[PATCH bpf-next v3 04/21] bpf: support bpf tracing/iter programs for BPF_LINK_UPDATE

[Yonghong Song]

Added BPF_LINK_UPDATE support for tracing/iter programs.
This way, a file based bpf iterator, which holds a reference
to the link, can have its bpf program updated without
creating new files.

Acked-by: Andrii Nakryiko <andr...@fb.com>
Signed-off-by: Yonghong Song <y...@fb.com>
---
 kernel/bpf/bpf_iter.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index 03f5832909db..0542a243b78c 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -23,6 +23,9 @@ struct bpf_iter_link {
 static struct list_head targets = LIST_HEAD_INIT(targets);
 static DEFINE_MUTEX(targets_mutex);
 
+/* protect bpf_iter_link changes */
+static DEFINE_MUTEX(link_mutex);
+
 int bpf_iter_reg_target(struct bpf_iter_reg *reg_info)
 {
        struct bpf_iter_target_info *tinfo;
@@ -111,9 +114,37 @@ static void bpf_iter_link_dealloc(struct bpf_link *link)
        kfree(iter_link);
 }
 
+static int bpf_iter_link_replace(struct bpf_link *link,
+                                struct bpf_prog *new_prog,
+                                struct bpf_prog *old_prog)
+{
+       int ret = 0;
+
+       mutex_lock(&link_mutex);
+       if (old_prog && link->prog != old_prog) {
+               ret = -EPERM;
+               goto out_unlock;
+       }
+
+       if (link->prog->type != new_prog->type ||
+           link->prog->expected_attach_type != new_prog->expected_attach_type 
||
+           link->prog->aux->attach_btf_id != new_prog->aux->attach_btf_id) {
+               ret = -EINVAL;
+               goto out_unlock;
+       }
+
+       old_prog = xchg(&link->prog, new_prog);
+       bpf_prog_put(old_prog);
+
+out_unlock:
+       mutex_unlock(&link_mutex);
+       return ret;
+}
+
 static const struct bpf_link_ops bpf_iter_link_lops = {
        .release = bpf_iter_link_release,
        .dealloc = bpf_iter_link_dealloc,
+       .update_prog = bpf_iter_link_replace,
 };
 
 int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
-- 
2.24.1