netdev - Re: [PATCH bpf-next v2 08/20] bpf: implement common macros/helpers for target iterators

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Date:   Tue, 5 May 2020 13:30:44 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next v2 08/20] bpf: implement common macros/helpers
 for target iterators



On 5/5/20 1:25 PM, Andrii Nakryiko wrote:
> On Sun, May 3, 2020 at 11:28 PM Yonghong Song <yhs@...com> wrote:
>>
>> Macro DEFINE_BPF_ITER_FUNC is implemented so target
>> can define an init function to capture the BTF type
>> which represents the target.
>>
>> The bpf_iter_meta is a structure holding meta data, common
>> to all targets in the bpf program.
>>
>> Additional marker functions are called before/after
>> bpf_seq_read() show() and stop() callback functions
>> to help calculate precise seq_num and whether call bpf_prog
>> inside stop().
>>
>> Two functions, bpf_iter_get_info() and bpf_iter_run_prog(),
>> are implemented so target can get needed information from
>> bpf_iter infrastructure and can run the program.
>>
>> Signed-off-by: Yonghong Song <yhs@...com>
>> ---
>>   include/linux/bpf.h   | 11 +++++
>>   kernel/bpf/bpf_iter.c | 94 ++++++++++++++++++++++++++++++++++++++++---
>>   2 files changed, 100 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 26daf85cba10..70c71c3cd9e8 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1129,6 +1129,9 @@ int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>>   int bpf_obj_get_user(const char __user *pathname, int flags);
>>
>>   #define BPF_ITER_FUNC_PREFIX "__bpf_iter__"
>> +#define DEFINE_BPF_ITER_FUNC(target, args...)                  \
>> +       extern int __bpf_iter__ ## target(args);                \
>> +       int __init __bpf_iter__ ## target(args) { return 0; }
> 
> Why is extern declaration needed here? Doesn't the same macro define

Silence sparse warning. Apparently in kernel, any global function, they 
want a declaration?

> global function itself? I'm probably missing some C semantics thingy,
> sorry...
> 
>>
>>   typedef int (*bpf_iter_init_seq_priv_t)(void *private_data);
>>   typedef void (*bpf_iter_fini_seq_priv_t)(void *private_data);
>> @@ -1141,11 +1144,19 @@ struct bpf_iter_reg {
>>          u32 seq_priv_size;
>>   };
>>
>> +struct bpf_iter_meta {
>> +       __bpf_md_ptr(struct seq_file *, seq);
>> +       u64 session_id;
>> +       u64 seq_num;
>> +};
>> +
> 
> [...]
> 
>>   /* bpf_seq_read, a customized and simpler version for bpf iterator.
>>    * no_llseek is assumed for this file.
>>    * The following are differences from seq_read():
>> @@ -83,12 +119,15 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
>>          if (!p || IS_ERR(p))
>>                  goto Stop;
>>
>> +       bpf_iter_inc_seq_num(seq);
> 
> so seq_num is one-based, not zero-based? So on first show() call it
> will be set to 1, not 0, right?

It is 1 based, we need to document this clearly. I forgot to adjust my 
bpf program for this. Will adjust them properly in the next revision.
> 
>>          err = seq->op->show(seq, p);
>>          if (seq_has_overflowed(seq)) {
>> +               bpf_iter_dec_seq_num(seq);
>>                  err = -E2BIG;
>>                  goto Error_show;
>>          } else if (err) {
>>                  /* < 0: go out, > 0: skip */
>> +               bpf_iter_dec_seq_num(seq);
>>                  if (likely(err < 0))
>>                          goto Error_show;
>>                  seq->count = 0;
> 
> [...]
>