tdep.c b/gdb/aarch64-linux-tdep.c
index 235b35bcfb4..1b2d4435088 100644
--- a/gdb/aarch64-linux-tdep.c
+++ b/gdb/aarch64-linux-tdep.c
@@ -53,6 +53,7 @@
 
 #include "arch/aarch64-fpmr-linux.h"
 #include "arch/aarch64-gcs-linux.h"
+#include "arch/aarch64-poe-linux.h"
 #include "arch/aarch64-mte.h"
 #include "arch/aarch64-mte-linux.h"
 #include "arch/aarch64-pauth-linux.h"
@@ -2675,11 +2676,12 @@ aarch64_linux_report_signal_info (struct gdbarch *gdbarch,
 {
   aarch64_gdbarch_tdep *tdep = gdbarch_tdep<aarch64_gdbarch_tdep> (gdbarch);
 
-  if (!(tdep->has_mte () || tdep->has_gcs ()) || siggnal != GDB_SIGNAL_SEGV)
+  if (!(tdep->has_mte () || tdep->has_gcs () || tdep->has_poe ())
+      || siggnal != GDB_SIGNAL_SEGV)
     return;
 
   CORE_ADDR fault_addr = 0;
-  long si_code = 0, si_errno = 0;
+  long si_code = 0, si_errno = 0, si_pkey = -1;
 
   try
     {
@@ -2689,6 +2691,8 @@ aarch64_linux_report_signal_info (struct gdbarch *gdbarch,
 	 violation.  */
       si_code = parse_and_eval_long (gdb_si::get (si_key::siginfo_code));
       si_errno = parse_and_eval_long (gdb_si::get (si_key::siginfo_errno));
+      if (tdep->has_poe ())
+	si_pkey = parse_and_eval_long (gdb_si::get (si_key::siginfo_pkey));
 
       fault_addr
 	= parse_and_eval_long (gdb_si::get (si_key::siginfo_addr));
@@ -2705,6 +2709,8 @@ aarch64_linux_report_signal_info (struct gdbarch *gdbarch,
     meaning = _("Memory tag violation");
   else if (si_code == AARCH64_SEGV_CPERR && si_errno == 0)
     meaning = _("Guarded Control Stack error");
+  else if (si_code == AARCH64_SEGV_PKUERR)
+    meaning = _("Protection Key Violation");
   else
     return;
 
@@ -2736,6 +2742,15 @@ aarch64_linux_report_signal_info (struct gdbarch *gdbarch,
 	  uiout->field_string ("logical-tag", hex_string (ltag));
 	}
     }
+  /* Additional information for SIGSEGV caused by a permission overlay
+     violation.  */
+  else if (si_code == AARCH64_SEGV_PKUERR)
+    {
+      uiout->text (_(" while accessing address "));
+      uiout->field_core_addr ("fault-addr", gdbarch, fault_addr);
+      uiout->text (_(" with Protection Key = "));
+      uiout->field_signed ("protection-key", si_pkey);
+    }
   else if (si_code != AARCH64_SEGV_CPERR)
     {
       uiout->text ("\n");
@@ -3056,7 +3071,7 @@ aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
 					 aarch64_linux_decode_memtag_section);
     }
 
-  if (tdep->has_mte () || tdep->has_gcs ())
+  if (tdep->has_mte () || tdep->has_gcs () || tdep->has_poe ())
     set_gdbarch_report_signal_info (gdbarch, aarch64_linux_report_signal_info);
 
   /* Initialize the aarch64_linux_record_tdep.  */
diff --git a/gdb/arch/aarch64-poe-linux.h b/gdb/arch/aarch64-poe-linux.h
new file mode 100644
index 00000000000..d6e4a001cbf
--- /dev/null
+++ b/gdb/arch/aarch64-poe-linux.h
@@ -0,0 +1,29 @@
+/* Common Linux target-dependent definitions for AArch64 POE
+
+   Copyright (C) 2026 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_ARCH_AARCH64_POE_LINUX_H
+#define GDB_ARCH_AARCH64_POE_LINUX_H
+
+/* Feature check for Permission Overlay Extension.  */
+#define AARCH64_HWCAP2_POE (1ULL << 63)
+
+/* Data or instruction abort caused by Protection Key Violation.  */
+#define AARCH64_SEGV_PKUERR 4
+
+#endif /* GDB_ARCH_AARCH64_POE_LINUX_H */
-- 
2.43.0