Add checks for Spectre v2 mitigation (retpoline)

This adds checks for gcc and clang flags for mitigations for Spectre
variant 2, ie "retpoline".  It'll automatically enabled if the compiler
supports it as part of toolchain hardening flag.  ok djm@

1 files changed, 4 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 0476398ac..71174571b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -163,6 +163,10 @@ 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
+        OSSH_CHECK_CFLAG_COMPILE([-mretpoline]) # clang
+        OSSH_CHECK_CFLAG_LINK([-z retpolineplt])
         OSSH_CHECK_CFLAG_COMPILE([-D_FORTIFY_SOURCE=2])
         OSSH_CHECK_LDFLAG_LINK([-Wl,-z,relro])
         OSSH_CHECK_LDFLAG_LINK([-Wl,-z,now])