Check that SIGSEGV handlers work

Tools such as ASAN may trap violations instead of our handlers,
making the sodium_utils{2,3} test fail.

This has been documented for a long time, but it's probably better
to detect this and ignore tests depending on working signal handlers.

configure.ac

@@ -295,6 +295,8 @@ AX_CHECK_LINK_FLAG([-Wl,-z,relro], [LDFLAGS="$LDFLAGS -Wl,-z,relro"])
 AX_CHECK_LINK_FLAG([-Wl,-z,now], [LDFLAGS="$LDFLAGS -Wl,-z,now"])
 AX_CHECK_LINK_FLAG([-Wl,-z,noexecstack], [LDFLAGS="$LDFLAGS -Wl,-z,noexecstack"])
 
+AX_CHECK_CATCHABLE_SEGV
+
 LT_INIT
 AC_SUBST(LIBTOOL_DEPS)
 

m4/ax_check_catchable_segv.m4

@@ -0,0 +1,42 @@
+# SYNOPSIS
+#
+#   AX_CHECK_CATCHABLE_SEGV
+#
+# DESCRIPTION
+#
+#  Check whether segmentation violations can be caught using signal handlers.
+
+#serial 1
+
+AC_DEFUN([AX_CHECK_CATCHABLE_SEGV], [dnl
+    AC_PREREQ(2.64)
+    AS_VAR_PUSHDEF([CACHEVAR], [ax_cv_check_[]_AC_LANG_ABBREV[]CATCHABLE_SEGV])dnl
+    AC_CACHE_CHECK([whether segmentation violations can be caught when using the _AC_LANG compiler], CACHEVAR, [
+        AC_RUN_IFELSE([
+            AC_LANG_PROGRAM([[
+#include <signal.h>
+#include <stdlib.h>
+static void sig(int _) { exit(0); }
+            ]], [[
+volatile unsigned char * volatile x = malloc(8);
+size_t i;
+
+signal(SIGSEGV, sig);
+signal(SIGBUS, sig);
+#ifndef __SANITIZE_ADDRESS__
+for (i = 0; i < 10000000; i += 1024) { x[-i] = x[i] = (unsigned char) i; }
+#endif
+free((void *) x);
+exit(1)
+            ]])],
+            [AS_VAR_SET(CACHEVAR, [yes])],
+            [AS_VAR_SET(CACHEVAR, [no])],
+            [AS_VAR_SET(CACHEVAR, [unknown])]
+        )
+    ])
+    AS_VAR_IF(CACHEVAR, yes,
+        [AC_DEFINE([HAVE_CATCHABLE_SEGV], [1], [Define if segmentation violations can be caught using signal handlers])],
+        [AC_MSG_WARN([On this platform, segmentation violations cannot be caught using signal handlers. This is expected if you enabled a tool such as Address Sanitizer (-fsanitize=address), but be aware that using Address Sanitizer may also significantly reduce performance.])]
+    )
+    AS_VAR_POPDEF([CACHEVAR])dnl
+])

test/default/sodium_utils2.c

@@ -81,7 +81,7 @@ main(void)
     assert(buf != NULL);
     sodium_mprotect_readonly(buf);
     sodium_mprotect_readwrite(buf);
-#ifndef __EMSCRIPTEN__
+#if defined(HAVE_CATCHABLE_SEGV) && !defined(__EMSCRIPTEN__) && !defined(__SANITIZE_ADDRESS__)
     sodium_memzero(((unsigned char *) buf) + size, 1U);
     sodium_mprotect_noaccess(buf);
     sodium_free(buf);

test/default/sodium_utils3.c

@@ -51,7 +51,7 @@ main(void)
     assert(buf != NULL);
     sodium_mprotect_noaccess(buf);
     sodium_mprotect_readwrite(buf);
-#ifndef __EMSCRIPTEN__
+#if defined(HAVE_CATCHABLE_SEGV) && !defined(__EMSCRIPTEN__) && !defined(__SANITIZE_ADDRESS__)
     sodium_memzero(((unsigned char *) buf) - 8, 8U);
     sodium_mprotect_readonly(buf);
     sodium_free(buf);