linux-kernel - Re: [PATCH] sample: bpf: syscall_tp_user: print result of verify

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Message-ID: <306ab457-9f3d-4d90-bb31-e6fb08b6a5ad@189.cn>
Date:   Fri, 1 Apr 2022 11:37:07 +0800
From:   Song Chen <chensong_2000@....cn>
To:     Yonghong Song <yhs@...com>, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kafai@...com, songliubraving@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sample: bpf: syscall_tp_user: print result of verify_map



在 2022/4/1 11:01, Yonghong Song 写道:
> 
> 
> On 3/31/22 6:41 PM, Song Chen wrote:
>> syscall_tp only prints the map id and messages when something goes wrong,
>> but it doesn't print the value passed from bpf map. I think it's better
>> to show that value to users.
>>
>> What's more, i also added a 2-second sleep before calling verify_map,
>> to make the value more obvious.
>>
>> Signed-off-by: Song Chen <chensong_2000@....cn>
>> ---
>>   samples/bpf/syscall_tp_user.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/samples/bpf/syscall_tp_user.c 
>> b/samples/bpf/syscall_tp_user.c
>> index a0ebf1833ed3..1faa7f08054e 100644
>> --- a/samples/bpf/syscall_tp_user.c
>> +++ b/samples/bpf/syscall_tp_user.c
>> @@ -36,6 +36,9 @@ static void verify_map(int map_id)
>>           fprintf(stderr, "failed: map #%d returns value 0\n", map_id);
>>           return;
>>       }
>> +
>> +    printf("verify map:%d val: %d\n", map_id, val);
> 
> I am not sure how useful it is or anybody really cares.
> This is just a sample to demonstrate how bpf tracepoint works.
> The error path has error print out already.
> 
>> +
>>       val = 0;
>>       if (bpf_map_update_elem(map_id, &key, &val, BPF_ANY) != 0) {
>>           fprintf(stderr, "map_update failed: %s\n", strerror(errno));
>> @@ -98,6 +101,7 @@ static int test(char *filename, int num_progs)
>>       }
>>       close(fd);
>> +    sleep(2);
> 
> The commit message mentioned this sleep(2) is
> to make the value more obvious. I don't know what does this mean.
> sleep(2) can be added only if it fixed a bug.

The value in bpf map means how many times trace_enter_open_at are 
triggered with tracepoint,sys_enter_openat. Sleep(2) is to enlarge the 
result, tell the user how many files are opened in the last 2 seconds.

It shows like this:

sudo ./samples/bpf/syscall_tp
prog #0: map ids 4 5
verify map:4 val: 253
verify map:5 val: 252

If we work harder, we can also print those files' name and opened by 
which process.

It's just an improvement instead of a bug fix, i will drop it if 
reviewers think it's unnecessary.

Thanks.

BR

chensong
> 
>>       /* verify the map */
>>       for (i = 0; i < num_progs; i++) {
>>           verify_map(map0_fds[i]);
>