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Subject: Re: [PATCH bpf 3/3] selftest/bpf: Test the read of vsyscall page
 under x86-64
To: Yonghong Song <yonghong.song@linux.dev>, x86@kernel.org,
 bpf@vger.kernel.org
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
 Andy Lutomirski <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
 linux-kernel@vger.kernel.org, xingwei lee <xrivendell7@gmail.com>,
 Jann Horn <jannh@google.com>, houtao1@huawei.com
References: <20240119073019.1528573-1-houtao@huaweicloud.com>
 <20240119073019.1528573-4-houtao@huaweicloud.com>
 <f227d88b-963a-4df0-a6bc-ad3b12abe6dd@linux.dev>
From: Hou Tao <houtao@huaweicloud.com>
Message-ID: <f83cd515-1492-229a-913a-aa542ecd1d74@huaweicloud.com>
Date: Thu, 25 Jan 2024 14:37:10 +0800
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Hi,

On 1/22/2024 2:30 PM, Yonghong Song wrote:
>
> 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.

Yes. These comment are highly coupled with the implementation.
>
> I suggest remove the above comments and add more
> detailed explanation in commit messages with callstack
> indicating where the fail/error return happens.

Will do in v2. Thanks for the suggestions.
>
>> +};
>> +
>> +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);
>> +}
> [...]