Re: [PATCH bpf-next v4 17/27] bpf: Add multi trampoline attach support

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Aug 26, 2021 at 12:41 PM Jiri Olsa <jolsa@redhat.com> wrote:
>
> Adding new multi trampoline link (BPF_LINK_TYPE_TRACING_MULTI)
> as an interface to attach program to multiple functions.
>
> The link_create bpf_attr interface already has 'bpf_prog' file
> descriptor, that defines the program to be attached. It must be
> loaded with BPF_F_MULTI_FUNC flag.
>
> Adding new multi_btf_ids/multi_btf_ids_cnt link_create bpf_attr
> fields that provides BTF ids.
>
> The new link gets multi trampoline (via bpf_trampoline_multi_get)
> and links the provided program with embedded trampolines and the
> 'main' trampoline with new multi link/unlink functions:
>
>   int bpf_trampoline_multi_link_prog(struct bpf_prog *prog,
>                                      struct bpf_trampoline_multi *tr);
>   int bpf_trampoline_multi_unlink_prog(struct bpf_prog *prog,
>                                        struct bpf_trampoline_multi *tr);
>
> If embedded trampoline contains fexit programs, we need to switch
> its model to the multi trampoline model (because of the final 'ret'
> argument). We keep the count of attached multi func programs for each
> trampoline, so we can tell when to switch the model.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  include/linux/bpf.h            |   5 ++
>  include/uapi/linux/bpf.h       |   5 ++
>  kernel/bpf/core.c              |   1 +
>  kernel/bpf/syscall.c           | 120 +++++++++++++++++++++++++++++++++
>  kernel/bpf/trampoline.c        |  87 ++++++++++++++++++++++--
>  tools/include/uapi/linux/bpf.h |   5 ++
>  6 files changed, 219 insertions(+), 4 deletions(-)
>

[...]

> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 1f9d336861f0..9533200ffadf 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1008,6 +1008,7 @@ enum bpf_link_type {
>         BPF_LINK_TYPE_NETNS = 5,
>         BPF_LINK_TYPE_XDP = 6,
>         BPF_LINK_TYPE_PERF_EVENT = 7,
> +       BPF_LINK_TYPE_TRACING_MULTI = 8,
>
>         MAX_BPF_LINK_TYPE,
>  };
> @@ -1462,6 +1463,10 @@ union bpf_attr {
>                                  */
>                                 __u64           bpf_cookie;
>                         } perf_event;
> +                       struct {
> +                               __aligned_u64   multi_btf_ids;          /* addresses to attach */
> +                               __u32           multi_btf_ids_cnt;      /* addresses count */
> +                       };

Please follow the pattern of perf_event, name this struct "multi".

>                 };
>         } link_create;
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index bad03dde97a2..6c16ac43dd91 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c

[...]

> +
> +       bpf_link_init(&link->link, BPF_LINK_TYPE_TRACING_MULTI,
> +                     &bpf_tracing_multi_link_lops, prog);
> +       link->attach_type = prog->expected_attach_type;
> +       link->multi = multi;
> +
> +       err = bpf_link_prime(&link->link, &link_primer);
> +       if (err)
> +               goto out_free;
> +       err = bpf_trampoline_multi_link_prog(prog, multi);
> +       if (err)
> +               goto out_free;

bpf_link_cleanup(), can't free link after priming. Look at other
places using bpf_link.


> +       return bpf_link_settle(&link_primer);
> +
> +out_free:
> +       bpf_trampoline_multi_put(multi);
> +       kfree(link);
> +out_free_ids:
> +       kfree(btf_ids);
> +       return err;
> +}
> +

[...]