Re: [C PATCH] Disallow subtracting pointers to empty structs (PR c/58346)

[Marek Polacek <polacek@redhat.com>]

On Wed, Jan 15, 2014 at 09:23:06PM +0000, Joseph S. Myers wrote:
> On Wed, 15 Jan 2014, Marek Polacek wrote:
> 
> > +/* Return true if T is a pointer to a zero-sized struct/union.  */
> > +
> > +bool
> > +pointer_to_zero_sized_aggr_p (tree t)
> > +{
> > +  t = strip_pointer_operator (t);
> > +  return ((RECORD_OR_UNION_TYPE_P (t) || TREE_CODE (t) == ARRAY_TYPE)
> > +	  && TYPE_SIZE (t)
> > +	  && integer_zerop (TYPE_SIZE (t)));
> 
> Why have the (RECORD_OR_UNION_TYPE_P (t) || TREE_CODE (t) == ARRAY_TYPE) 
> check at all?  It may well be the case that those are the only kinds of 
> types that can have zero size here, but the principle of this error 
> applies to anything with zero size so it would seem best not to have that 
> part of the check at all.

Ok, changed.
 
> strip_pointer_operator seems wrong here.  It recursively removes an 
> arbitrary number of pointer type derivations - but where the types are 
> pointer to pointer to zero-size, arithmetic is perfectly valid (so you 
> should have a test that such cases are still accepted, where this patch 
> version would have rejected them).  I believe this function should return 
> true if the argument is a pointer (to anything) and after removal of 
> exactly one level of pointer type derivation, the result has zero size 
> (constant zero - also add a test that the array case where the size is a 
> const int initialized to 0 is not, for C, rejected, as those are VLAs in C 
> terms).

Ah, right, sorry about that.  Hopefully all done in the following.
The if (!POINTER_TYPE_P (t)) check in pointer_to_zero_sized_aggr_p
could perhaps go; I added it mostly for paranoia sake.

2014-01-16  Marek Polacek  <polacek@redhat.com>

	PR c/58346
c-family/
	* c-common.c (pointer_to_zero_sized_aggr_p): New function.
	* c-common.h: Declare it.
cp/
	* typeck.c (pointer_diff): Give an error on arithmetic on pointer to
	an empty aggregate.
c/
	* c-typeck.c (pointer_diff): Give an error on arithmetic on pointer to
	an empty aggregate.
testsuite/
	* c-c++-common/pr58346-1.c: New test.
	* c-c++-common/pr58346-2.c: New test.
	* c-c++-common/pr58346-3.c: New test.

--- gcc/c-family/c-common.h.mp	2014-01-15 11:24:41.438608787 +0100
+++ gcc/c-family/c-common.h	2014-01-16 16:40:06.105688048 +0100
@@ -789,6 +789,7 @@ extern bool keyword_is_storage_class_spe
 extern bool keyword_is_type_qualifier (enum rid);
 extern bool keyword_is_decl_specifier (enum rid);
 extern bool cxx_fundamental_alignment_p (unsigned);
+extern bool pointer_to_zero_sized_aggr_p (tree);
 
 #define c_sizeof(LOC, T)  c_sizeof_or_alignof_type (LOC, T, true, false, 1)
 #define c_alignof(LOC, T) c_sizeof_or_alignof_type (LOC, T, false, false, 1)
--- gcc/c-family/c-common.c.mp	2014-01-16 16:39:17.093487218 +0100
+++ gcc/c-family/c-common.c	2014-01-16 17:15:37.466403444 +0100
@@ -11823,4 +11823,15 @@ cxx_fundamental_alignment_p  (unsigned a
 			 TYPE_ALIGN (long_double_type_node)));
 }
 
+/* Return true if T is a pointer to a zero-sized aggregate.  */
+
+bool
+pointer_to_zero_sized_aggr_p (tree t)
+{
+  if (!POINTER_TYPE_P (t))
+    return false;
+  t = TREE_TYPE (t);
+  return (TYPE_SIZE (t) && integer_zerop (TYPE_SIZE (t)));
+}
+
 #include "gt-c-family-c-common.h"
--- gcc/cp/typeck.c.mp	2014-01-15 11:24:48.940639724 +0100
+++ gcc/cp/typeck.c	2014-01-16 16:40:06.120688120 +0100
@@ -5043,6 +5043,14 @@ pointer_diff (tree op0, tree op1, tree p
 	return error_mark_node;
     }
 
+  if (pointer_to_zero_sized_aggr_p (TREE_TYPE (op1)))
+    {
+      if (complain & tf_error)
+	error ("arithmetic on pointer to an empty aggregate");
+      else
+	return error_mark_node;
+    }
+
   op1 = (TYPE_PTROB_P (ptrtype)
 	 ? size_in_bytes (target_type)
 	 : integer_one_node);
--- gcc/c/c-typeck.c.mp	2014-01-15 11:24:53.699659333 +0100
+++ gcc/c/c-typeck.c	2014-01-16 16:40:06.130688170 +0100
@@ -3536,6 +3536,9 @@ pointer_diff (location_t loc, tree op0,
   /* This generates an error if op0 is pointer to incomplete type.  */
   op1 = c_size_in_bytes (target_type);
 
+  if (pointer_to_zero_sized_aggr_p (TREE_TYPE (orig_op1)))
+    error_at (loc, "arithmetic on pointer to an empty aggregate");
+
   /* Divide by the size, in easiest possible way.  */
   result = fold_build2_loc (loc, EXACT_DIV_EXPR, inttype,
 			    op0, convert (inttype, op1));
--- gcc/testsuite/c-c++-common/pr58346-3.c.mp	2014-01-16 17:13:26.184800572 +0100
+++ gcc/testsuite/c-c++-common/pr58346-3.c	2014-01-16 17:30:15.503506328 +0100
@@ -0,0 +1,16 @@
+/* PR c/58346 */
+/* { dg-do compile } */
+
+void
+foo (void)
+{
+  __PTRDIFF_TYPE__ d;
+  const int i = 0;
+  int a1[2][0], a2[2][0];
+  int b1[3][i], b2[4][i];
+  d = a1 - a2; /* { dg-error "arithmetic on pointer to an empty aggregate" } */
+  __asm volatile ("" : "+g" (d));
+  /* No error here for C.  */
+  d = b1 - b2; /* { dg-error "arithmetic on pointer to an empty aggregate" "" { target c++ } } */
+  __asm volatile ("" : "+g" (d));
+}
--- gcc/testsuite/c-c++-common/pr58346-1.c.mp	2014-01-15 14:04:27.159428834 +0100
+++ gcc/testsuite/c-c++-common/pr58346-1.c	2014-01-16 16:59:13.428236540 +0100
@@ -0,0 +1,24 @@
+/* PR c/58346 */
+/* { dg-do compile } */
+
+struct U {
+#ifdef __cplusplus
+  char a[0];
+#endif
+};
+static struct U b[6];
+static struct U *u1, *u2;
+
+int
+foo (struct U *p, struct U *q)
+{
+  return q - p; /* { dg-error "arithmetic on pointer to an empty aggregate" } */
+}
+
+void
+bar (void)
+{
+  __PTRDIFF_TYPE__ d = u1 - u2; /* { dg-error "arithmetic on pointer to an empty aggregate" } */
+  __asm volatile ("" : "+g" (d));
+  foo (&b[0], &b[4]);
+}
--- gcc/testsuite/c-c++-common/pr58346-2.c.mp	2014-01-13 15:48:20.011420141 +0100
+++ gcc/testsuite/c-c++-common/pr58346-2.c	2014-01-15 14:05:57.042817401 +0100
@@ -0,0 +1,8 @@
+/* PR c/58346 */
+/* { dg-do compile } */
+
+__PTRDIFF_TYPE__
+foo (int p[3][0], int q[3][0])
+{
+  return p - q; /* { dg-error "arithmetic on pointer to an empty aggregate" } */
+}

	Marek