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
2017-06-21 Jakub Jelinek <ja...@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),
2017-06-21 Jakub Jelinek <ja...@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),
