Re: [PATCH 6/6] KVM: selftests: Verify that faulting in private guest_memfd memory fails

Sean Christopherson
2025-09-29T18:35:09Z

On Mon, Sep 29, 2025, Sean Christopherson wrote:
> How's this look?
>
> static void test_fault_sigbus(int fd, size_t accessible_size, size_t mmap_size)
> {
> struct sigaction sa_old, sa_new = {
> .sa_handler = fault_sigbus_handler,
> };
> const uint8_t val = 0xaa;
> uint8_t *mem;
> size_t i;
>
> mem = kvm_mmap(mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
>
> sigaction(SIGBUS, &sa_new, &sa_old);
> if (sigsetjmp(jmpbuf, 1) == 0) {
> memset(mem, val, mmap_size);
> TEST_FAIL("memset() should have triggered SIGBUS");
> }
> if (sigsetjmp(jmpbuf, 1) == 0) {
> (void)READ_ONCE(mem[accessible_size]);
> TEST_FAIL("load at first unaccessible byte should have triggered SIGBUS");
> }
> sigaction(SIGBUS, &sa_old, NULL);
>
> for (i = 0; i < accessible_size; i++)
> TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);
>
> kvm_munmap(mem, mmap_size);
> }
>
> static void test_fault_overflow(int fd, size_t total_size)
> {
> test_fault_sigbus(fd, total_size, total_size * 4);
> }
>
> static void test_fault_private(int fd, size_t total_size)
> {
> test_fault_sigbus(fd, 0, total_size);
> }

And if I don't wantonly change variable names/types, the diff is much cleaner:

diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index 8ed08be72c43..8e375de2d7d8 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -83,12 +83,11 @@ void fault_sigbus_handler(int signum)
siglongjmp(jmpbuf, 1);
}

-static void test_fault_overflow(int fd, size_t total_size)
+static void test_fault_sigbus(int fd, size_t accessible_size, size_t map_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;
size_t i;
@@ -102,12 +101,22 @@ static void test_fault_overflow(int fd, size_t total_size)
}
sigaction(SIGBUS, &sa_old, NULL);

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

kvm_munmap(mem, map_size);
}

+static void test_fault_overflow(int fd, size_t total_size)
+{
+ test_fault_sigbus(fd, total_size, total_size * 4);
+}
+
+static void test_fault_private(int fd, size_t total_size)
+{
+ test_fault_sigbus(fd, 0, total_size);
+}
+
static void test_mmap_not_supported(int fd, size_t total_size)
{
char *mem;
@@ -279,10 +288,13 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)

gmem_test(file_read_write, vm, flags);

- if (flags & GUEST_MEMFD_FLAG_MMAP) {
+ if (flags & GUEST_MEMFD_FLAG_MMAP &&
+ flags & GUEST_MEMFD_FLAG_DEFAULT_SHARED) {
gmem_test(mmap_supported, vm, flags);
gmem_test(mmap_cow, vm, flags);
gmem_test(fault_overflow, vm, flags);
+ } else if (flags & GUEST_MEMFD_FLAG_MMAP) {
+ gmem_test(fault_private, vm, flags);
} else {
gmem_test(mmap_not_supported, vm, flags);
}
@@ -300,9 +312,11 @@ static void test_guest_memfd(unsigned long vm_type)

__test_guest_memfd(vm, 0);

- if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
+ if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP)) {
+ __test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP);
__test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP |
GUEST_MEMFD_FLAG_DEFAULT_SHARED);
+ }

kvm_vm_free(vm);
}