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 13:25:25 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...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 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
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?

>         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;

[...]