Re: [PATCH bpf-next v3 08/21] bpf: implement common macros/helpers for target iterators

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	Martin KaFai Lau <kafai@fb.com>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next v3 08/21] bpf: implement common macros/helpers for target iterators
Date: Fri, 8 May 2020 12:07:48 -0700	[thread overview]
Message-ID: <CAEf4BzZ1vD_F74gy5mx_s8+cbw4OuZwJxpW36CijE-RWxOf__g@mail.gmail.com> (raw)
In-Reply-To: <20200507053924.1543103-1-yhs@fb.com>

On Wed, May 6, 2020 at 10:40 PM Yonghong Song <yhs@fb.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 or after
> bpf_seq_read() show()/next()/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@fb.com>
> ---
>  include/linux/bpf.h   | 11 ++++++
>  kernel/bpf/bpf_iter.c | 86 ++++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 92 insertions(+), 5 deletions(-)
>

Looks good. I was worried about re-using seq_num when element is
skipped, but this could already happen that same seq_num is associated
with different objects: overflow + retry returns different object
(because iteration is not a snapshot, so the element could be gone on
retry). Both cases will have to be handled in about the same fashion,
so it's fine.

Hm... Could this be a problem for start() implementation? E.g., if
object is still there, but iterator wants to skip it permanently.
Re-using seq_num will mean that start() will keep trying to fetch same
to-be-skipped element? Not sure, please think about it, but we can fix
it up later, if necessary.

Acked-by: Andrii Nakryiko <andriin@fb.com>

[...]

> @@ -112,11 +143,16 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
>                         err = PTR_ERR(p);
>                         break;
>                 }
> +
> +               /* get a valid next object, increase seq_num */

typo: get -> got

> +               bpf_iter_inc_seq_num(seq);
> +
>                 if (seq->count >= size)
>                         break;
>
>                 err = seq->op->show(seq, p);
>                 if (err > 0) {
> +                       bpf_iter_dec_seq_num(seq);
>                         seq->count = offs;
>                 } else if (err < 0 || seq_has_overflowed(seq)) {
>                         seq->count = offs;

[...]