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.

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

On Tue, Nov 26, 2019 at 5:54 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> 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.
>
> -----
>  configure    | 23 ++++++++++++++++++++++-
>  configure.ac |  9 ++++++++-
>  2 files changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/configure b/configure
> index 2f44b66656..b959d2d988 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 e69c88c543..49b900c1ed 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
>

LGTM.

Thanks.