[PATCH] Disable type demotion for sanitizer (PR sanitizer/82072)

[Marek Polacek]

Here, do_narrow and convert_to_integer_1 is demoting signed types to unsigned,
e.g. for
  i = i - lmin
where i is int and lmin is long int, so what we should produce is
  i = (int) ((long int) i - lmin)
but instead it produces
  i = (int) ((unsigned int) i - (unsigned int) lmin);
which hides the overflow.  Similarly for NEGATE_EXPR.  This patch prevents
such demoting when the sanitizer is on.

There still might be a similar issue with division or shifting, but I couldn't
trigger that.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2017-09-01  Marek Polacek  <pola...@redhat.com>

        PR sanitizer/82072
        * convert.c (do_narrow): When sanitizing signed integer overflows,
        bail out for signed types.
        (convert_to_integer_1) <case NEGATE_EXPR>: Likewise.

        * c-c++-common/ubsan/pr82072.c: New test.

diff --git gcc/convert.c gcc/convert.c
index 22152cae79b..139d790fd98 100644
--- gcc/convert.c
+++ gcc/convert.c
@@ -434,6 +434,13 @@ do_narrow (location_t loc,
     typex = lang_hooks.types.type_for_size (TYPE_PRECISION (typex),
                                            TYPE_UNSIGNED (typex));
 
+  /* The type demotion below might cause doing unsigned arithmetic
+     instead of signed, and thus hide overflow bugs.  */
+  if ((ex_form == PLUS_EXPR || ex_form == MINUS_EXPR)
+      && !TYPE_UNSIGNED (typex)
+      && sanitize_flags_p (SANITIZE_SI_OVERFLOW))
+    return NULL_TREE;
+
   /* But now perhaps TYPEX is as wide as INPREC.
      In that case, do nothing special here.
      (Otherwise would recurse infinitely in convert.  */
@@ -895,7 +902,12 @@ convert_to_integer_1 (tree type, tree expr, bool dofold)
                                                    TYPE_UNSIGNED (typex));
 
              if (!TYPE_UNSIGNED (typex))
-               typex = unsigned_type_for (typex);
+               {
+                 /* Using unsigned arithmetic may hide overflow bugs.  */
+                 if (sanitize_flags_p (SANITIZE_SI_OVERFLOW))
+                   break;
+                 typex = unsigned_type_for (typex);
+               }
              return convert (type,
                              fold_build1 (ex_form, typex,
                                           convert (typex,
diff --git gcc/testsuite/c-c++-common/ubsan/pr82072.c 
gcc/testsuite/c-c++-common/ubsan/pr82072.c
index e69de29bb2d..d5683406b14 100644
--- gcc/testsuite/c-c++-common/ubsan/pr82072.c
+++ gcc/testsuite/c-c++-common/ubsan/pr82072.c
@@ -0,0 +1,19 @@
+/* PR sanitizer/82072 */
+/* { dg-do run } */
+/* { dg-options "-fsanitize=signed-integer-overflow" } */
+
+int
+main ()
+{
+  long long l = -__LONG_LONG_MAX__ - 1;
+  int i = 0;
+  asm volatile ("" : "+r" (i));
+  i -= l;
+  asm volatile ("" : "+r" (i));
+  i = -l;
+  asm volatile ("" : "+r" (i));
+  return 0;
+}
+
+/* { dg-output "signed integer overflow: 0 - -9223372036854775808 cannot be 
represented in type 'long long int'\[^\n\r]*(\n|\r\n|\r)" } */
+/* { dg-output "\[^\n\r]*negation of -9223372036854775808 cannot be 
represented in type 'long long int'\[^\n\r]*; cast to an unsigned type to 
negate this value to itself" } */

        Marek