[glibc] x86: Assume --enable-cet if GCC defaults to CET [BZ #25225]

[Florian Weimer <fw@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=9fb8139079ef0bb1aa33a4ae418cbb113b9b9da7

commit 9fb8139079ef0bb1aa33a4ae418cbb113b9b9da7
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Dec 3 18:48:18 2019 +0100

    x86: Assume --enable-cet if GCC defaults to CET [BZ #25225]
    
    This links in CET support if GCC defaults to CET.  Otherwise, __CET__
    is defined, yet CET functionality is not compiled and linked into the
    dynamic loader, resulting in a linker failure due to undefined
    references to _dl_cet_check and _dl_open_check.

Diff:
---
 configure    | 23 ++++++++++++++++++++++-
 configure.ac |  9 ++++++++-
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/configure b/configure
index 2f44b66..b959d2d 100755
--- a/configure
+++ b/configure
@@ -3777,11 +3777,32 @@ else
 fi
 
 
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+#ifndef __CET__
+#error no CET compiler support
+#endif
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  libc_cv_compiler_default_cet=yes
+else
+  libc_cv_compiler_default_cet=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
 # Check whether --enable-cet was given.
 if test "${enable_cet+set}" = set; then :
   enableval=$enable_cet; enable_cet=$enableval
 else
-  enable_cet=no
+  enable_cet=$libc_cv_compiler_default_cet
 fi
 
 
diff --git a/configure.ac b/configure.ac
index e69c88c..49b900c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -473,11 +473,18 @@ AC_ARG_ENABLE([mathvec],
 	      [build_mathvec=$enableval],
 	      [build_mathvec=notset])
 
+AC_TRY_COMPILE([], [
+#ifndef __CET__
+# error no CET compiler support
+#endif],
+	       [libc_cv_compiler_default_cet=yes],
+	       [libc_cv_compiler_default_cet=no])
+
 AC_ARG_ENABLE([cet],
 	      AC_HELP_STRING([--enable-cet],
 			     [enable Intel Control-flow Enforcement Technology (CET), x86 only]),
 	      [enable_cet=$enableval],
-	      [enable_cet=no])
+	      [enable_cet=$libc_cv_compiler_default_cet])
 
 # We keep the original values in `$config_*' and never modify them, so we
 # can write them unchanged into config.make.  Everything else uses