linux-kernel - Re: [PATCH 6/6] KVM: selftests: Verify that faulting in private guest

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Message-ID: <aNrRW7RtUgFU8ivs@google.com>
Date: Mon, 29 Sep 2025 11:35:07 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ackerley Tng <ackerleytng@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>, 
	Fuad Tabba <tabba@...gle.com>
Subject: Re: [PATCH 6/6] KVM: selftests: Verify that faulting in private
 guest_memfd memory fails

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);
 }