bpf: Support bpf tracing/iter programs for BPF_LINK_CREATE

Given a bpf program, the step to create an anonymous bpf iterator is:
  - create a bpf_iter_link, which combines bpf program and the target.
    In the future, there could be more information recorded in the link.
    A link_fd will be returned to the user space.
  - create an anonymous bpf iterator with the given link_fd.

The bpf_iter_link can be pinned to bpffs mount file system to
create a file based bpf iterator as well.

The benefit to use of bpf_iter_link:
  - using bpf link simplifies design and implementation as bpf link
    is used for other tracing bpf programs.
  - for file based bpf iterator, bpf_iter_link provides a standard
    way to replace underlying bpf programs.
  - for both anonymous and free based iterators, bpf link query
    capability can be leveraged.

The patch added support of tracing/iter programs for BPF_LINK_CREATE.
A new link type BPF_LINK_TYPE_ITER is added to facilitate link
querying. Currently, only prog_id is needed, so there is no
additional in-kernel show_fdinfo() and fill_link_info() hook
is needed for BPF_LINK_TYPE_ITER link.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200509175901.2475084-1-yhs@fb.com

1 files changed, 62 insertions, 0 deletions

diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index dec182d8395a..03f5832909db 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -15,6 +15,11 @@ struct bpf_iter_target_info {
 	u32 btf_id;	/* cached value */
 };
 
+struct bpf_iter_link {
+	struct bpf_link link;
+	struct bpf_iter_target_info *tinfo;
+};
+
 static struct list_head targets = LIST_HEAD_INIT(targets);
 static DEFINE_MUTEX(targets_mutex);
 
@@ -93,3 +98,60 @@ bool bpf_iter_prog_supported(struct bpf_prog *prog)
 
 	return supported;
 }
+
+static void bpf_iter_link_release(struct bpf_link *link)
+{
+}
+
+static void bpf_iter_link_dealloc(struct bpf_link *link)
+{
+	struct bpf_iter_link *iter_link =
+		container_of(link, struct bpf_iter_link, link);
+
+	kfree(iter_link);
+}
+
+static const struct bpf_link_ops bpf_iter_link_lops = {
+	.release = bpf_iter_link_release,
+	.dealloc = bpf_iter_link_dealloc,
+};
+
+int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
+{
+	struct bpf_link_primer link_primer;
+	struct bpf_iter_target_info *tinfo;
+	struct bpf_iter_link *link;
+	bool existed = false;
+	u32 prog_btf_id;
+	int err;
+
+	if (attr->link_create.target_fd || attr->link_create.flags)
+		return -EINVAL;
+
+	prog_btf_id = prog->aux->attach_btf_id;
+	mutex_lock(&targets_mutex);
+	list_for_each_entry(tinfo, &targets, list) {
+		if (tinfo->btf_id == prog_btf_id) {
+			existed = true;
+			break;
+		}
+	}
+	mutex_unlock(&targets_mutex);
+	if (!existed)
+		return -ENOENT;
+
+	link = kzalloc(sizeof(*link), GFP_USER | __GFP_NOWARN);
+	if (!link)
+		return -ENOMEM;
+
+	bpf_link_init(&link->link, BPF_LINK_TYPE_ITER, &bpf_iter_link_lops, prog);
+	link->tinfo = tinfo;
+
+	err  = bpf_link_prime(&link->link, &link_primer);
+	if (err) {
+		kfree(link);
+		return err;
+	}
+
+	return bpf_link_settle(&link_primer);
+}