Message-ID: <71c53be6-add4-2557-bc8f-8acb8e4a2f39@fb.com>
Date:   Tue, 17 Dec 2019 04:13:44 +0000
From:   Yonghong Song <yhs@...com>
To:     Wenbo Zhang <ethercflow@...il.com>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v12 2/2] selftests/bpf: test for
 bpf_get_file_path() from tracepoint



On 12/16/19 8:01 PM, Wenbo Zhang wrote:
>> In non-bpf .c file, typically we do not add 'inline' attribute.
>> It is up to compiler to decide whether it should be inlined.
> 
> Thank you, I'll fix this.
> 
>>> +struct sys_enter_newfstat_args {
>>> +     unsigned long long pad1;
>>> +     unsigned long long pad2;
>>> +     unsigned int fd;
>>> +};
> 
>> The BTF generated vmlinux.h has the following structure,
>> struct trace_entry {
>>           short unsigned int type;
>>           unsigned char flags;
>>           unsigned char preempt_count;
>>           int pid;
>> };
>> struct trace_event_raw_sys_enter {
>>           struct trace_entry ent;
>>           long int id;
>>           long unsigned int args[6];
>>           char __data[0];
>> };
> 
>> The third parameter type should be long, otherwise,
>> it may have issue on big endian machines?
> 
> Sorry, I don't understand why there is a problem on big-endian machines.
> Would you please explain that in more detail? Thank you.

The kernel will actually have 8 bytes of memory to store fd
based on trace_event_raw_sys_enter.

For little endian machine, the lower 4 bytes are read based on
your sys_enter_newfstat_args, which is "accidentally" the lower
4 bytes in u64, so you get the correct answer.

For big endian machine, the lower 4 bytes read based on
your sys_enter_newfstat_args will be high 4 bytes in u64, which
is incorrect.

> 
> Yonghong Song <yhs@...com> 于2019年12月16日周一 上午12:25写道：
>>
>>
>>
>> On 12/14/19 8:01 PM, Wenbo Zhang wrote:
>>> trace fstat events by tracepoint syscalls/sys_enter_newfstat, and handle
>>> events only produced by test_file_get_path, which call fstat on several
>>> different types of files to test bpf_get_file_path's feature.
>>>
>>> v4->v5: addressed Andrii's feedback
>>> - pass NULL for opts as bpf_object__open_file's PARAM2, as not really
>>> using any
>>> - modify patch subject to keep up with test code
>>> - as this test is single-threaded, so use getpid instead of SYS_gettid
>>> - remove unnecessary parens around check which after if (i < 3)
>>> - in kern use bpf_get_current_pid_tgid() >> 32 to fit getpid() in
>>> userspace part
>>> - with the patch adding helper as one patch series
>>>
>>> v3->v4: addressed Andrii's feedback
>>> - use a set of fd instead of fds array
>>> - use global variables instead of maps (in v3, I mistakenly thought that
>>> the bpf maps are global variables.)
>>> - remove uncessary global variable path_info_index
>>> - remove fd compare as the fstat's order is fixed
>>>
>>> v2->v3: addressed Andrii's feedback
>>> - use global data instead of perf_buffer to simplified code
>>>
>>> v1->v2: addressed Daniel's feedback
>>> - rename bpf_fd2path to bpf_get_file_path to be consistent with other
>>> helper's names
>>>
>>> Signed-off-by: Wenbo Zhang <ethercflow@...il.com>
>>> ---
>>>    .../selftests/bpf/prog_tests/get_file_path.c  | 171 ++++++++++++++++++
>>>    .../selftests/bpf/progs/test_get_file_path.c  |  43 +++++
>>>    2 files changed, 214 insertions(+)
>>>    create mode 100644 tools/testing/selftests/bpf/prog_tests/get_file_path.c
>>>    create mode 100644 tools/testing/selftests/bpf/progs/test_get_file_path.c
>>>
>>> diff --git a/tools/testing/selftests/bpf/prog_tests/get_file_path.c b/tools/testing/selftests/bpf/prog_tests/get_file_path.c
>>> new file mode 100644
>>> index 000000000000..7ec11e43e0fc
>>> --- /dev/null
>>> +++ b/tools/testing/selftests/bpf/prog_tests/get_file_path.c
>>> @@ -0,0 +1,171 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +#define _GNU_SOURCE
>>> +#include <test_progs.h>
>>> +#include <sys/stat.h>
>>> +#include <linux/sched.h>
>>> +#include <sys/syscall.h>
>>> +
>>> +#define MAX_PATH_LEN         128
>>> +#define MAX_FDS                      7
>>> +#define MAX_EVENT_NUM                16
>>> +
>>> +static struct file_path_test_data {
>>> +     pid_t pid;
>>> +     __u32 cnt;
>>> +     __u32 fds[MAX_EVENT_NUM];
>>> +     char paths[MAX_EVENT_NUM][MAX_PATH_LEN];
>>> +} src, dst;
>>> +
>>> +static inline int set_pathname(int fd)
>>
>> In non-bpf .c file, typically we do not add 'inline' attribute.
>> It is up to compiler to decide whether it should be inlined.
>>
>>> +{
>>> +     char buf[MAX_PATH_LEN];
>>> +
>>> +     snprintf(buf, MAX_PATH_LEN, "/proc/%d/fd/%d", src.pid, fd);
>>> +     src.fds[src.cnt] = fd;
>>> +     return readlink(buf, src.paths[src.cnt++], MAX_PATH_LEN);
>>> +}
>>> +
>> [...]
>>> diff --git a/tools/testing/selftests/bpf/progs/test_get_file_path.c b/tools/testing/selftests/bpf/progs/test_get_file_path.c
>>> new file mode 100644
>>> index 000000000000..eae663c1262a
>>> --- /dev/null
>>> +++ b/tools/testing/selftests/bpf/progs/test_get_file_path.c
>>> @@ -0,0 +1,43 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +
>>> +#include <linux/bpf.h>
>>> +#include <linux/ptrace.h>
>>> +#include <string.h>
>>> +#include <unistd.h>
>>> +#include "bpf_helpers.h"
>>> +#include "bpf_tracing.h"
>>> +
>>> +#define MAX_PATH_LEN         128
>>> +#define MAX_EVENT_NUM                16
>>> +
>>> +static struct file_path_test_data {
>>> +     pid_t pid;
>>> +     __u32 cnt;
>>> +     __u32 fds[MAX_EVENT_NUM];
>>> +     char paths[MAX_EVENT_NUM][MAX_PATH_LEN];
>>> +} data;
>>> +
>>> +struct sys_enter_newfstat_args {
>>> +     unsigned long long pad1;
>>> +     unsigned long long pad2;
>>> +     unsigned int fd;
>>> +};
>>
>> The BTF generated vmlinux.h has the following structure,
>> struct trace_entry {
>>           short unsigned int type;
>>           unsigned char flags;
>>           unsigned char preempt_count;
>>           int pid;
>> };
>> struct trace_event_raw_sys_enter {
>>           struct trace_entry ent;
>>           long int id;
>>           long unsigned int args[6];
>>           char __data[0];
>> };
>>
>> The third parameter type should be long, otherwise,
>> it may have issue on big endian machines?

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