Check linking of hardening options, give better warnings if it fails.

Previously we'd only check whether the hardening options succeeded
at the compile step. Now we'll try to link with them too, and tell
the user in advance if something seems likely to go wrong.

Closes ticket 18895.

changes/bug18895

+  o Minor features (build):
+    - When building on a system without runtime support for some of the
+      runtime hardening options, try to log a useful warning at configuration
+      time, rather than an incomprehensible warning at link time.
+      If expensive hardening was requested, this warning becomes an error.
+      Closes ticket 18895.

configure.ac

@@ -787,14 +787,29 @@ m4_ifdef([AS_VAR_IF],[
        TOR_CHECK_CFLAGS(-fPIE)
        TOR_CHECK_LDFLAGS(-pie, "$all_ldflags_for_check", "$all_libs_for_check")
     fi
-    TOR_TRY_COMPILE_WITH_CFLAGS(-ftrapv, , CFLAGS_FTRAPV="-ftrapv", true)
-    TOR_TRY_COMPILE_WITH_CFLAGS(-fwrapv, , CFLAGS_FWRAPV="-fwrapv", true)
+    TOR_TRY_COMPILE_WITH_CFLAGS(-ftrapv, also_link, CFLAGS_FTRAPV="-ftrapv", true)
+    TOR_TRY_COMPILE_WITH_CFLAGS(-fwrapv, also_link, CFLAGS_FWRAPV="-fwrapv", true)
+    if test "$tor_cv_cflags__ftrapv" = "yes" && test "$tor_can_link__ftrapv" != "yes"; then
+      AC_MSG_WARN([The compiler supports -ftrapv, but for some reason I was not able to link with -ftrapv. Are you missing run-time support? Run-time hardening will not work as well as it should.])
+    fi
 fi
 
 if test "x$enable_expensive_hardening" = "xyes"; then
-   TOR_TRY_COMPILE_WITH_CFLAGS([-fsanitize=address], , CFLAGS_ASAN="-fsanitize=address", true)
-   TOR_TRY_COMPILE_WITH_CFLAGS([-fsanitize=undefined], , CFLAGS_UBSAN="-fsanitize=undefined", true)
-   TOR_CHECK_CFLAGS([-fno-omit-frame-pointer])
+   if test "$tor_cv_cflags__ftrapv" != "yes"; then
+     AC_MSG_ERROR([You requested expensive hardening, but the compiler does not seem to support -ftrapv.])
+   fi
+
+   TOR_TRY_COMPILE_WITH_CFLAGS([-fsanitize=address], also_link, CFLAGS_ASAN="-fsanitize=address", true)
+    if test "$tor_cv_cflags__fsanitize_address" = "yes" && test "$tor_can_link__fsanitize_address" != "yes"; then
+      AC_MSG_ERROR([The compiler supports -fsanitize=address, but for some reason I was not able to link when using it. Are you missing run-time support? With GCC you need libubsan.so, and with Clang you need libclang_rt.ubsan*])
+    fi
+
+   TOR_TRY_COMPILE_WITH_CFLAGS([-fsanitize=undefined], also_link, CFLAGS_UBSAN="-fsanitize=undefined", true)
+    if test "$tor_cv_cflags__fsanitize_address" = "yes" && test "$tor_can_link__fsanitize_address" != "yes"; then
+      AC_MSG_ERROR([The compiler supports -fsanitize=undefined, but for some reason I was not able to link when using it. Are you missing run-time support? With GCC you need libasan.so, and with Clang you need libclang_rt.ubsan*])
+    fi
+
+TOR_CHECK_CFLAGS([-fno-omit-frame-pointer])
 fi
 
 CFLAGS_BUGTRAP="$CFLAGS_FTRAPV $CFLAGS_ASAN $CFLAGS_UBSAN"