[glibc] x86/cet: Don't set CET active by default

[H.J. Lu <hjl@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=55d63e731253de82e96ed4ddca2e294076cd0bc5

commit 55d63e731253de82e96ed4ddca2e294076cd0bc5
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Dec 29 08:43:53 2023 -0800

    x86/cet: Don't set CET active by default
    
    Not all CET enabled applications and libraries have been properly tested
    in CET enabled environments.  Some CET enabled applications or libraries
    will crash or misbehave when CET is enabled.  Don't set CET active by
    default so that all applications and libraries will run normally regardless
    of whether CET is active or not.  Shadow stack can be enabled by
    
    $ export GLIBC_TUNABLES=glibc.cpu.hwcaps=SHSTK
    
    at run-time if shadow stack can be enabled by kernel.
    
    NB: This commit can be reverted if it is OK to enable CET by default for
    all applications and libraries.

Diff:
---
 sysdeps/x86/cpu-features.c |  2 +-
 sysdeps/x86/cpu-tunables.c | 15 ++++++++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
index 097868c1d9..80a07ac589 100644
--- a/sysdeps/x86/cpu-features.c
+++ b/sysdeps/x86/cpu-features.c
@@ -110,7 +110,7 @@ update_active (struct cpu_features *cpu_features)
   if (!CPU_FEATURES_CPU_P (cpu_features, RTM_ALWAYS_ABORT))
     CPU_FEATURE_SET_ACTIVE (cpu_features, RTM);
 
-#if CET_ENABLED
+#if CET_ENABLED && 0
   CPU_FEATURE_SET_ACTIVE (cpu_features, IBT);
   CPU_FEATURE_SET_ACTIVE (cpu_features, SHSTK);
 #endif
diff --git a/sysdeps/x86/cpu-tunables.c b/sysdeps/x86/cpu-tunables.c
index 142c6b9240..1742400525 100644
--- a/sysdeps/x86/cpu-tunables.c
+++ b/sysdeps/x86/cpu-tunables.c
@@ -35,6 +35,17 @@
       break;								\
     }
 
+#define CHECK_GLIBC_IFUNC_CPU_BOTH(f, cpu_features, name, len)		\
+  _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
+  if (tunable_str_comma_strcmp_cte (&f, #name))				\
+    {									\
+      if (f.disable)							\
+	CPU_FEATURE_UNSET (cpu_features, name)				\
+      else								\
+	CPU_FEATURE_SET_ACTIVE (cpu_features, name)			\
+      break;								\
+    }
+
 /* Disable a preferred feature NAME.  We don't enable a preferred feature
    which isn't available.  */
 #define CHECK_GLIBC_IFUNC_PREFERRED_OFF(f, cpu_features, name, len)	\
@@ -131,11 +142,13 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	    }
 	  break;
 	case 5:
+	  {
+	    CHECK_GLIBC_IFUNC_CPU_BOTH (n, cpu_features, SHSTK, 5);
+	  }
 	  if (n.disable)
 	    {
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, LZCNT, 5);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, MOVBE, 5);
-	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, SHSTK, 5);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, SSSE3, 5);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, XSAVE, 5);
 	    }