netdev - Re: [PATCH v2 bpf-next 1/3] bpf: Introduce bpf

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Date:   Mon, 14 Jun 2021 20:33:17 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: Introduce bpf_timer

On Fri, Jun 11, 2021 at 3:12 PM Yonghong Song <yhs@...com> wrote:
> > +struct bpf_hrtimer {
> > +     struct hrtimer timer;
> > +     struct bpf_map *map;
> > +     struct bpf_prog *prog;
> > +     void *callback_fn;
> > +     void *value;
> > +};
> > +
> > +/* the actual struct hidden inside uapi struct bpf_timer */
> > +struct bpf_timer_kern {
> > +     struct bpf_hrtimer *timer;
> > +     struct bpf_spin_lock lock;
> > +};
>
> Looks like in 32bit system, sizeof(struct bpf_timer_kern) is 64
> and sizeof(struct bpf_timer) is 128.
>
> struct bpf_spin_lock {
>          __u32   val;
> };
>
> struct bpf_timer {
>         __u64 :64;
>         __u64 :64;
> };
>
> Checking the code, we may not have issues as structure
> "bpf_timer" is only used to reserve spaces and
> map copy value routine handles that properly.
>
> Maybe we can still make it consistent with
> two fields in bpf_timer_kern mapping to
> two fields in bpf_timer?
>
> struct bpf_timer_kern {
>         __bpf_md_ptr(struct bpf_hrtimer *, timer);
>         struct bpf_spin_lock lock;
> };

Such alignment of fields is not necessary,
since the fields are not accessible directly from bpf prog.
struct bpf_timer_kern needs to fit into struct bpf_timer and
alignof these two structs needs to be the same.
That's all. I'll add build_bug_on to make sure.