[PATCH v8 09/20] aarch64: Add glibc.cpu.aarch64_gcs tunable

[Yury Khrustalev]

From: Szabolcs Nagy <szabolcs.nagy@arm.com>

This tunable is for controlling the GCS status. It is used to work
out the argument to the PR_SET_SHADOW_STACK_STATUS prctl syscall.

By default it is 0, so GCS is disabled, value 1 will enable GCS.

The status is stored into GL(dl_aarch64_gcs) early and only applied
later, since enabling GCS is tricky: it must happen on a top level
stack frame. Using GL instead of GLRO because it may need updates
depending on loaded libraries that happen after readonly protection
is applied, however library marking based GCS setting is not yet
implemented.

Describe new tunable in the manual.
---
 manual/tunables.texi                          |  9 +++++
 sysdeps/aarch64/dl-tunables.list              |  5 +++
 .../unix/sysv/linux/aarch64/cpu-features.c    |  8 ++++
 .../unix/sysv/linux/aarch64/dl-procruntime.c  | 37 +++++++++++++++++++
 4 files changed, 59 insertions(+)
 create mode 100644 sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c

diff --git a/manual/tunables.texi b/manual/tunables.texi
index 1ac58b9ae9..118f490010 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -669,6 +669,15 @@ This tunable is specific to x86-64 and effective only when the lazy
 binding is disabled.
 @end deftp
 
+@deftp Tunable glibc.cpu.aarch64_gcs
+This tunable enables or disables Guarded Control Stack for a process.
+Value @code{0} will leave GCS disabled and value @code{1} will enable
+GCS via a @code{prctl} system call. Default is @code{0}, so GCS is
+disabled by default.
+
+This tunable is specific to AArch64.
+@end deftp
+
 @node Memory Related Tunables
 @section Memory Related Tunables
 @cindex memory related tunables
diff --git a/sysdeps/aarch64/dl-tunables.list b/sysdeps/aarch64/dl-tunables.list
index 5c1a172559..4b28341b72 100644
--- a/sysdeps/aarch64/dl-tunables.list
+++ b/sysdeps/aarch64/dl-tunables.list
@@ -21,5 +21,10 @@ glibc {
     name {
       type: STRING
     }
+    aarch64_gcs {
+      type: UINT_64
+      minval: 0
+      default: 0
+    }
   }
 }
diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
index 7ac228303f..1ecf6cd176 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
@@ -176,4 +176,12 @@ init_cpu_features (struct cpu_features *cpu_features)
 
   /* Check if MOPS is supported.  */
   cpu_features->mops = GLRO (dl_hwcap2) & HWCAP2_MOPS;
+
+#ifndef HWCAP_GCS
+#define HWCAP_GCS (1UL << 32)
+#endif
+
+  if (GLRO (dl_hwcap) & HWCAP_GCS)
+    /* GCS status may be updated later by binary compatibility checks.  */
+    GL (dl_aarch64_gcs) = TUNABLE_GET (glibc, cpu, aarch64_gcs, uint64_t, 0);
 }
diff --git a/sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c b/sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c
new file mode 100644
index 0000000000..044544aa78
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c
@@ -0,0 +1,37 @@
+/* Data for processor runtime information.  AArch64 version.
+   Copyright (C) 2024-2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef PROCINFO_CLASS
+# define PROCINFO_CLASS
+#endif
+
+#if !IS_IN (ldconfig)
+# if !defined PROCINFO_DECL && defined SHARED
+  ._dl_aarch64_gcs
+# else
+PROCINFO_CLASS unsigned long _dl_aarch64_gcs
+# endif
+# ifndef PROCINFO_DECL
+= 0
+# endif
+# if !defined SHARED || defined PROCINFO_DECL
+;
+# else
+,
+# endif
+#endif
-- 
2.39.5