[PATCH 1/2] libstdc++: Validate user-provided stride values for layout_stride.

[Tomasz Kaminski]

On Tue, Jul 7, 2026 at 2:08 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:

>
>
> On Tue, Jul 7, 2026 at 1:59 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>
>> On Tue, 7 Jul 2026 at 11:21, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>> >
>> > Converting the __strides values using __index_type_cast asserts
>> > that they are non-negative and each value is representable as
>> > index_type.
>> >
>> > libstdc++-v3/ChangeLog:
>> >
>> >         * include/std/mdspan
>> >         (layout_stride::mapping::mapping(const extent_type&, span<...>):
>> >         Convert strides using __index_type_cast, which bring asserts
>> >         for negative and unrepresentable values.
>> >         * testsuite/23_containers/mdspan/layouts/stride_neg.cc: New
>> test.
>> > ---
>> > Tested on x86_64-linux locally. OK for trunk?
>>
>> OK for trunk. I think this makes sense for gcc-16 too?
>>
> Yes, once LWG4603 is approved. I will submit PR for it this week.
>
As Mark and I are still clarifying the uniqueness condition, I will
backport only this
patch to GCC-16.

>
>> >
>> >  libstdc++-v3/include/std/mdspan               |  3 +-
>> >  .../mdspan/layouts/stride_neg.cc              | 31 +++++++++++++++++++
>> >  2 files changed, 33 insertions(+), 1 deletion(-)
>> >  create mode 100644
>> libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
>> >
>> > diff --git a/libstdc++-v3/include/std/mdspan
>> b/libstdc++-v3/include/std/mdspan
>> > index f5556f35fa1..5e1c4d8b1a4 100644
>> > --- a/libstdc++-v3/include/std/mdspan
>> > +++ b/libstdc++-v3/include/std/mdspan
>> > @@ -1891,7 +1891,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >         : _M_extents(__exts)
>> >         {
>> >           for (size_t __i = 0; __i < extents_type::rank(); ++__i)
>> > -           _M_strides[__i] = index_type(as_const(__strides[__i]));
>> > +           _M_strides[__i]
>> > +              =
>> __mdspan::__index_type_cast<index_type>(__strides[__i]);
>> >         }
>> >
>> >        template<typename _OIndexType>
>> > diff --git
>> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
>> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
>> > new file mode 100644
>> > index 00000000000..153560bc82f
>> > --- /dev/null
>> > +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
>> > @@ -0,0 +1,31 @@
>> > +// { dg-do compile { target c++23 } }
>> > +#include <mdspan>
>> > +
>> > +#include "../layout_traits.h"
>> > +#include <cstdint>
>> > +
>> > +constexpr size_t dyn = std::dynamic_extent;
>> > +
>> > +constexpr bool
>> > +test_stride_overflow()
>> > +{
>> > +  auto exts = std::extents<uint8_t, dyn, dyn>(1, 3);
>> > +  auto n = size_t(1) << 9;
>> > +  auto m = std::layout_stride::mapping(exts, std::array{n, 1zu}); // {
>> dg-error "expansion of" }
>> > +  (void) m;
>> > +  return true;
>> > +}
>> > +static_assert(test_stride_overflow()); // { dg-error "expansion of" }
>> > +
>> > +constexpr bool
>> > +test_stride_negative()
>> > +{
>> > +  auto exts = std::extents<std::size_t, dyn, dyn>(1, 3);
>> > +  auto m = std::layout_stride::mapping(exts, std::array{1, -4}); // {
>> dg-error "expansion of" }
>> > +  (void) m;
>> > +  return true;
>> > +}
>> > +static_assert(test_stride_negative()); // { dg-error "expansion of" }
>> > +
>> > +// { dg-prune-output "non-constant condition for static assertion" }
>> > +// { dg-prune-output "__glibcxx_assert_fail()" }
>> > --
>> > 2.54.0
>> >
>>
>>
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