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

[Ackerley Tng <ackerleytng@google.com>]

Sean Christopherson <seanjc@google.com> writes:

> Add a guest_memfd testcase to verify that faulting in private memory gets
> a SIGBUS.  For now, test only the case where memory is private by default
> since KVM doesn't yet support in-place conversion.
>
> Cc: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  .../testing/selftests/kvm/guest_memfd_test.c  | 62 ++++++++++++++-----
>  1 file changed, 46 insertions(+), 16 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 5dd40b77dc07..b5a631aca933 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -40,17 +40,26 @@ static void test_file_read_write(int fd, size_t total_size)
>  		    "pwrite on a guest_mem fd should fail");
>  }
>  

I feel that the tests should be grouped by concepts being tested

+ test_cow_not_supported()
    + mmap() should fail
+ test_mmap_supported()
    + kvm_mmap()
    + regular, successful accesses to offsets within the size of the fd
    + kvm_munmap()
+ test_fault_overflow()
    + kvm_mmap()
    + a helper (perhaps "assert_fault_sigbus(char *mem)"?) that purely
      tries to access beyond the size of the fd and catches SIGBUS
    + regular, successful accesses to offsets within the size of the fd
    + kvm_munmap()
+ test_fault_private()
    + kvm_mmap()
    + a helper (perhaps "assert_fault_sigbus(char *mem)"?) that purely
      tries to access within the size of the fd and catches SIGBUS
    + kvm_munmap()

I think some code duplication in tests is okay if it makes the test flow
more obvious.

> -static void test_mmap_supported(int fd, size_t total_size)
> +static void *test_mmap_common(int fd, size_t size)
>  {
> -	const char val = 0xaa;
> -	char *mem;
> -	size_t i;
> -	int ret;
> +	void *mem;
>  
> -	mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
> +	mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
>  	TEST_ASSERT(mem == MAP_FAILED, "Copy-on-write not allowed by guest_memfd.");
>

When grouped this way, test_mmap_common() tests that MAP_PRIVATE or COW
is not allowed twice, once in test_mmap_supported() and once in
test_fault_sigbus(). Is that intentional?

> -	mem = kvm_mmap(total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
> +	mem = kvm_mmap(size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
> +
> +	return mem;

I feel that returning (and using) the userspace address from a test
(test_mmap_common()) is a little hard to follow.

> +}
> +
> +static void test_mmap_supported(int fd, size_t total_size)
> +{
> +	const char val = 0xaa;
> +	char *mem;
> +	size_t i;
> +	int ret;
> +
> +	mem = test_mmap_common(fd, total_size);
>  
>  	memset(mem, val, total_size);
>  	for (i = 0; i < total_size; i++)
> @@ -78,31 +87,47 @@ 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 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;
> +	void *mem;
>  
> -	mem = kvm_mmap(map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
> +	mem = test_mmap_common(fd, size);
>  
>  	sigaction(SIGBUS, &sa_new, &sa_old);
>  	if (sigsetjmp(jmpbuf, 1) == 0) {
> -		memset(mem, 0xaa, map_size);
> +		memset(mem, 0xaa, size);
>  		TEST_ASSERT(false, "memset() should have triggered SIGBUS.");
>  	}
>  	sigaction(SIGBUS, &sa_old, NULL);
>  
> +	return mem;

I think returning the userspace address from a test is a little hard to
follow. This one feels even more unexpected because a valid address is
being returned (and used) from a test that has sigbus in its name.

> +}
> +
> +static void test_fault_overflow(int fd, size_t total_size)
> +{
> +	size_t map_size = total_size * 4;
> +	const char val = 0xaa;
> +	char *mem;
> +	size_t i;
> +
> +	mem = test_fault_sigbus(fd, map_size);
> +
>  	for (i = 0; i < total_size; i++)
>  		TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);
>  
>  	kvm_munmap(mem, map_size);
>  }
>  
> +static void test_fault_private(int fd, size_t total_size)
> +{
> +	void *mem = test_fault_sigbus(fd, total_size);
> +
> +	kvm_munmap(mem, total_size);
> +}
> +

Testing that faults fail when GUEST_MEMFD_FLAG_DEFAULT_SHARED is not set
is a good idea. Perhaps it could be even clearer if further split up:

+ test_mmap_supported()
    + kvm_mmap()
    + kvm_munmap()
+ test_mmap_supported_fault_supported()
    + kvm_mmap()
    + successful accesses to offsets within the size of the fd
    + kvm_munmap()
+ test_mmap_supported_fault_sigbus()
    + kvm_mmap()
    + expect SIGBUS from accesses to offsets within the size of the fd
    + kvm_munmap()

>  static void test_mmap_not_supported(int fd, size_t total_size)
>  {
>  	char *mem;
> @@ -274,9 +299,12 @@ 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(fault_overflow, vm, flags);
> +	} else if (flags & GUEST_MEMFD_FLAG_MMAP) {
> +		gmem_test(fault_private, vm, flags);

test_fault_private() makes me think the test is testing for private
faults, but there's nothing private about this fault, and the fault
doesn't even come from the guest.

>  	} else {
>  		gmem_test(mmap_not_supported, vm, flags);
>  	}

If split up as described above, this could be

	if (flags & GUEST_MEMFD_FLAG_MMAP &&
	    flags & GUEST_MEMFD_FLAG_DEFAULT_SHARED) {
		gmem_test(mmap_supported_fault_supported, vm, flags);
		gmem_test(fault_overflow, vm, flags);
	} else if (flags & GUEST_MEMFD_FLAG_MMAP) {
		gmem_test(mmap_supported_fault_sigbus, vm, flags);
	} else {
		gmem_test(mmap_not_supported, vm, flags);
	}

> @@ -294,9 +322,11 @@ 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);
>  }

I could send a revision, if you agree/prefer!

Reviewed-by: Ackerley Tng <ackerleytng@google.com>

> -- 
> 2.51.0.536.g15c5d4f767-goog