[bpf-next,4/5] libbpf: Introduce bpf_prog_attach_xattr

Introduce a new bpf_prog_attach_xattr function that accepts an
extendable structure and supports passing a new attribute to
BPF_PROG_ATTACH command: replace_prog_fd that is fd of previously
attached cgroup-bpf program to replace if recently introduced
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).

NOTE: DECLARE_LIBBPF_OPTS macro is not used here because it's available
in libbpf.h, and unavailable in bpf.h. Please let me know if the macro
should be shared in a common place and used here instead of declaring
struct bpf_prog_attach_attr directly.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
---
 tools/lib/bpf/bpf.c      | 22 ++++++++++++++++++----
 tools/lib/bpf/bpf.h      | 10 ++++++++++
 tools/lib/bpf/libbpf.map |  5 +++++
 3 files changed, 33 insertions(+), 4 deletions(-)

On Tue, Dec 10, 2019 at 6:35 PM Andrey Ignatov <rdna@fb.com> wrote:
>
> Introduce a new bpf_prog_attach_xattr function that accepts an
> extendable structure and supports passing a new attribute to
> BPF_PROG_ATTACH command: replace_prog_fd that is fd of previously
> attached cgroup-bpf program to replace if recently introduced
> 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).
>
> NOTE: DECLARE_LIBBPF_OPTS macro is not used here because it's available
> in libbpf.h, and unavailable in bpf.h. Please let me know if the macro
> should be shared in a common place and used here instead of declaring
> struct bpf_prog_attach_attr directly.
>

I think doing opts is a better way to go forward. With current
approach, next time we need another extra field, we'd need to add yet
another function and/or symbol version existing one. BTW, with
xxx_opts approach, we tried to keep mandatory arguments that are going
to be always specified as first few arguments of a function, and other
stuff that's optional (e.g., flags or replace_prog_fd seem to be good
candidates), would go under opts. This differs from xattr way, which
is why I'm pointing this out.

> Signed-off-by: Andrey Ignatov <rdna@fb.com>
> ---
>  tools/lib/bpf/bpf.c      | 22 ++++++++++++++++++----
>  tools/lib/bpf/bpf.h      | 10 ++++++++++
>  tools/lib/bpf/libbpf.map |  5 +++++
>  3 files changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index 98596e15390f..5a2830fac227 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -466,14 +466,28 @@ 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)
> +{
> +       struct bpf_prog_attach_attr attach_attr;
> +
> +       memset(&attach_attr, 0, sizeof(attach_attr));
> +       attach_attr.target_fd   = target_fd;
> +       attach_attr.prog_fd     = prog_fd;
> +       attach_attr.type        = type;
> +       attach_attr.flags       = flags;
> +
> +       return bpf_prog_attach_xattr(&attach_attr);
> +}
> +
> +int bpf_prog_attach_xattr(const struct bpf_prog_attach_attr *attach_attr)
>  {
>         union bpf_attr attr;
>
>         memset(&attr, 0, sizeof(attr));
> -       attr.target_fd     = target_fd;
> -       attr.attach_bpf_fd = prog_fd;
> -       attr.attach_type   = type;
> -       attr.attach_flags  = flags;
> +       attr.target_fd     = attach_attr->target_fd;
> +       attr.attach_bpf_fd = attach_attr->prog_fd;
> +       attr.attach_type   = attach_attr->type;
> +       attr.attach_flags  = attach_attr->flags;
> +       attr.replace_bpf_fd = attach_attr->replace_prog_fd;
>
>         return sys_bpf(BPF_PROG_ATTACH, &attr, sizeof(attr));
>  }
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index 3c791fa8e68e..4b7269d3bae7 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -128,8 +128,18 @@ LIBBPF_API int bpf_map_get_next_key(int fd, const void *key, void *next_key);
>  LIBBPF_API int bpf_map_freeze(int fd);
>  LIBBPF_API int bpf_obj_pin(int fd, const char *pathname);
>  LIBBPF_API int bpf_obj_get(const char *pathname);
> +
> +struct bpf_prog_attach_attr {
> +       int target_fd;
> +       int prog_fd;
> +       enum bpf_attach_type type;
> +       unsigned int flags;
> +       int replace_prog_fd;
> +};
> +
>  LIBBPF_API int bpf_prog_attach(int prog_fd, int attachable_fd,
>                                enum bpf_attach_type type, unsigned int flags);
> +LIBBPF_API int bpf_prog_attach_xattr(const struct bpf_prog_attach_attr *attr);
>  LIBBPF_API int bpf_prog_detach(int attachable_fd, enum bpf_attach_type type);
>  LIBBPF_API int bpf_prog_detach2(int prog_fd, int attachable_fd,
>                                 enum bpf_attach_type type);
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 8ddc2c40e482..42b065454031 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -208,3 +208,8 @@ LIBBPF_0.0.6 {
>                 btf__find_by_name_kind;
>                 libbpf_find_vmlinux_btf_id;
>  } LIBBPF_0.0.5;
> +
> +LIBBPF_0.0.7 {
> +       global:
> +               bpf_prog_attach_xattr;
> +} LIBBPF_0.0.6;
> --
> 2.17.1
>