KVM: Avoid a lurking guest_memfd ABI mess

[PATCH 4/6] KVM: selftests: Add test coverage for guest_memfd without GUEST_MEMFD_FLAG_MMAP

Posted by Sean Christopherson 5 days, 6 hours ago

From: Ackerley Tng <ackerleytng@google.com>

If a VM type supports KVM_CAP_GUEST_MEMFD_MMAP, the guest_memfd test will
run all test cases with GUEST_MEMFD_FLAG_MMAP set.  This leaves the code
path for creating a non-mmap()-able guest_memfd on a VM that supports
mappable guest memfds untested.

Refactor the test to run the main test suite with a given set of flags.
Then, for VM types that support the mappable capability, invoke the test
suite twice: once with no flags, and once with GUEST_MEMFD_FLAG_MMAP
set.

This ensures both creation paths are properly exercised on capable VMs.

test_guest_memfd_flags() tests valid flags, hence it can be run just once
per VM type, and valid flag identification can be moved into the test
function.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
[sean: use double-underscores for the inner helper]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 .../testing/selftests/kvm/guest_memfd_test.c  | 30 ++++++++++++-------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index 60c6dec63490..5a50a28ce1fa 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -239,11 +239,16 @@ static void test_create_guest_memfd_multiple(struct kvm_vm *vm)
 	close(fd1);
 }
 
-static void test_guest_memfd_flags(struct kvm_vm *vm, uint64_t valid_flags)
+static void test_guest_memfd_flags(struct kvm_vm *vm)
 {
+	uint64_t valid_flags = 0;
 	uint64_t flag;
 	int fd;
 
+	if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
+		valid_flags |= GUEST_MEMFD_FLAG_MMAP |
+			       GUEST_MEMFD_FLAG_DEFAULT_SHARED;
+
 	for (flag = BIT(0); flag; flag <<= 1) {
 		fd = __vm_create_guest_memfd(vm, page_size, flag);
 		if (flag & valid_flags) {
@@ -267,16 +272,8 @@ do {									\
 	close(fd);							\
 } while (0)
 
-static void test_guest_memfd(unsigned long vm_type)
+static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
 {
-	uint64_t flags = 0;
-	struct kvm_vm *vm;
-
-	vm = vm_create_barebones_type(vm_type);
-
-	if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
-		flags |= GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_DEFAULT_SHARED;
-
 	test_create_guest_memfd_multiple(vm);
 	test_create_guest_memfd_invalid_sizes(vm, flags);
 
@@ -292,8 +289,19 @@ static void test_guest_memfd(unsigned long vm_type)
 	gmem_test(file_size, vm, flags);
 	gmem_test(fallocate, vm, flags);
 	gmem_test(invalid_punch_hole, vm, flags);
+}
 
-	test_guest_memfd_flags(vm, flags);
+static void test_guest_memfd(unsigned long vm_type)
+{
+	struct kvm_vm *vm = vm_create_barebones_type(vm_type);
+
+	test_guest_memfd_flags(vm);
+
+	__test_guest_memfd(vm, 0);
+
+	if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
+		__test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP |
+				       GUEST_MEMFD_FLAG_DEFAULT_SHARED);
 
 	kvm_vm_free(vm);
 }
-- 
2.51.0.536.g15c5d4f767-goog

Re: [PATCH 4/6] KVM: selftests: Add test coverage for guest_memfd without GUEST_MEMFD_FLAG_MMAP

Posted by Fuad Tabba 2 days, 13 hours ago

On Fri, 26 Sept 2025 at 17:31, Sean Christopherson <seanjc@google.com> wrote:
>
> From: Ackerley Tng <ackerleytng@google.com>
>
> If a VM type supports KVM_CAP_GUEST_MEMFD_MMAP, the guest_memfd test will
> run all test cases with GUEST_MEMFD_FLAG_MMAP set.  This leaves the code
> path for creating a non-mmap()-able guest_memfd on a VM that supports
> mappable guest memfds untested.
>
> Refactor the test to run the main test suite with a given set of flags.
> Then, for VM types that support the mappable capability, invoke the test
> suite twice: once with no flags, and once with GUEST_MEMFD_FLAG_MMAP
> set.
>
> This ensures both creation paths are properly exercised on capable VMs.
>
> test_guest_memfd_flags() tests valid flags, hence it can be run just once
> per VM type, and valid flag identification can be moved into the test
> function.
>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> [sean: use double-underscores for the inner helper]
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>

Cheers,
/fuad



> ---
>  .../testing/selftests/kvm/guest_memfd_test.c  | 30 ++++++++++++-------
>  1 file changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 60c6dec63490..5a50a28ce1fa 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -239,11 +239,16 @@ static void test_create_guest_memfd_multiple(struct kvm_vm *vm)
>         close(fd1);
>  }
>
> -static void test_guest_memfd_flags(struct kvm_vm *vm, uint64_t valid_flags)
> +static void test_guest_memfd_flags(struct kvm_vm *vm)
>  {
> +       uint64_t valid_flags = 0;
>         uint64_t flag;
>         int fd;
>
> +       if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
> +               valid_flags |= GUEST_MEMFD_FLAG_MMAP |
> +                              GUEST_MEMFD_FLAG_DEFAULT_SHARED;
> +
>         for (flag = BIT(0); flag; flag <<= 1) {
>                 fd = __vm_create_guest_memfd(vm, page_size, flag);
>                 if (flag & valid_flags) {
> @@ -267,16 +272,8 @@ do {                                                                       \
>         close(fd);                                                      \
>  } while (0)
>
> -static void test_guest_memfd(unsigned long vm_type)
> +static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
>  {
> -       uint64_t flags = 0;
> -       struct kvm_vm *vm;
> -
> -       vm = vm_create_barebones_type(vm_type);
> -
> -       if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
> -               flags |= GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_DEFAULT_SHARED;
> -
>         test_create_guest_memfd_multiple(vm);
>         test_create_guest_memfd_invalid_sizes(vm, flags);
>
> @@ -292,8 +289,19 @@ static void test_guest_memfd(unsigned long vm_type)
>         gmem_test(file_size, vm, flags);
>         gmem_test(fallocate, vm, flags);
>         gmem_test(invalid_punch_hole, vm, flags);
> +}
>
> -       test_guest_memfd_flags(vm, flags);
> +static void test_guest_memfd(unsigned long vm_type)
> +{
> +       struct kvm_vm *vm = vm_create_barebones_type(vm_type);
> +
> +       test_guest_memfd_flags(vm);
> +
> +       __test_guest_memfd(vm, 0);
> +
> +       if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
> +               __test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP |
> +                                      GUEST_MEMFD_FLAG_DEFAULT_SHARED);
>
>         kvm_vm_free(vm);
>  }
> --
> 2.51.0.536.g15c5d4f767-goog
>

Re: [PATCH 4/6] KVM: selftests: Add test coverage for guest_memfd without GUEST_MEMFD_FLAG_MMAP

Posted by David Hildenbrand 2 days, 13 hours ago

On 26.09.25 18:31, Sean Christopherson wrote:
> From: Ackerley Tng <ackerleytng@google.com>
> 
> If a VM type supports KVM_CAP_GUEST_MEMFD_MMAP, the guest_memfd test will
> run all test cases with GUEST_MEMFD_FLAG_MMAP set.  This leaves the code
> path for creating a non-mmap()-able guest_memfd on a VM that supports
> mappable guest memfds untested.
> 
> Refactor the test to run the main test suite with a given set of flags.
> Then, for VM types that support the mappable capability, invoke the test
> suite twice: once with no flags, and once with GUEST_MEMFD_FLAG_MMAP
> set.
> 
> This ensures both creation paths are properly exercised on capable VMs.
> 
> test_guest_memfd_flags() tests valid flags, hence it can be run just once
> per VM type, and valid flag identification can be moved into the test
> function.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> [sean: use double-underscores for the inner helper]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   .../testing/selftests/kvm/guest_memfd_test.c  | 30 ++++++++++++-------
>   1 file changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 60c6dec63490..5a50a28ce1fa 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -239,11 +239,16 @@ static void test_create_guest_memfd_multiple(struct kvm_vm *vm)
>   	close(fd1);
>   }
>   
> -static void test_guest_memfd_flags(struct kvm_vm *vm, uint64_t valid_flags)
> +static void test_guest_memfd_flags(struct kvm_vm *vm)
>   {
> +	uint64_t valid_flags = 0;
>   	uint64_t flag;
>   	int fd;
>   
> +	if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
> +		valid_flags |= GUEST_MEMFD_FLAG_MMAP |
> +			       GUEST_MEMFD_FLAG_DEFAULT_SHARED;
> +
>   	for (flag = BIT(0); flag; flag <<= 1) {
>   		fd = __vm_create_guest_memfd(vm, page_size, flag);
>   		if (flag & valid_flags) {
> @@ -267,16 +272,8 @@ do {									\
>   	close(fd);							\
>   } while (0)
>   
> -static void test_guest_memfd(unsigned long vm_type)
> +static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
>   {
> -	uint64_t flags = 0;
> -	struct kvm_vm *vm;
> -
> -	vm = vm_create_barebones_type(vm_type);
> -
> -	if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
> -		flags |= GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_DEFAULT_SHARED;
> -
>   	test_create_guest_memfd_multiple(vm);
>   	test_create_guest_memfd_invalid_sizes(vm, flags);
>   
> @@ -292,8 +289,19 @@ static void test_guest_memfd(unsigned long vm_type)
>   	gmem_test(file_size, vm, flags);
>   	gmem_test(fallocate, vm, flags);
>   	gmem_test(invalid_punch_hole, vm, flags);
> +}
>   
> -	test_guest_memfd_flags(vm, flags);
> +static void test_guest_memfd(unsigned long vm_type)
> +{
> +	struct kvm_vm *vm = vm_create_barebones_type(vm_type);
> +
> +	test_guest_memfd_flags(vm);
> +
> +	__test_guest_memfd(vm, 0);

Having a simple test_guest_memfd_noflags() wrapper might make this 
easier to read.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb

[PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject user page faults if not set
[PATCH 2/6] KVM: selftests: Stash the host page size in a global in the guest_memfd test
[PATCH 3/6] KVM: selftests: Create a new guest_memfd for each testcase
[PATCH 4/6] KVM: selftests: Add test coverage for guest_memfd without GUEST_MEMFD_FLAG_MMAP
[PATCH 5/6] KVM: selftests: Add wrappers for mmap() and munmap() to assert success
[PATCH 6/6] KVM: selftests: Verify that faulting in private guest_memfd memory fails