configure: work around GCC shortcoming on Cygwin

Cygwin's latest 7.x GCC allows to specify -mfunction-return=thunk
as well as -mindirect-branch=thunk on the command line, albeit
producing invalid code, leading to an error at link stage.

The check in configure.ac only checks if the option is present,
but not if it produces valid code.

This patch fixes it by special-casing Cygwin.  Another solution
may be to change these to linker checks.

Signed-off-by: Corinna Vinschen <vinschen@redhat.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/configure.ac b/configure.ac
index c4c759d4e..83e530750 100644
--- a/configure.ac
+++ b/configure.ac
@@ -164,8 +164,15 @@ if test "$GCC" = "yes" || test "$GCC" = "egcs"; then
         OSSH_CHECK_CFLAG_COMPILE([-Wunused-result], [-Wno-unused-result])
         OSSH_CHECK_CFLAG_COMPILE([-fno-strict-aliasing])
     if test "x$use_toolchain_hardening" = "x1"; then
-        OSSH_CHECK_CFLAG_COMPILE([-mfunction-return=thunk]) # gcc
-        OSSH_CHECK_CFLAG_COMPILE([-mindirect-branch=thunk]) # gcc
+        # Cygwin GCC 7.x allows thunking on the CLI, but produces non-working
+        # code.  Unfortunately you only notice this at link time.
+        case "$host" in
+        *-*-cygwin*) ;;
+        *)
+            OSSH_CHECK_CFLAG_COMPILE([-mfunction-return=thunk]) # gcc
+            OSSH_CHECK_CFLAG_COMPILE([-mindirect-branch=thunk]) # gcc
+            ;;
+        esac
         OSSH_CHECK_CFLAG_COMPILE([-mretpoline]) # clang
         OSSH_CHECK_LDFLAG_LINK([-Wl,-z,retpolineplt])
         OSSH_CHECK_CFLAG_COMPILE([-D_FORTIFY_SOURCE=2])