[PATCH v4 10/13] gdb: Implement the hook 'is_no_return_shadow_stack_address' for amd64 linux.

[Christina Schimpe <christina.schimpe@intel.com>]

There can be elements on the shadow stack which are not return addresses.
This can happen, for instance, in case of signals on amd64 linux.
The old shadow stack pointer is pushed in a special format with bit 63 set.

|1...old SSP| - Pointer to old pre-signal ssp in sigframe token format
                (bit 63 set to 1)

Linux kernel documentation: https://docs.kernel.org/arch/x86/shstk.html

Implement the gdbarch hook is_no_return_shadow_stack_address to detect
this scenario to print the shadow stack backtrace correctly.
---
 gdb/amd64-linux-tdep.c                        | 59 +++++++++++++++++++
 .../amd64-shadow-stack-backtrace-signal.exp   | 49 +++++++++++++++
 .../gdb.arch/amd64-shadow-stack-signal.c      | 31 ++++++++++
 3 files changed, 139 insertions(+)
 create mode 100644 gdb/testsuite/gdb.arch/amd64-shadow-stack-backtrace-signal.exp
 create mode 100644 gdb/testsuite/gdb.arch/amd64-shadow-stack-signal.c

diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c
index 42a62eaa975..d98415b8989 100644
--- a/gdb/amd64-linux-tdep.c
+++ b/gdb/amd64-linux-tdep.c
@@ -1960,6 +1960,62 @@ amd64_linux_top_addr_empty_shadow_stack
   return addr == range.second;
 }
 
+/* Return a shadow stack frame info, if the shadow stack pointer SSP
+   belongs to a valid shadow stack frame while the element on the shadow
+   stack VALUE does not refer to a return address.  This can happen, for
+   instance, in case of signals.  The old shadow stack pointer is pushed
+   in a special format with bit 63 set.  In case this is true, a valid
+   shadow stack frame info is returned with its attributes frame_type and
+   non_return_description configured to ssp_frame_type::non_return_frame
+   and "<sigframe token>", respectively.  */
+
+static std::optional<shadow_stack_frame_info>
+amd64_linux_is_no_return_shadow_stack_address
+  (gdbarch *gdbarch, const CORE_ADDR ssp, const CORE_ADDR value,
+   const unsigned long level)
+{
+  /* SSP must belong to the shadow stack memory range.  */
+  std::pair<CORE_ADDR, CORE_ADDR> range;
+  gdb_assert (gdbarch_address_in_shadow_stack_memory_range (gdbarch,
+							    ssp,
+							    &range));
+
+  /* In case bit 63 is not configured, the address on the shadow stack
+     should be a return address.  */
+  constexpr CORE_ADDR mask = (CORE_ADDR) 1 << 63;
+  if ((value & mask) == 0)
+    return {};
+
+  /* To compare the shadow stack pointer of the previous frame with the
+     value of FRAME, we must clear bit 63.  */
+  CORE_ADDR shadow_stack_val_cleared = (value & (~mask));
+
+  /* Compute the previous/old SSP.  The shadow stack grows downwards.  To
+     compute the previous shadow stack pointer, we need to increment
+     SSP.  */
+  CORE_ADDR prev_ssp
+    = ssp + gdbarch_shadow_stack_element_size_aligned (gdbarch);
+
+  /* We incremented SSP by one element to compute PREV_SSP before.  In
+     case SSP points to the first element of the shadow stack, PREV_SSP
+     must point to the bottom of the shadow stack (RANGE.SECOND), but not
+     beyond that address.  */
+  gdb_assert (prev_ssp > range.first && prev_ssp <= range.second);
+
+  if (shadow_stack_val_cleared == prev_ssp)
+    {
+      /* Assign the current gdbarch to the new shadow stack frame.  Since
+	 the token is no PC value, do not assign a SAL object.  */
+      return std::optional<shadow_stack_frame_info>
+	({ssp, value, level, gdbarch, {},
+	  ssp_frame_type::non_return_frame,
+	  {"<sigframe token>"},
+	  ssp_unwind_stop_reason::no_error});
+    }
+
+  return {};
+}
+
 static void
 amd64_linux_init_abi_common (struct gdbarch_info info, struct gdbarch *gdbarch,
 			     int num_disp_step_buffers)
@@ -2023,6 +2079,9 @@ amd64_linux_init_abi_common (struct gdbarch_info info, struct gdbarch *gdbarch,
 
   set_gdbarch_top_addr_empty_shadow_stack
     (gdbarch, amd64_linux_top_addr_empty_shadow_stack);
+
+  set_gdbarch_is_no_return_shadow_stack_address
+    (gdbarch, amd64_linux_is_no_return_shadow_stack_address);
 }
 
 static void
diff --git a/gdb/testsuite/gdb.arch/amd64-shadow-stack-backtrace-signal.exp b/gdb/testsuite/gdb.arch/amd64-shadow-stack-backtrace-signal.exp
new file mode 100644
index 00000000000..21373dc07f3
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/amd64-shadow-stack-backtrace-signal.exp
@@ -0,0 +1,49 @@
+# Copyright 2024-2026 Free Software Foundation, Inc.
+
+# 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/>.
+
+# Test shadow stack backtrace for signal handling on linux.
+
+require allow_ssp_tests {istarget "*-*-linux*"}
+
+standard_testfile amd64-shadow-stack-signal.c
+
+save_vars { ::env(GLIBC_TUNABLES) } {
+
+    append_environment GLIBC_TUNABLES "glibc.cpu.hwcaps" "SHSTK"
+
+    if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
+	  {debug additional_flags="-fcf-protection=return"}] } {
+	return
+    }
+
+    if { ![runto_main] } {
+	return
+    }
+
+    gdb_breakpoint "handler"
+    gdb_test "continue" \
+	".*Program received signal SIGUSR1, User defined signal 1.*" \
+	"continue until signal"
+    gdb_continue_to_breakpoint "continue to breakpoint in handler"
+
+    # Test shadow stack backtrace including <sigframe token>.
+    gdb_test "bt -shadow" \
+	[multi_line \
+	    "#0\[ \t\]*$hex in \[^\r\n\]+" \
+	    "#1\[ \t\]*<sigframe token>" \
+	    "#2\[ \t\]*$hex in \[^\r\n\]+" \
+	    ".*" ] \
+	"test shadow stack backtrace for signal handling."
+}
diff --git a/gdb/testsuite/gdb.arch/amd64-shadow-stack-signal.c b/gdb/testsuite/gdb.arch/amd64-shadow-stack-signal.c
new file mode 100644
index 00000000000..c726e05b224
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/amd64-shadow-stack-signal.c
@@ -0,0 +1,31 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2024-2026 Free Software Foundation, Inc.
+
+   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/>.  */
+
+#include <signal.h>
+
+void
+handler (int signo)
+{
+}
+
+int
+main (void)
+{
+  signal (SIGUSR1, handler);
+  raise (SIGUSR1);
+  return 0;
+}
-- 
2.34.1

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