Subject: Re: [PATCH bpf-next v3 01/16] bpf, netns: Handle multiple link attachments

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 10 Jul 2020 21:23:21 +0200

On Fri, Jul 10, 2020 at 12:02 AM CEST, Andrii Nakryiko wrote:
> On Thu, Jul 9, 2020 at 5:49 AM Jakub Sitnicki <[email-masked]> wrote:
>>
>> On Thu, Jul 09, 2020 at 05:44 AM CEST, Andrii Nakryiko wrote:
>> > On Thu, Jul 2, 2020 at 2:24 AM Jakub Sitnicki <[email-masked]> wrote:
>> >>
>> >> Extend the BPF netns link callbacks to rebuild (grow/shrink) or update the
>> >> prog_array at given position when link gets attached/updated/released.
>> >>
>> >> This let's us lift the limit of having just one link attached for the new
>> >> attach type introduced by subsequent patch.
>> >>
>> >> No functional changes intended.
>> >>
>> >> Signed-off-by: Jakub Sitnicki <[email-masked]>
>> >> ---
>> >>
>> >> Notes:
>> >>     v3:
>> >>     - New in v3 to support multi-prog attachments. (Alexei)
>> >>
>> >>  include/linux/bpf.h        |  4 ++
>> >>  kernel/bpf/core.c          | 22 ++++++++++
>> >>  kernel/bpf/net_namespace.c | 88 +++++++++++++++++++++++++++++++++++---
>> >>  3 files changed, 107 insertions(+), 7 deletions(-)
>> >>
>> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> >> index 3d2ade703a35..26bc70533db0 100644
>> >> --- a/include/linux/bpf.h
>> >> +++ b/include/linux/bpf.h
>> >> @@ -928,6 +928,10 @@ int bpf_prog_array_copy_to_user(struct bpf_prog_array *progs,
>> >>
>> >>  void bpf_prog_array_delete_safe(struct bpf_prog_array *progs,
>> >>                                 struct bpf_prog *old_prog);
>> >> +void bpf_prog_array_delete_safe_at(struct bpf_prog_array *array,
>> >> +                                  unsigned int index);
>> >> +void bpf_prog_array_update_at(struct bpf_prog_array *array, unsigned int index,
>> >> +                             struct bpf_prog *prog);
>> >>  int bpf_prog_array_copy_info(struct bpf_prog_array *array,
>> >>                              u32 *prog_ids, u32 request_cnt,
>> >>                              u32 *prog_cnt);
>> >> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> >> index 9df4cc9a2907..d4b3b9ee6bf1 100644
>> >> --- a/kernel/bpf/core.c
>> >> +++ b/kernel/bpf/core.c
>> >> @@ -1958,6 +1958,28 @@ void bpf_prog_array_delete_safe(struct bpf_prog_array *array,
>> >>                 }
>> >>  }
>> >>
>> >> +void bpf_prog_array_delete_safe_at(struct bpf_prog_array *array,
>> >> +                                  unsigned int index)
>> >> +{
>> >> +       bpf_prog_array_update_at(array, index, &dummy_bpf_prog.prog);
>> >> +}
>> >> +
>> >> +void bpf_prog_array_update_at(struct bpf_prog_array *array, unsigned int index,
>> >> +                             struct bpf_prog *prog)
>> >
>> > it's a good idea to mention it in a comment for both delete_safe_at
>> > and update_at that slots with dummy entries are ignored.
>>
>> I agree. These two need doc comments. update_at doesn't event hint that
>> this is not a regular update operation. Will add in v4.
>>
>> >
>> > Also, given that index can be out of bounds, should these functions
>> > actually return error if the slot is not found?
>>
>> That won't hurt. I mean, from bpf-netns PoV getting such an error would
>> indicate that there is a bug in the code that manages prog_array. But
>> perhaps other future users of this new prog_array API can benefit.
>>
>> >
>> >> +{
>> >> +       struct bpf_prog_array_item *item;
>> >> +
>> >> +       for (item = array->items; item->prog; item++) {
>> >> +               if (item->prog == &dummy_bpf_prog.prog)
>> >> +                       continue;
>> >> +               if (!index) {
>> >> +                       WRITE_ONCE(item->prog, prog);
>> >> +                       break;
>> >> +               }
>> >> +               index--;
>> >> +       }
>> >> +}
>> >> +
>> >>  int bpf_prog_array_copy(struct bpf_prog_array *old_array,
>> >>                         struct bpf_prog *exclude_prog,
>> >>                         struct bpf_prog *include_prog,
>> >> diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
>> >> index 247543380fa6..6011122c35b6 100644
>> >> --- a/kernel/bpf/net_namespace.c
>> >> +++ b/kernel/bpf/net_namespace.c
>> >> @@ -36,11 +36,51 @@ static void netns_bpf_run_array_detach(struct net *net,
>> >>         bpf_prog_array_free(run_array);
>> >>  }
>> >>
>> >> +static unsigned int link_index(struct net *net,
>> >> +                              enum netns_bpf_attach_type type,
>> >> +                              struct bpf_netns_link *link)
>> >> +{
>> >> +       struct bpf_netns_link *pos;
>> >> +       unsigned int i = 0;
>> >> +
>> >> +       list_for_each_entry(pos, &net->bpf.links[type], node) {
>> >> +               if (pos == link)
>> >> +                       return i;
>> >> +               i++;
>> >> +       }
>> >> +       return UINT_MAX;
>> >
>> > Why not return a negative error, if the slot is not found? Feels a bit
>> > unusual as far as error reporting goes.
>>
>> Returning uint played well with the consumer of link_index() return
>> value, that is bpf_prog_array_update_at(). update at takes an index into
>> the array, which must not be negative.
>
> Yeah, it did, but it's also quite implicit. I think just doing
> BUG_ON() for update_at or delete_at would be good enough there.

BUG_ON got deprecated [0], but I will WARN.

[0] https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on

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