Re: [RFC PATCH] -fsanitize=pointer-overflow support (PR sanitizer/80998)

[Jakub Jelinek <jakub@redhat.com>]

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On Tue, Jun 20, 2017 at 10:18:20AM +0200, Richard Biener wrote:
> > It would be an attempt to avoid sanitizing int foo (int *p) { return p[10] + p[-5]; }
> > (when the offset is constant and small and we dereference it).
> > If there is no page mapped at NULL or at the highest page in the virtual
> > address space, then the above will crash in case p + 10 or p - 5 wraps
> > around.
> 
> Ah, so merely an optimization to avoid excessive instrumentation then,
> yes, this might work (maybe make 4096 a --param configurable to be able
> to disable it).

Yes.  And I think it can be implemented incrementally.

> > > > I've bootstrapped/regtested the patch on x86_64-linux and i686-linux
> > > > and additionally bootstrapped/regtested with bootstrap-ubsan on both too.
> > > > The latter revealed a couple of issues I'd like to discuss:
> > > > 
> > > > 1) libcpp/symtab.c contains a couple of spots reduced into:
> > > > #define DELETED ((char *) -1)
> > > > void bar (char *);
> > > > void
> > > > foo (char *p)
> > > > {
> > > >   if (p && p != DELETED)
> > > >     bar (p);
> > > > }
> > > > where we fold it early into if ((p p+ -1) <= (char *) -3)
> > > > and as the instrumentation is done during ubsan pass, if p is NULL,
> > > > we diagnose this as invalid pointer overflow from NULL to 0xffff*f.
> > > > Shall we change the folder so that during GENERIC folding it
> > > > actually does the addition and comparison in pointer_sized_int
> > > > instead (my preference), or shall I move the UBSAN_PTR instrumentation
> > > > earlier into the FEs (but then I still risk stuff is folded earlier)?
> > > 
> > > Aww, so we turn the pointer test into a range test ;)  That it uses
> > > a pointer type rather than an unsigned integer type is a bug, probably
> > > caused by pointers being TYPE_UNSIGNED.
> > > 
> > > Not sure if the folding itself is worthwhile to keep though, thus an
> > > option would be to not generate range tests from pointers?
> > 
> > I'll have a look.  Maybe only do it during reassoc and not earlier.
> 
> It certainly looks somewhat premature in fold-const.c.

So for this, I have right now 2 variant patches:

The first one keeps doing what we were except for the
-fsanitize=pointer-overflow case and has been bootstrap-ubsan
bootstrapped/regtested on x86_64-linux and i686-linux.

The second one performs the addition and comparison in pointer sized
unsigned type instead (not bootstrapped yet).

I think the second one would be my preference.  Note build_range_check
is used not just during early folding, but e.g. during ifcombine, reassoc
etc.

Martin is contemplating instrumentation of pointer <=/</>=/> comparisons
and in that case we'd need some further build_range_check changes,
because while ptr == (void *) 0 || ptr == (void *) 1 || ptr == (void *) 2
would be without UB, ptr <= (void *) 2 would be UB, so we'd need to perform
all pointer range checks in integral type except the ones where we just do
EQ_EXPR/NE_EXPR.

	Jakub

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2017-06-21  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/80998
	* fold-const.c: Include asan.h.
	(build_range_check): For -fsanitize=pointer-overflow don't
	add pointer arithmetics for range test.

--- gcc/fold-const.c.jj	2017-06-14 18:07:47.000000000 +0200
+++ gcc/fold-const.c	2017-06-20 17:05:44.351608513 +0200
@@ -79,6 +79,7 @@ along with GCC; see the file COPYING3.
 #include "tree-vrp.h"
 #include "tree-ssanames.h"
 #include "selftest.h"
+#include "asan.h"
 
 /* Nonzero if we are folding constants inside an initializer; zero
    otherwise.  */
@@ -4906,6 +4907,14 @@ build_range_check (location_t loc, tree
     {
       if (value != 0 && !TREE_OVERFLOW (value))
 	{
+	  /* Avoid creating pointer arithmetics that is not present
+	     in the source when sanitizing.  */
+	  if (!integer_zerop (low)
+	      && current_function_decl
+	      && sanitize_flags_p (SANITIZE_POINTER_OVERFLOW,
+				   current_function_decl))
+	    return 0;
+
 	  low = fold_build1_loc (loc, NEGATE_EXPR, TREE_TYPE (low), low);
           return build_range_check (loc, type,
 			     	    fold_build_pointer_plus_loc (loc, exp, low),

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2017-06-21  Jakub Jelinek  <jakub@redhat.com>

	* fold-const.c (build_range_check): Compute pointer range check in
	integral type if pointer arithmetics would be needed.  Formatting
	fixes.

--- gcc/fold-const.c.jj	2017-06-20 21:38:04.000000000 +0200
+++ gcc/fold-const.c	2017-06-21 09:23:00.572404964 +0200
@@ -4818,21 +4818,21 @@ build_range_check (location_t loc, tree
 
   if (low == 0)
     return fold_build2_loc (loc, LE_EXPR, type, exp,
-			fold_convert_loc (loc, etype, high));
+			    fold_convert_loc (loc, etype, high));
 
   if (high == 0)
     return fold_build2_loc (loc, GE_EXPR, type, exp,
-			fold_convert_loc (loc, etype, low));
+			    fold_convert_loc (loc, etype, low));
 
   if (operand_equal_p (low, high, 0))
     return fold_build2_loc (loc, EQ_EXPR, type, exp,
-			fold_convert_loc (loc, etype, low));
+			    fold_convert_loc (loc, etype, low));
 
   if (TREE_CODE (exp) == BIT_AND_EXPR
       && maskable_range_p (low, high, etype, &mask, &value))
     return fold_build2_loc (loc, EQ_EXPR, type,
 			    fold_build2_loc (loc, BIT_AND_EXPR, etype,
-					      exp, mask),
+					     exp, mask),
 			    value);
 
   if (integer_zerop (low))
@@ -4864,7 +4864,7 @@ build_range_check (location_t loc, tree
 	      exp = fold_convert_loc (loc, etype, exp);
 	    }
 	  return fold_build2_loc (loc, GT_EXPR, type, exp,
-			      build_int_cst (etype, 0));
+				  build_int_cst (etype, 0));
 	}
     }
 
@@ -4895,25 +4895,15 @@ build_range_check (location_t loc, tree
 	return 0;
     }
 
+  if (POINTER_TYPE_P (etype))
+    etype = unsigned_type_for (etype);
+
   high = fold_convert_loc (loc, etype, high);
   low = fold_convert_loc (loc, etype, low);
   exp = fold_convert_loc (loc, etype, exp);
 
   value = const_binop (MINUS_EXPR, high, low);
 
-
-  if (POINTER_TYPE_P (etype))
-    {
-      if (value != 0 && !TREE_OVERFLOW (value))
-	{
-	  low = fold_build1_loc (loc, NEGATE_EXPR, TREE_TYPE (low), low);
-          return build_range_check (loc, type,
-			     	    fold_build_pointer_plus_loc (loc, exp, low),
-			            1, build_int_cst (etype, 0), value);
-	}
-      return 0;
-    }
-
   if (value != 0 && !TREE_OVERFLOW (value))
     return build_range_check (loc, type,
 			      fold_build2_loc (loc, MINUS_EXPR, etype, exp, low),