Re: [PATCH RFC 16/17] KVM: selftests: Test guest execution with tmpfs-backed guest_memfd

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] selftests: guest_memfd: tmpfs pool capacity mismatch causes ENOSPC
- [Medium] selftests: guest_memfd: missing backward compatibility check for USE_RESOURCE flag
--

commit 1a86f5779ff5f4e3456d291df1f061ef3218958b
Author: Ackerley Tng <ackerleytng@google.com>

KVM: selftests: Test guest execution with tmpfs-backed guest_memfd
    
Test that a guest vCPU can execute instructions and read/write memory
mapped into a guest_memfd instance that is backed by a tmpfs resource
pool descriptor.

> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 345b41c912a18..bf369874f2647 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c

[ ... ]

> @@ -829,6 +829,62 @@ static void test_guest_memfd_guest(void)
>  	kvm_vm_free(vm);
>  }
>  
> +static void test_guest_memfd_guest_resource(void)
> +{
> +	const gpa_t gpa = SZ_4G;
> +	const int slot = 1;
> +	struct kvm_vcpu *vcpu;
> +	struct kvm_vm *vm;
> +	int pool_fd, fd, i;
> +	size_t size;
> +	u8 *mem;
> +
> +	if (!kvm_check_cap(KVM_CAP_GUEST_MEMFD_FLAGS))
> +		return;
> +
> +	pool_fd = create_tmpfs_pool_fd("never", true, page_size);
> +	if (pool_fd < 0)
> +		TEST_REQUIRE(false);
> +
> +	vm = __vm_create_shape_with_one_vcpu(VM_SHAPE_DEFAULT, &vcpu, 1, guest_code);
> +
> +	TEST_ASSERT(vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS) & GUEST_MEMFD_FLAG_MMAP,
> +		    "Default VM type should support MMAP, supported flags = 0x%x",
> +		    vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS));
> +	TEST_ASSERT(vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS) & GUEST_MEMFD_FLAG_INIT_SHARED,
> +		    "Default VM type should support INIT_SHARED, supported flags = 0x%x",
> +		    vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS));
> +
> +	size = max_t(size_t, vm->page_size, page_size);

[Severity: Medium]
Will this capacity mismatch cause the test to fail unconditionally with
ENOSPC when the guest page size exceeds the host page size?

The tmpfs pool is sized statically to the host page_size earlier in the
function, before the required guest memory size is dynamically calculated
here. This could structurally prevent the test from passing on architectures
with larger guest page sizes (e.g. ARM64).

> +	fd = __vm_create_guest_memfd_resource(vm, size,
> +					      GUEST_MEMFD_FLAG_MMAP |
> +					      GUEST_MEMFD_FLAG_INIT_SHARED |
> +					      GUEST_MEMFD_FLAG_USE_RESOURCE,
> +					      pool_fd);
> +	TEST_ASSERT(fd >= 0, "guest_memfd with tmpfs pool should succeed");

[Severity: Medium]
Could this crash with an assertion failure instead of gracefully skipping
when run against older kernels?

The GUEST_MEMFD_FLAG_USE_RESOURCE flag is passed here without validating
whether it is present in the KVM_CAP_GUEST_MEMFD_FLAGS bitmask, which is
required for backward compatibility with kernels lacking support for this flag.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260925-gmem-tmpfs-backend-v1-0-d36159822d18@google.com?part=16