Re: [PATCH bpf 3/3] selftest/bpf: Test the read of vsyscall page under x86-64

[Yonghong Song <yonghong.song@linux.dev>]

On 1/18/24 11:30 PM, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
>
> Using bpf_probe_read_kernel{_str}() or bpf_probe_read{_str}() to read
> from vsyscall page under x86-64 will trigger oops, so add one test case
> to ensure that the problem is fixed.
>
> Beside those four bpf helpers mentioned above, testing the read of
> vsyscall page by using bpf_probe_read_user{_str} and
> bpf_copy_from_user{_task}() as well.
>
> vsyscall page could be disabled by CONFIG_LEGACY_VSYSCALL_NONE or
> vsyscall=none boot cmd-line, but it doesn't affect the reproduce of the
> problem and the returned error codes.
>
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
>   .../selftests/bpf/prog_tests/read_vsyscall.c  | 61 +++++++++++++++++++
>   .../selftests/bpf/progs/read_vsyscall.c       | 45 ++++++++++++++
>   2 files changed, 106 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/read_vsyscall.c
>   create mode 100644 tools/testing/selftests/bpf/progs/read_vsyscall.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/read_vsyscall.c b/tools/testing/selftests/bpf/prog_tests/read_vsyscall.c
> new file mode 100644
> index 0000000000000..d9247cc89cf3e
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/read_vsyscall.c
> @@ -0,0 +1,61 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (C) 2024. Huawei Technologies Co., Ltd */
> +#include "test_progs.h"
> +#include "read_vsyscall.skel.h"
> +
> +#if defined(__x86_64__)
> +/* For VSYSCALL_ADDR */
> +#include <asm/vsyscall.h>
> +#else
> +/* To prevent build failure on non-x86 arch */
> +#define VSYSCALL_ADDR 0UL
> +#endif
> +
> +struct read_ret_desc {
> +	const char *name;
> +	int ret;
> +} all_read[] = {
> +	{ .name = "probe_read_kernel", .ret = -ERANGE },
> +	{ .name = "probe_read_kernel_str", .ret = -ERANGE },
> +	{ .name = "probe_read", .ret = -ERANGE },
> +	{ .name = "probe_read_str", .ret = -ERANGE },
> +	/* __access_ok() will fail */
> +	{ .name = "probe_read_user", .ret = -EFAULT },
> +	/* __access_ok() will fail */
> +	{ .name = "probe_read_user_str", .ret = -EFAULT },
> +	/* access_ok() will fail */
> +	{ .name = "copy_from_user", .ret = -EFAULT },
> +	/* both vma_lookup() and expand_stack() will fail */
> +	{ .name = "copy_from_user_task", .ret = -EFAULT },

The above comments are not clear enough. For example,
'__access_ok() will fail', user will need to
check the source code where __access_ok() is and
this could be hard e.g., for probe_read_user_str().
Another example, 'both vma_lookup() and expand_stack() will fail',
where is vma_lookup()/expand_stack()? User needs to further
check to make sense.

I suggest remove the above comments and add more
detailed explanation in commit messages with callstack
indicating where the fail/error return happens.

> +};
> +
> +void test_read_vsyscall(void)
> +{
> +	struct read_vsyscall *skel;
> +	unsigned int i;
> +	int err;
> +
> +#if !defined(__x86_64__)
> +	test__skip();
> +	return;
> +#endif
> +	skel = read_vsyscall__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "read_vsyscall open_load"))
> +		return;
> +
> +	skel->bss->target_pid = getpid();
> +	err = read_vsyscall__attach(skel);
> +	if (!ASSERT_EQ(err, 0, "read_vsyscall attach"))
> +		goto out;
> +
> +	/* userspace may don't have vsyscall page due to LEGACY_VSYSCALL_NONE,
> +	 * but it doesn't affect the returned error codes.
> +	 */
> +	skel->bss->user_ptr = (void *)VSYSCALL_ADDR;
> +	usleep(1);
> +
> +	for (i = 0; i < ARRAY_SIZE(all_read); i++)
> +		ASSERT_EQ(skel->bss->read_ret[i], all_read[i].ret, all_read[i].name);
> +out:
> +	read_vsyscall__destroy(skel);
> +}
[...]