x86-64/cet: Make CET feature check specific to Linux/x86

CET feature bits in TCB, which are Linux specific, are used to check if
CET features are active.  Move CET feature check to Linux/x86 directory.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

sysdeps/unix/sysv/linux/x86/bits/platform/features.h

@@ -0,0 +1,48 @@
+/* Inline functions for x86 CPU features.
+   This file is part of the GNU C Library.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   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 _SYS_PLATFORM_X86_H
+# error "Never include <bits/platform/features.h> directly; use <sys/platform/x86.h> instead."
+#endif
+
+/* Bits in the feature_1 field in TCB.  */
+
+enum
+{
+  x86_feature_1_ibt		= 1U << 0,
+  x86_feature_1_shstk		= 1U << 1
+};
+
+static __inline__ _Bool
+x86_cpu_cet_active (unsigned int __index)
+{
+#ifdef __x86_64__
+  unsigned int __feature_1;
+# ifdef __LP64__
+  __asm__ ("mov %%fs:72, %0" : "=r" (__feature_1));
+# else
+  __asm__ ("mov %%fs:40, %0" : "=r" (__feature_1));
+# endif
+  if (__index == x86_cpu_IBT)
+    return __feature_1 & x86_feature_1_ibt;
+  else
+    return __feature_1 & x86_feature_1_shstk;
+#else
+  return false;
+#endif
+}

sysdeps/unix/sysv/linux/x86_64/dl-cet.h

@@ -54,6 +54,14 @@ dl_cet_get_cet_status (void)
   return status;
 }
 
+static __always_inline bool
+dl_cet_ibt_enabled (void)
+{
+  unsigned int feature_1 = THREAD_GETMEM (THREAD_SELF,
+					  header.feature_1);
+  return (feature_1 & GNU_PROPERTY_X86_FEATURE_1_IBT) != 0;
+}
+
 /* Enable shadow stack with a macro to avoid shadow stack underflow.  */
 #define ENABLE_X86_CET(cet_feature)				\
   if ((cet_feature & GNU_PROPERTY_X86_FEATURE_1_SHSTK))		\

sysdeps/unix/sysv/linux/x86_64/get-cpuid-feature-leaf.c

@@ -0,0 +1,24 @@
+/* Get CPUID feature leaf.  Linux/x86-64 version.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   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/>.  */
+
+#include <sysdeps/x86/get-cpuid-feature-leaf.c>
+
+#ifdef __LP64__
+_Static_assert (FEATURE_1_OFFSET == 72, "FEATURE_1_OFFSET != 72");
+#else
+_Static_assert (FEATURE_1_OFFSET == 40, "FEATURE_1_OFFSET != 40");
+#endif

sysdeps/x86/Makefile

@@ -5,7 +5,11 @@ endif
 ifeq ($(subdir),elf)
 sysdep_routines += get-cpuid-feature-leaf
 sysdep-dl-routines += dl-get-cpu-features
-sysdep_headers += sys/platform/x86.h bits/platform/x86.h
+sysdep_headers += \
+  bits/platform/features.h \
+  bits/platform/x86.h \
+  sys/platform/x86.h \
+# sysdep_headers
 
 CFLAGS-dl-get-cpu-features.os += $(rtld-early-cflags)
 CFLAGS-get-cpuid-feature-leaf.o += $(no-stack-protector)

sysdeps/x86/bits/platform/features.h

@@ -0,0 +1,27 @@
+/* Inline functions for x86 CPU features.
+   This file is part of the GNU C Library.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   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 _SYS_PLATFORM_X86_H
+# error "Never include <bits/platform/features.h> directly; use <sys/platform/x86.h> instead."
+#endif
+
+static __inline__ _Bool
+x86_cpu_cet_active (unsigned int __index)
+{
+  return false;
+}

sysdeps/x86/bits/platform/x86.h

@@ -337,11 +337,3 @@ enum
   x86_cpu_AVX10_YMM = x86_cpu_index_24_ecx_0_ebx + 17,
   x86_cpu_AVX10_ZMM = x86_cpu_index_24_ecx_0_ebx + 18,
 };
-
-/* Bits in the feature_1 field in TCB.  */
-
-enum
-{
-  x86_feature_1_ibt		= 1U << 0,
-  x86_feature_1_shstk		= 1U << 1
-};

sysdeps/x86/get-cpuid-feature-leaf.c

@@ -19,14 +19,6 @@
 #include <tcb-offsets.h>
 #include <ldsodefs.h>
 
-#ifdef __x86_64__
-# ifdef __LP64__
-_Static_assert (FEATURE_1_OFFSET == 72, "FEATURE_1_OFFSET != 72");
-# else
-_Static_assert (FEATURE_1_OFFSET == 40, "FEATURE_1_OFFSET != 40");
-# endif
-#endif
-
 const struct cpuid_feature *
 __x86_get_cpuid_feature_leaf (unsigned int leaf)
 {

sysdeps/x86/sys/platform/x86.h

@@ -22,6 +22,7 @@
 #include <features.h>
 #include <stdbool.h>
 #include <bits/platform/x86.h>
+#include <bits/platform/features.h>
 
 __BEGIN_DECLS
 
@@ -46,22 +47,8 @@ static __inline__ _Bool
 x86_cpu_active (unsigned int __index)
 {
   if (__index == x86_cpu_IBT || __index == x86_cpu_SHSTK)
-    {
+    return x86_cpu_cet_active (__index);
-#ifdef __x86_64__
+
-      unsigned int __feature_1;
-# ifdef __LP64__
-      __asm__ ("mov %%fs:72, %0" : "=r" (__feature_1));
-# else
-      __asm__ ("mov %%fs:40, %0" : "=r" (__feature_1));
-# endif
-      if (__index == x86_cpu_IBT)
-	return __feature_1 & x86_feature_1_ibt;
-      else
-	return __feature_1 & x86_feature_1_shstk;
-#else
-      return false;
-#endif
-    }
   const struct cpuid_feature *__ptr = __x86_get_cpuid_feature_leaf
     (__index / (8 * sizeof (unsigned int) * 4));
   unsigned int __reg

sysdeps/x86_64/dl-machine.h

@@ -587,10 +587,11 @@ x86_64_rewrite_plt (struct link_map *map, ElfW(Addr) plt_rewrite)
   const ElfW(Rela) *reloc = (const void *) start;
   const ElfW(Rela) *reloc_end = (const void *) (start + size);
 
-  unsigned int feature_1 = THREAD_GETMEM (THREAD_SELF,
+# ifdef __CET__
-					  header.feature_1);
+  bool ibt_enabled_p = dl_cet_ibt_enabled ();
-  bool ibt_enabled_p
+# else
-    = (feature_1 & GNU_PROPERTY_X86_FEATURE_1_IBT) != 0;
+  bool ibt_enabled_p = false;
+# endif
 
   if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
     _dl_debug_printf ("\nchanging PLT in '%s' to direct branch\n",