Re: [PATCH v3 bpf-next 4/6] libbpf: Introduce bpf_prog_attach_xattr

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

On Wed, Dec 18, 2019 at 6:56 PM Andrey Ignatov <rdna@fb.com> wrote:
>
> Introduce a new bpf_prog_attach_xattr function that, in addition to
> program fd, target fd and attach type, accepts an extendable struct
> bpf_prog_attach_opts.
>
> bpf_prog_attach_opts relies on DECLARE_LIBBPF_OPTS macro to maintain
> backward and forward compatibility and has the following "optional"
> attach attributes:
>
> * existing attach_flags, since it's not required when attaching in NONE
>   mode. Even though it's quite often used in MULTI and OVERRIDE mode it
>   seems to be a good idea to reduce number of arguments to
>   bpf_prog_attach_xattr;
>
> * newly introduced attribute of BPF_PROG_ATTACH command: replace_prog_fd
>   that is fd of previously attached cgroup-bpf program to replace if
>   BPF_F_REPLACE flag is used.
>
> The new function is named to be consistent with other xattr-functions
> (bpf_prog_test_run_xattr, bpf_create_map_xattr, bpf_load_program_xattr).
>
> The struct bpf_prog_attach_opts is supposed to be used with
> DECLARE_LIBBPF_OPTS macro.
>
> Signed-off-by: Andrey Ignatov <rdna@fb.com>
> ---
>  tools/lib/bpf/bpf.c      | 16 +++++++++++++++-
>  tools/lib/bpf/bpf.h      | 11 +++++++++++
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index 98596e15390f..ebb4f8d71bdb 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -466,6 +466,17 @@ int bpf_obj_get(const char *pathname)
>
>  int bpf_prog_attach(int prog_fd, int target_fd, enum bpf_attach_type type,
>                     unsigned int flags)
> +{
> +       DECLARE_LIBBPF_OPTS(bpf_prog_attach_opts, opts,
> +               .flags = flags,
> +       );
> +
> +       return bpf_prog_attach_xattr(prog_fd, target_fd, type, &opts);
> +}
> +
> +int bpf_prog_attach_xattr(int prog_fd, int target_fd,
> +                         enum bpf_attach_type type,
> +                         const struct bpf_prog_attach_opts *opts)
>  {
>         union bpf_attr attr;
>

You need to validate opts with OPTS_VALID macro (see
btf_dump__emit_type_decl() for simple example).

> @@ -473,7 +484,10 @@ int bpf_prog_attach(int prog_fd, int target_fd, enum bpf_attach_type type,
>         attr.target_fd     = target_fd;
>         attr.attach_bpf_fd = prog_fd;
>         attr.attach_type   = type;
> -       attr.attach_flags  = flags;
> +       if (opts) {
> +               attr.attach_flags = opts->flags;
> +               attr.replace_bpf_fd = opts->replace_prog_fd;
> +       }

Please use OPTS_GET() macro to fetch values from opts struct and
provide default value if they are not specified.


>
>         return sys_bpf(BPF_PROG_ATTACH, &attr, sizeof(attr));
>  }

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