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

Andrii Nakryiko
2020-05-12T03:16:38Z

On Fri, May 8, 2020 at 8:18 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 5/8/20 12:07 PM, Andrii Nakryiko wrote:
> > 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.
>
> This is what I thought as well.
>
> >
> > 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.
>
> The seq_num is for bpf_program context. It does not affect how start()
> behaves. The start() MAY retry the same element over and over again
> if show() overflows or returns <0, but in which case, user space
> should check the return error code to decide to retry or give up.

ah, right seq_num vs internal id, makes sense, never mind :)

>
> >
> > 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
>
> Ack.
>
> >
> >> + 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;
> >
> > [...]
> >