Re: [PATCH v11 18/18] KVM: selftests: guest_memfd mmap() test when mapping is allowed

[James Houghton <jthoughton@google.com>]

On Thu, Jun 5, 2025 at 8:38 AM Fuad Tabba <tabba@google.com> wrote:
>
> Expand the guest_memfd selftests to include testing mapping guest
> memory for VM types that support it.
>
> Co-developed-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>

Feel free to add:

Reviewed-by: James Houghton <jthoughton@google.com>

> ---
>  .../testing/selftests/kvm/guest_memfd_test.c  | 201 ++++++++++++++++--
>  1 file changed, 180 insertions(+), 21 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 341ba616cf55..1612d3adcd0d 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -13,6 +13,8 @@
>
>  #include <linux/bitmap.h>
>  #include <linux/falloc.h>
> +#include <setjmp.h>
> +#include <signal.h>
>  #include <sys/mman.h>
>  #include <sys/types.h>
>  #include <sys/stat.h>
> @@ -34,12 +36,83 @@ static void test_file_read_write(int fd)
>                     "pwrite on a guest_mem fd should fail");
>  }
>
> -static void test_mmap(int fd, size_t page_size)
> +static void test_mmap_supported(int fd, size_t page_size, size_t total_size)
> +{
> +       const char val = 0xaa;
> +       char *mem;

This must be `volatile char *` to ensure that the compiler doesn't
elide the accesses you have written.

> +       size_t i;
> +       int ret;
> +
> +       mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
> +       TEST_ASSERT(mem == MAP_FAILED, "Copy-on-write not allowed by guest_memfd.");
> +
> +       mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       TEST_ASSERT(mem != MAP_FAILED, "mmap() for shared guest memory should succeed.");
> +
> +       memset(mem, val, total_size);

Now unfortunately, `memset` and `munmap` will complain about the
volatile qualification. So...

memset((char *)mem, val, total_size);

Eh... wish they just wouldn't complain, but this is a small price to
pay for correctness. :)

> +       for (i = 0; i < total_size; i++)
> +               TEST_ASSERT_EQ(mem[i], val);

The compiler is allowed to[1] elide the read of `mem[i]` and just
assume that it is `val`.

[1]: https://godbolt.org/z/Wora54bP6

Feel free to add `volatile` to that snippet to see how the code changes.

> +
> +       ret = fallocate(fd, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE, 0,
> +                       page_size);
> +       TEST_ASSERT(!ret, "fallocate the first page should succeed.");
> +
> +       for (i = 0; i < page_size; i++)
> +               TEST_ASSERT_EQ(mem[i], 0x00);
> +       for (; i < total_size; i++)
> +               TEST_ASSERT_EQ(mem[i], val);
> +
> +       memset(mem, val, page_size);
> +       for (i = 0; i < total_size; i++)
> +               TEST_ASSERT_EQ(mem[i], val);
> +
> +       ret = munmap(mem, total_size);
> +       TEST_ASSERT(!ret, "munmap() should succeed.");
> +}
> +
> +static sigjmp_buf jmpbuf;
> +void fault_sigbus_handler(int signum)
> +{
> +       siglongjmp(jmpbuf, 1);
> +}
> +
> +static void test_fault_overflow(int fd, size_t page_size, size_t total_size)
> +{
> +       struct sigaction sa_old, sa_new = {
> +               .sa_handler = fault_sigbus_handler,
> +       };
> +       size_t map_size = total_size * 4;
> +       const char val = 0xaa;
> +       char *mem;

`volatile` here as well.

> +       size_t i;
> +       int ret;
> +
> +       mem = mmap(NULL, map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       TEST_ASSERT(mem != MAP_FAILED, "mmap() for shared guest memory should succeed.");
> +
> +       sigaction(SIGBUS, &sa_new, &sa_old);
> +       if (sigsetjmp(jmpbuf, 1) == 0) {
> +               memset(mem, 0xaa, map_size);
> +               TEST_ASSERT(false, "memset() should have triggered SIGBUS.");
> +       }
> +       sigaction(SIGBUS, &sa_old, NULL);
> +
> +       for (i = 0; i < total_size; i++)
> +               TEST_ASSERT_EQ(mem[i], val);
> +
> +       ret = munmap(mem, map_size);
> +       TEST_ASSERT(!ret, "munmap() should succeed.");
> +}
> +
> +static void test_mmap_not_supported(int fd, size_t page_size, size_t total_size)
>  {
>         char *mem;
>
>         mem = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
>         TEST_ASSERT_EQ(mem, MAP_FAILED);
> +
> +       mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       TEST_ASSERT_EQ(mem, MAP_FAILED);
>  }
>
>  static void test_file_size(int fd, size_t page_size, size_t total_size)
> @@ -120,26 +193,19 @@ static void test_invalid_punch_hole(int fd, size_t page_size, size_t total_size)
>         }
>  }
>
> -static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
> +static void test_create_guest_memfd_invalid_sizes(struct kvm_vm *vm,
> +                                                 uint64_t guest_memfd_flags,
> +                                                 size_t page_size)
>  {
> -       size_t page_size = getpagesize();
> -       uint64_t flag;
>         size_t size;
>         int fd;
>
>         for (size = 1; size < page_size; size++) {
> -               fd = __vm_create_guest_memfd(vm, size, 0);
> -               TEST_ASSERT(fd == -1 && errno == EINVAL,
> +               fd = __vm_create_guest_memfd(vm, size, guest_memfd_flags);
> +               TEST_ASSERT(fd < 0 && errno == EINVAL,
>                             "guest_memfd() with non-page-aligned page size '0x%lx' should fail with EINVAL",
>                             size);
>         }
> -
> -       for (flag = BIT(0); flag; flag <<= 1) {
> -               fd = __vm_create_guest_memfd(vm, page_size, flag);
> -               TEST_ASSERT(fd == -1 && errno == EINVAL,
> -                           "guest_memfd() with flag '0x%lx' should fail with EINVAL",
> -                           flag);
> -       }
>  }
>
>  static void test_create_guest_memfd_multiple(struct kvm_vm *vm)
> @@ -171,30 +237,123 @@ static void test_create_guest_memfd_multiple(struct kvm_vm *vm)
>         close(fd1);
>  }
>
> -int main(int argc, char *argv[])
> +static bool check_vm_type(unsigned long vm_type)
>  {
> -       size_t page_size;
> +       /*
> +        * Not all architectures support KVM_CAP_VM_TYPES. However, those that
> +        * support guest_memfd have that support for the default VM type.
> +        */
> +       if (vm_type == VM_TYPE_DEFAULT)
> +               return true;
> +
> +       return kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(vm_type);
> +}
> +
> +static void test_with_type(unsigned long vm_type, uint64_t guest_memfd_flags,
> +                          bool expect_mmap_allowed)
> +{
> +       struct kvm_vm *vm;
>         size_t total_size;
> +       size_t page_size;
>         int fd;
> -       struct kvm_vm *vm;
>
> -       TEST_REQUIRE(kvm_has_cap(KVM_CAP_GUEST_MEMFD));
> +       if (!check_vm_type(vm_type))
> +               return;
>
>         page_size = getpagesize();
>         total_size = page_size * 4;
>
> -       vm = vm_create_barebones();
> +       vm = vm_create_barebones_type(vm_type);
>
> -       test_create_guest_memfd_invalid(vm);
>         test_create_guest_memfd_multiple(vm);
> +       test_create_guest_memfd_invalid_sizes(vm, guest_memfd_flags, page_size);
>
> -       fd = vm_create_guest_memfd(vm, total_size, 0);
> +       fd = vm_create_guest_memfd(vm, total_size, guest_memfd_flags);
>
>         test_file_read_write(fd);
> -       test_mmap(fd, page_size);
> +
> +       if (expect_mmap_allowed) {
> +               test_mmap_supported(fd, page_size, total_size);
> +               test_fault_overflow(fd, page_size, total_size);
> +
> +       } else {
> +               test_mmap_not_supported(fd, page_size, total_size);
> +       }
> +
>         test_file_size(fd, page_size, total_size);
>         test_fallocate(fd, page_size, total_size);
>         test_invalid_punch_hole(fd, page_size, total_size);
>
>         close(fd);
> +       kvm_vm_release(vm);

I think kvm_vm_free() is probably more appropriate?

> +}
> +
> +static void test_vm_type_gmem_flag_validity(unsigned long vm_type,
> +                                           uint64_t expected_valid_flags)
> +{
> +       size_t page_size = getpagesize();
> +       struct kvm_vm *vm;
> +       uint64_t flag = 0;
> +       int fd;
> +
> +       if (!check_vm_type(vm_type))
> +               return;
> +
> +       vm = vm_create_barebones_type(vm_type);
> +
> +       for (flag = BIT(0); flag; flag <<= 1) {
> +               fd = __vm_create_guest_memfd(vm, page_size, flag);
> +
> +               if (flag & expected_valid_flags) {
> +                       TEST_ASSERT(fd >= 0,
> +                                   "guest_memfd() with flag '0x%lx' should be valid",
> +                                   flag);
> +                       close(fd);
> +               } else {
> +                       TEST_ASSERT(fd < 0 && errno == EINVAL,
> +                                   "guest_memfd() with flag '0x%lx' should fail with EINVAL",
> +                                   flag);
> +               }
> +       }
> +
> +       kvm_vm_release(vm);

Same here.

> +}
> +
> +static void test_gmem_flag_validity(void)
> +{
> +       uint64_t non_coco_vm_valid_flags = 0;
> +
> +       if (kvm_has_cap(KVM_CAP_GMEM_SHARED_MEM))
> +               non_coco_vm_valid_flags = GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> +
> +       test_vm_type_gmem_flag_validity(VM_TYPE_DEFAULT, non_coco_vm_valid_flags);
> +
> +#ifdef __x86_64__
> +       test_vm_type_gmem_flag_validity(KVM_X86_SW_PROTECTED_VM, non_coco_vm_valid_flags);
> +       test_vm_type_gmem_flag_validity(KVM_X86_SEV_VM, 0);
> +       test_vm_type_gmem_flag_validity(KVM_X86_SEV_ES_VM, 0);
> +       test_vm_type_gmem_flag_validity(KVM_X86_SNP_VM, 0);
> +       test_vm_type_gmem_flag_validity(KVM_X86_TDX_VM, 0);
> +#endif
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +       TEST_REQUIRE(kvm_has_cap(KVM_CAP_GUEST_MEMFD));
> +
> +       test_gmem_flag_validity();
> +
> +       test_with_type(VM_TYPE_DEFAULT, 0, false);
> +       if (kvm_has_cap(KVM_CAP_GMEM_SHARED_MEM)) {
> +               test_with_type(VM_TYPE_DEFAULT, GUEST_MEMFD_FLAG_SUPPORT_SHARED,
> +                              true);
> +       }
> +
> +#ifdef __x86_64__
> +       test_with_type(KVM_X86_SW_PROTECTED_VM, 0, false);
> +       if (kvm_has_cap(KVM_CAP_GMEM_SHARED_MEM)) {
> +               test_with_type(KVM_X86_SW_PROTECTED_VM,
> +                              GUEST_MEMFD_FLAG_SUPPORT_SHARED, true);
> +       }
> +#endif
>  }
> --
> 2.49.0.1266.g31b7d2e469-goog
>