From 6d6c8287d5e060117bfa066f87d4e2fc08ab708d Mon Sep 17 00:00:00 2001
From: Nick Mathewson <nickm@torproject.org>
Date: Wed, 18 May 2016 09:44:05 -0400
Subject: [PATCH] Include __mulodi4 in libor_ctime when it fixes clang -m32
 -ftrapv

We use a pretty specific pair of autoconf tests here to make sure
that we only add this code when:
   a) a 64-bit signed multiply fails to link,
 AND
   b) the same 64-bit signed multiply DOES link correctly when
      __mulodi4 is defined.

Closes ticket 19079.
---
 changes/bug19079      |  4 ++++
 configure.ac          | 40 ++++++++++++++++++++++++++++++++++++++++
 src/common/include.am |  7 +++++++
 3 files changed, 51 insertions(+)
 create mode 100644 changes/bug19079

diff --git a/changes/bug19079 b/changes/bug19079
new file mode 100644
index 0000000000..6cbc6cee3b
--- /dev/null
+++ b/changes/bug19079
@@ -0,0 +1,4 @@
+  o Minor features (build):
+    - Detect and work around a libclang_rt problem that prevents clang from
+      finding __mulodi4() on some 32-bit platforms. This clang bug would keep
+      -ftrapv from linking on those systems. Closes ticket 19079.
diff --git a/configure.ac b/configure.ac
index de59c752bc..8465b43262 100644
--- a/configure.ac
+++ b/configure.ac
@@ -801,6 +801,46 @@ fi
 CFLAGS_BUGTRAP="$CFLAGS_FTRAPV $CFLAGS_ASAN $CFLAGS_UBSAN"
 CFLAGS_CONSTTIME="$CFLAGS_FWRAPV"
 
+if test "$have_clang" = "yes"; then
+  saved_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS $CFLAGS_FTRAPV"
+  AC_MSG_CHECKING([whether clang -ftrapv can link a 64-bit int multiply])
+  AC_LINK_IFELSE([
+      AC_LANG_SOURCE([[
+          #include <stdint.h>
+          #include <stdlib.h>
+	  int main(int argc, char **argv)
+	  {
+            int64_t x = ((int64_t)atoi(argv[1])) * (int64_t)atoi(argv[2])
+	                * (int64_t)argv[3];
+	    return x == 9;
+	  } ]])],
+	  [ftrapv_can_link=yes; AC_MSG_RESULT([yes])],
+	  [ftrapv_can_link=no; AC_MSG_RESULT([no])])
+  mulodi_fixes_ftrapv=no
+  if test "$ftrapv_can_link" = "no"; then
+    AC_MSG_CHECKING([whether defining __mulodi4 fixes that])
+    AC_LINK_IFELSE([
+      AC_LANG_SOURCE([[
+          #include <stdint.h>
+          #include <stdlib.h>
+	  int64_t __mulodi4(int64_t a, int64_t b, int *overflow) {
+             *overflow=0;
+	     return a;
+          }
+	  int main(int argc, char **argv)
+	  {
+            int64_t x = ((int64_t)atoi(argv[1])) * (int64_t)atoi(argv[2])
+	                * (int64_t)argv[3];
+	    return x == 9;
+	  } ]])],
+	  [mulodi_fixes_ftrapv=yes; AC_MSG_RESULT([yes])],
+	  [mulodi_fixes_ftrapv=no; AC_MSG_RESULT([no])])
+  fi
+  AM_CONDITIONAL(ADD_MULODI4, test "$mulodi_fixes_ftrapv" = "yes")
+  CFLAGS="$saved_CFLAGS"
+fi
+
 dnl These cflags add bunches of branches, and we haven't been able to
 dnl persuade ourselves that they're suitable for code that needs to be
 dnl constant time.
diff --git a/src/common/include.am b/src/common/include.am
index c39ad8d16b..222afe0291 100644
--- a/src/common/include.am
+++ b/src/common/include.am
@@ -62,7 +62,14 @@ else
 readpassphrase_source=
 endif
 
+if ADD_MULODI4
+mulodi4_source=src/ext/mulodi/mulodi4.c
+else
+mulodi4_source=
+endif
+
 LIBOR_CTIME_A_SRC = \
+   $(mulodi4_source) \
    src/ext/csiphash.c   \
    src/common/di_ops.c