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Date: Wed, 16 Sep 2020 23:21:11 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
Andrii Nakryiko <andriin@...com>,
John Fastabend <john.fastabend@...il.com>,
Jiri Olsa <jolsa@...hat.com>,
Eelco Chaudron <echaudro@...hat.com>,
KP Singh <kpsingh@...omium.org>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 6/8] libbpf: add support for freplace
attachment in bpf_link_create
Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
> On Wed, Sep 16, 2020 at 1:37 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
>>
>> On Tue, Sep 15, 2020 at 5:50 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>> >
>> > From: Toke Høiland-Jørgensen <toke@...hat.com>
>> >
>> > This adds support for supplying a target btf ID for the bpf_link_create()
>> > operation, and adds a new bpf_program__attach_freplace() high-level API for
>> > attaching freplace functions with a target.
>> >
>> > Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> > ---
>> > tools/lib/bpf/bpf.c | 1 +
>> > tools/lib/bpf/bpf.h | 3 ++-
>> > tools/lib/bpf/libbpf.c | 24 ++++++++++++++++++------
>> > tools/lib/bpf/libbpf.h | 3 +++
>> > tools/lib/bpf/libbpf.map | 1 +
>> > 5 files changed, 25 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
>> > index 82b983ff6569..e928456c0dd6 100644
>> > --- a/tools/lib/bpf/bpf.c
>> > +++ b/tools/lib/bpf/bpf.c
>> > @@ -599,6 +599,7 @@ int bpf_link_create(int prog_fd, int target_fd,
>> > attr.link_create.iter_info =
>> > ptr_to_u64(OPTS_GET(opts, iter_info, (void *)0));
>> > attr.link_create.iter_info_len = OPTS_GET(opts, iter_info_len, 0);
>> > + attr.link_create.target_btf_id = OPTS_GET(opts, target_btf_id, 0);
>> >
>> > return sys_bpf(BPF_LINK_CREATE, &attr, sizeof(attr));
>> > }
>> > diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
>> > index 015d13f25fcc..f8dbf666b62b 100644
>> > --- a/tools/lib/bpf/bpf.h
>> > +++ b/tools/lib/bpf/bpf.h
>> > @@ -174,8 +174,9 @@ struct bpf_link_create_opts {
>> > __u32 flags;
>> > union bpf_iter_link_info *iter_info;
>> > __u32 iter_info_len;
>> > + __u32 target_btf_id;
>> > };
>> > -#define bpf_link_create_opts__last_field iter_info_len
>> > +#define bpf_link_create_opts__last_field target_btf_id
>> >
>> > LIBBPF_API int bpf_link_create(int prog_fd, int target_fd,
>> > enum bpf_attach_type attach_type,
>> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> > index 550950eb1860..165131c73f40 100644
>> > --- a/tools/lib/bpf/libbpf.c
>> > +++ b/tools/lib/bpf/libbpf.c
>> > @@ -9322,12 +9322,14 @@ static struct bpf_link *attach_iter(const struct bpf_sec_def *sec,
>> >
>> > static struct bpf_link *
>> > bpf_program__attach_fd(struct bpf_program *prog, int target_fd,
>> > - const char *target_name)
>> > + int target_btf_id, const char *target_name)
>> > {
>> > enum bpf_attach_type attach_type;
>> > char errmsg[STRERR_BUFSIZE];
>> > struct bpf_link *link;
>> > int prog_fd, link_fd;
>> > + DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts,
>> > + .target_btf_id = target_btf_id);
>> >
>> > prog_fd = bpf_program__fd(prog);
>> > if (prog_fd < 0) {
>> > @@ -9340,8 +9342,12 @@ bpf_program__attach_fd(struct bpf_program *prog, int target_fd,
>> > return ERR_PTR(-ENOMEM);
>> > link->detach = &bpf_link__detach_fd;
>> >
>> > - attach_type = bpf_program__get_expected_attach_type(prog);
>> > - link_fd = bpf_link_create(prog_fd, target_fd, attach_type, NULL);
>> > + if (bpf_program__get_type(prog) == BPF_PROG_TYPE_EXT)
>> > + attach_type = BPF_TRACE_FREPLACE;
>>
>> doing this unconditionally will break an old-style freplace without
>> target_fd/btf_id on older kernels. Safe and simple way would be to
>> continue using raw_tracepoint_open when there is no target_fd/btf_id,
>> and use LINK_CREATE for newer stuff. Alternatively, you'd need to do
>> feature detection, but it's still would be nice to handle old-style
>> attach through raw_tracepoint_open for bpf_program__attach_freplace.
>>
>> so I suggest leaving bpf_program__attach_fd() as is and to create a
>> custom bpf_program__attach_freplace() implementation.
Sure, I'll take another pass at this. Not sure how useful feature
detection in libbpf is; if the caller passes a target, libbpf can't
really do much if the kernel doesn't support it...
>> > + else
>> > + attach_type = bpf_program__get_expected_attach_type(prog);
>> > +
>> > + link_fd = bpf_link_create(prog_fd, target_fd, attach_type, &opts);
>> > if (link_fd < 0) {
>> > link_fd = -errno;
>> > free(link);
>> > @@ -9357,19 +9363,25 @@ bpf_program__attach_fd(struct bpf_program *prog, int target_fd,
>> > struct bpf_link *
>> > bpf_program__attach_cgroup(struct bpf_program *prog, int cgroup_fd)
>> > {
>> > - return bpf_program__attach_fd(prog, cgroup_fd, "cgroup");
>> > + return bpf_program__attach_fd(prog, cgroup_fd, 0, "cgroup");
>> > }
>> >
>> > struct bpf_link *
>> > bpf_program__attach_netns(struct bpf_program *prog, int netns_fd)
>> > {
>> > - return bpf_program__attach_fd(prog, netns_fd, "netns");
>> > + return bpf_program__attach_fd(prog, netns_fd, 0, "netns");
>> > }
>> >
>> > struct bpf_link *bpf_program__attach_xdp(struct bpf_program *prog, int ifindex)
>> > {
>> > /* target_fd/target_ifindex use the same field in LINK_CREATE */
>> > - return bpf_program__attach_fd(prog, ifindex, "xdp");
>> > + return bpf_program__attach_fd(prog, ifindex, 0, "xdp");
>> > +}
>> > +
>> > +struct bpf_link *bpf_program__attach_freplace(struct bpf_program *prog,
>> > + int target_fd, int target_btf_id)
>> > +{
>> > + return bpf_program__attach_fd(prog, target_fd, target_btf_id, "freplace");
>> > }
>> >
>> > struct bpf_link *
>> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> > index a750f67a23f6..ce5add9b9203 100644
>> > --- a/tools/lib/bpf/libbpf.h
>> > +++ b/tools/lib/bpf/libbpf.h
>> > @@ -261,6 +261,9 @@ LIBBPF_API struct bpf_link *
>> > bpf_program__attach_netns(struct bpf_program *prog, int netns_fd);
>> > LIBBPF_API struct bpf_link *
>> > bpf_program__attach_xdp(struct bpf_program *prog, int ifindex);
>> > +LIBBPF_API struct bpf_link *
>> > +bpf_program__attach_freplace(struct bpf_program *prog,
>> > + int target_fd, int target_btf_id);
>>
>> maybe a const char * function name instead of target_btf_id would be a
>> nicer API? Users won't have to deal with fetching target prog's BTF,
>> searching it, etc. That's all pretty straightforward for libbpf to do,
>> leaving users with more natural and simpler API.
>>
>
> bpf_program__set_attach_target() uses string name for target
> functions, we should definitely be consistent here
All right, fair enough :)
-Toke
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