Re: [PATCH bpf-next] libbpf: Add ability to clear per-program load flags

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

On Thu, Nov 18, 2021 at 2:33 AM Florent Revest <revest@chromium.org> wrote:
>
> Recently, bpf_program__flags and bpf_program__set_extra_flags were
> introduced to the libbpf API but they only allow adding load flags.
>
> We have a use-case where we construct a skeleton with a sleepable
> program and if it fails to load then we want to make it non-sleepable by
> clearing BPF_F_SLEEPABLE.

I'd say the better way to do this is to have two programs (that share
the logic, of course) and pick one or another at runtime:

static int whatever_logic(bool sleepable) { ... }

SEC("fentry.s/whatever")
int BPF_PROG(whatever_sleepable, ...)
{
    return whatever_logic(true);
}

SEC("fentry/whatever")
int BPF_PROG(whatever_nonsleepable, ...)
{
    return whatever_logic(false);
}


Then at runtime you can bpf_program__autoload(..., false) for a
variant you don't want to load.

This clear_flags business seems too low-level and too limited. Next
thing we'll be adding a few more bit manipulation variants (e.g, reset
flags). Let's see how far you can get with the use of existing
features. I'd set_extra_flags() to be almost never used, btw. And they
shouldn't, if can be avoided. So I'm hesitant to keep extending
operations around prog_flags.

But given we just added set_extra_flags() and it's already too
limiting, let's change set_extra flags to just set_flags() that will
override the flags with whatever user provides. Then with
bpf_program__flags() and bpf_program__set_flags() you can express
whatever you want without adding extra APIs. Care to fix that?

>
> Signed-off-by: Florent Revest <revest@chromium.org>
> ---
>  tools/lib/bpf/libbpf.c   | 9 +++++++++
>  tools/lib/bpf/libbpf.h   | 1 +
>  tools/lib/bpf/libbpf.map | 1 +
>  3 files changed, 11 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index de7e09a6b5ec..dcb7fced5fd2 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8305,6 +8305,15 @@ int bpf_program__set_extra_flags(struct bpf_program *prog, __u32 extra_flags)
>         return 0;
>  }
>
> +int bpf_program__clear_flags(struct bpf_program *prog, __u32 flags)
> +{
> +       if (prog->obj->loaded)
> +               return libbpf_err(-EBUSY);
> +
> +       prog->prog_flags &= ~flags;
> +       return 0;
> +}
> +
>  #define SEC_DEF(sec_pfx, ptype, atype, flags, ...) {                       \
>         .sec = sec_pfx,                                                     \
>         .prog_type = BPF_PROG_TYPE_##ptype,                                 \
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 4ec69f224342..08f108e49841 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -495,6 +495,7 @@ bpf_program__set_expected_attach_type(struct bpf_program *prog,
>
>  LIBBPF_API __u32 bpf_program__flags(const struct bpf_program *prog);
>  LIBBPF_API int bpf_program__set_extra_flags(struct bpf_program *prog, __u32 extra_flags);
> +LIBBPF_API int bpf_program__clear_flags(struct bpf_program *prog, __u32 flags);
>
>  LIBBPF_API int
>  bpf_program__set_attach_target(struct bpf_program *prog, int attach_prog_fd,
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 6a59514a48cf..eeff700240dc 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -401,6 +401,7 @@ LIBBPF_0.6.0 {
>                 bpf_program__insn_cnt;
>                 bpf_program__insns;
>                 bpf_program__set_extra_flags;
> +               bpf_program__clear_flags;
>                 btf__add_btf;
>                 btf__add_decl_tag;
>                 btf__add_type_tag;
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>