Re: [PATCH v8 43/46] KVM: selftests: Check fd/flags provided to mmap() when setting up memslot

[Fuad Tabba]

On Fri, 19 Jun 2026 at 01:32, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
>
> Check that a valid fd provided to mmap() must be accompanied by MAP_SHARED.
>
> With an invalid fd (usually used for anonymous mappings), there are no
> constraints on mmap() flags.
>
> Add this check to make sure that when a guest_memfd is used as region->fd,
> the flag provided to mmap() will include MAP_SHARED.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> [Rephrase assertion message.]
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad

> ---
>  tools/testing/selftests/kvm/lib/kvm_util.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index 0b2256ea65ff9..6b304e8a0e0d5 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -1110,6 +1110,9 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>                                              src_type == VM_MEM_SRC_SHARED_HUGETLB);
>         }
>
> +       TEST_ASSERT(region->fd == -1 || backing_src_is_shared(src_type),
> +                   "A valid fd provided to mmap() must be accompanied by MAP_SHARED.");
> +
>         region->mmap_start = __kvm_mmap(region->mmap_size, PROT_READ | PROT_WRITE,
>                                         vm_mem_backing_src_alias(src_type)->flag,
>                                         region->fd, mmap_offset);
>
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>
>