On Mon, Mar 23, 2020 at 4:02 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Andrii Nakryiko <andriin@xxxxxx> writes:
>
> > Add bpf_program__attach_cgroup(), which uses BPF_LINK_CREATE subcommand to
> > create an FD-based kernel bpf_link. Also add low-level bpf_link_create() API.
> >
> > If expected_attach_type is not specified explicitly with
> > bpf_program__set_expected_attach_type(), libbpf will try to determine proper
> > attach type from BPF program's section definition.
> >
> > Also add support for bpf_link's underlying BPF program replacement:
> > - unconditional through high-level bpf_link__update_program() API;
> > - cmpxchg-like with specifying expected current BPF program through
> > low-level bpf_link_update() API.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
> > ---
> > tools/include/uapi/linux/bpf.h | 12 +++++++++
> > tools/lib/bpf/bpf.c | 34 +++++++++++++++++++++++++
> > tools/lib/bpf/bpf.h | 19 ++++++++++++++
> > tools/lib/bpf/libbpf.c | 46 ++++++++++++++++++++++++++++++++++
> > tools/lib/bpf/libbpf.h | 8 +++++-
> > tools/lib/bpf/libbpf.map | 4 +++
> > 6 files changed, 122 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index fad9f79bb8f1..fa944093f9fc 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -112,6 +112,7 @@ enum bpf_cmd {
> > BPF_MAP_UPDATE_BATCH,
> > BPF_MAP_DELETE_BATCH,
> > BPF_LINK_CREATE,
> > + BPF_LINK_UPDATE,
> > };
> >
> > enum bpf_map_type {
> > @@ -574,6 +575,17 @@ union bpf_attr {
> > __u32 target_fd; /* object to attach to */
> > __u32 attach_type; /* attach type */
> > } link_create;
> > +
> > + struct { /* struct used by BPF_LINK_UPDATE command */
> > + __u32 link_fd; /* link fd */
> > + /* new program fd to update link with */
> > + __u32 new_prog_fd;
> > + __u32 flags; /* extra flags */
> > + /* expected link's program fd; is specified only if
> > + * BPF_F_REPLACE flag is set in flags */
> > + __u32 old_prog_fd;
> > + } link_update;
> > +
> > } __attribute__((aligned(8)));
> >
> > /* The description below is an attempt at providing documentation to eBPF
> > diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> > index c6dafe563176..35c34fc81bd0 100644
> > --- a/tools/lib/bpf/bpf.c
> > +++ b/tools/lib/bpf/bpf.c
> > @@ -584,6 +584,40 @@ int bpf_prog_detach2(int prog_fd, int target_fd, enum bpf_attach_type type)
> > return sys_bpf(BPF_PROG_DETACH, &attr, sizeof(attr));
> > }
> >
> > +int bpf_link_create(int prog_fd, int target_fd,
> > + enum bpf_attach_type attach_type,
> > + const struct bpf_link_create_opts *opts)
> > +{
> > + union bpf_attr attr;
> > +
> > + if (!OPTS_VALID(opts, bpf_link_create_opts))
> > + return -EINVAL;
> > +
> > + memset(&attr, 0, sizeof(attr));
> > + attr.link_create.prog_fd = prog_fd;
> > + attr.link_create.target_fd = target_fd;
> > + attr.link_create.attach_type = attach_type;
> > +
> > + return sys_bpf(BPF_LINK_CREATE, &attr, sizeof(attr));
> > +}
> > +
> > +int bpf_link_update(int link_fd, int new_prog_fd,
> > + const struct bpf_link_update_opts *opts)
> > +{
> > + union bpf_attr attr;
> > +
> > + if (!OPTS_VALID(opts, bpf_link_update_opts))
> > + return -EINVAL;
> > +
> > + memset(&attr, 0, sizeof(attr));
> > + attr.link_update.link_fd = link_fd;
> > + attr.link_update.new_prog_fd = new_prog_fd;
> > + attr.link_update.flags = OPTS_GET(opts, flags, 0);
> > + attr.link_update.old_prog_fd = OPTS_GET(opts, old_prog_fd, 0);
> > +
> > + return sys_bpf(BPF_LINK_UPDATE, &attr, sizeof(attr));
> > +}
> > +
> > int bpf_prog_query(int target_fd, enum bpf_attach_type type, __u32 query_flags,
> > __u32 *attach_flags, __u32 *prog_ids, __u32 *prog_cnt)
> > {
> > diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> > index b976e77316cc..46d47afdd887 100644
> > --- a/tools/lib/bpf/bpf.h
> > +++ b/tools/lib/bpf/bpf.h
> > @@ -168,6 +168,25 @@ 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);
> >
> > +struct bpf_link_create_opts {
> > + size_t sz; /* size of this struct for forward/backward compatibility */
> > +};
> > +#define bpf_link_create_opts__last_field sz
> > +
> > +LIBBPF_API int bpf_link_create(int prog_fd, int target_fd,
> > + enum bpf_attach_type attach_type,
> > + const struct bpf_link_create_opts *opts);
> > +
> > +struct bpf_link_update_opts {
> > + size_t sz; /* size of this struct for forward/backward compatibility */
> > + __u32 flags; /* extra flags */
> > + __u32 old_prog_fd; /* expected old program FD */
> > +};
> > +#define bpf_link_update_opts__last_field old_prog_fd
> > +
> > +LIBBPF_API int bpf_link_update(int link_fd, int new_prog_fd,
> > + const struct bpf_link_update_opts *opts);
> > +
> > struct bpf_prog_test_run_attr {
> > int prog_fd;
> > int repeat;
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 085e41f9b68e..8b23c70033d3 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -6951,6 +6951,12 @@ struct bpf_link {
> > bool disconnected;
> > };
> >
> > +/* Replace link's underlying BPF program with the new one */
> > +int bpf_link__update_program(struct bpf_link *link, struct bpf_program *prog)
> > +{
> > + return bpf_link_update(bpf_link__fd(link), bpf_program__fd(prog), NULL);
> > +}
>
> I would expect bpf_link to keep track of the previous program and
> automatically fill it in with this operation. I.e., it should be
> possible to do something like:
>
> link = bpf_link__open("/sys/fs/bpf/my_link");
> prog = bpf_link__get_prog(link);
I don't think libbpf is able to construct struct bpf_program from link
info. It can get program FD, of course, but struct bpf_program is much
more than that and not sure kernel has all the necessary info. Some
parts of bpf_program is coming from ELF file, which is gone by this
time.
> new_prog = enhance_prog(prog);
> err = bpf_link__update_program(link, new_prog);
>
> and have atomic replacement "just work". This obviously implies that
> bpf_link__open() should use that BPF_LINK_QUERY operation I was
> requesting in my comment to the previous patch :)
This will depend on which way we go with mandatory/default expected
program FD vs optional.
>
> -Toke
>
