On Fri, 27 May 2022, Jakub Jelinek wrote:
> 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?
OK.
Richard.
> 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
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
