Hi!
The following testcase triggers a false positive UBSan binding a reference
to null diagnostics.
In the FE we instrument conversions from pointer to reference type
to diagnose at runtime if the operand of such a conversion is 0.
The problem is that a GENERIC folding folds
((const struct Bar *) ((const struct Foo *) this)->data) + (sizetype)
range_check (x)
conversion to const struct Bar & by converting to that the first
operand of the POINTER_PLUS_EXPR. But that changes when the -fsanitize=null
binding to reference runtime check occurs. Without the optimization,
it is invoked on the result of the POINTER_PLUS_EXPR, and as range_check
call throws, that means it never triggers in the testcase.
With the optimization, it checks whether this->data is NULL and it is.
The following patch avoids that optimization during GENERIC folding when
-fsanitize=null is enabled and it is a cast from non-REFERENCE_TYPE to
REFERENCE_TYPE.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2022-05-27 Jakub Jelinek <ja...@redhat.com>
PR sanitizer/105729
* fold-const.cc (fold_unary_loc): Don't optimize (X &) ((Y *) z + w)
to (X &) z + w if -fsanitize=null during GENERIC folding.
* g++.dg/ubsan/pr105729.C: New test.
--- gcc/fold-const.cc.jj 2022-05-13 09:34:23.967147706 +0200
+++ gcc/fold-const.cc 2022-05-26 17:12:50.145526094 +0200
@@ -9516,6 +9516,16 @@ fold_unary_loc (location_t loc, enum tre
> min_align_of_type (TREE_TYPE (TREE_TYPE (arg00)))))
return NULL_TREE;
+ /* Similarly, avoid this optimization in GENERIC for -fsanitize=null
+ when type is a reference type and arg00's type is not,
+ because arg00 could be validly nullptr and if arg01 doesn't return,
+ we don't want false positive binding of reference to nullptr. */
+ if (TREE_CODE (type) == REFERENCE_TYPE
+ && !in_gimple_form
+ && sanitize_flags_p (SANITIZE_NULL)
+ && TREE_CODE (TREE_TYPE (arg00)) != REFERENCE_TYPE)
+ return NULL_TREE;
+
arg00 = fold_convert_loc (loc, type, arg00);
return fold_build_pointer_plus_loc (loc, arg00, arg01);
}
--- gcc/testsuite/g++.dg/ubsan/pr105729.C.jj 2022-05-26 19:54:58.725009300
+0200
+++ gcc/testsuite/g++.dg/ubsan/pr105729.C 2022-05-26 19:56:50.785848337
+0200
@@ -0,0 +1,29 @@
+// PR sanitizer/105729
+// { dg-do run }
+// { dg-options "-fsanitize=null -fno-sanitize-recover=null" }
+
+int
+foo (int x)
+{
+ throw 0;
+}
+
+struct S {};
+struct T {
+ S *data;
+ T () : data (0) {}
+ const S &bar (int x) const { return data[foo (x)]; }
+};
+
+int
+main ()
+{
+ T t;
+ try
+ {
+ t.bar (-1);
+ }
+ catch (...)
+ {
+ }
+}
Jakub
