On 2026/1/1 21:52 Jiri Olsa [off-list ref] write:

On Wed, Dec 24, 2025 at 09:07:26PM +0800, Menglong Dong wrote:

SNIP

Hi, Jiri. Happy New Year!

quoted

+struct bpf_fsession_link {
+	struct bpf_tracing_link link;
+	struct bpf_tramp_link fexit;
+};
+
 struct bpf_raw_tp_link {
 	struct bpf_link link;
 	struct bpf_raw_event_map *btp;

@@ -2114,6 +2120,20 @@ static inline void bpf_struct_ops_desc_release(struct bpf_struct_ops_desc *st_op
 
 #endif
 
+static inline int bpf_fsession_cnt(struct bpf_tramp_links *links)
+{
+	struct bpf_tramp_links fentries = links[BPF_TRAMP_FENTRY];
+	int cnt = 0;
+
+	for (int i = 0; i < links[BPF_TRAMP_FENTRY].nr_links; i++) {
+		if (fentries.links[i]->link.prog->expected_attach_type ==
+		    BPF_TRACE_FSESSION)

let's keep it on the single line ?

OK

quoted

+			cnt++;
+	}
+
+	return cnt;
+}
+

[......]

quoted

@@ -3628,7 +3629,21 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 		key = bpf_trampoline_compute_key(tgt_prog, NULL, btf_id);
 	}
 
-	link = kzalloc(sizeof(*link), GFP_USER);
+	if (prog->expected_attach_type == BPF_TRACE_FSESSION) {
+		struct bpf_fsession_link *fslink;
+
+		fslink = kzalloc(sizeof(*fslink), GFP_USER);
+		if (fslink) {
+			bpf_link_init(&fslink->fexit.link, BPF_LINK_TYPE_TRACING,
+				      &bpf_tracing_link_lops, prog, attach_type);

I don't think we need the extra exit struct bpf_link, we just need
hlist_node hook for exit program, so this should perhaps be:

struct bpf_fsession_link {
	struct bpf_tracing_link link;
	struct hlist_node tramp_hlist;
};

I think we can't do it this way according to how we manager
the bpf_link in trampoline, as you can see in
bpf_trampoline_get_progs() and the struct of bpf_tramp_links.

In bpf_trampoline_get_progs(), it will lookup all the bpf_link
in the trampoline. If we simply add the bpf_fsession_link->tramp_hlist,
the struct in the progs_hlist will be inconsistent.

SNIP

quoted

@@ -596,6 +598,8 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 {
 	enum bpf_tramp_prog_type kind;
 	struct bpf_tramp_link *link_exiting;
+	struct bpf_fsession_link *fslink;
+	struct hlist_head *prog_list;
 	int err = 0;
 	int cnt = 0, i;
 

@@ -621,24 +625,44 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 					  BPF_MOD_JUMP, NULL,
 					  link->link.prog->bpf_func);
 	}
+	if (kind == BPF_TRAMP_FSESSION) {
+		prog_list = &tr->progs_hlist[BPF_TRAMP_FENTRY];
+		cnt++;
+	} else {
+		prog_list = &tr->progs_hlist[kind];
+	}
 	if (cnt >= BPF_MAX_TRAMP_LINKS)
 		return -E2BIG;
 	if (!hlist_unhashed(&link->tramp_hlist))
 		/* prog already linked */
 		return -EBUSY;
-	hlist_for_each_entry(link_exiting, &tr->progs_hlist[kind], tramp_hlist) {
+	hlist_for_each_entry(link_exiting, prog_list, tramp_hlist) {
 		if (link_exiting->link.prog != link->link.prog)
 			continue;
 		/* prog already linked */
 		return -EBUSY;
 	}
 
-	hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
-	tr->progs_cnt[kind]++;
+	hlist_add_head(&link->tramp_hlist, prog_list);
+	if (kind == BPF_TRAMP_FSESSION) {
+		tr->progs_cnt[BPF_TRAMP_FENTRY]++;
+		fslink = container_of(link, struct bpf_fsession_link, link.link);
+		hlist_add_head(&fslink->fexit.tramp_hlist,
+			       &tr->progs_hlist[BPF_TRAMP_FEXIT]);
+		tr->progs_cnt[BPF_TRAMP_FEXIT]++;
+	} else {
+		tr->progs_cnt[kind]++;
+	}
 	err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
 	if (err) {
 		hlist_del_init(&link->tramp_hlist);
-		tr->progs_cnt[kind]--;
+		if (kind == BPF_TRAMP_FSESSION) {
+			tr->progs_cnt[BPF_TRAMP_FENTRY]--;
+			hlist_del_init(&fslink->fexit.tramp_hlist);
+			tr->progs_cnt[BPF_TRAMP_FEXIT]--;
+		} else {
+			tr->progs_cnt[kind]--;
+		}
 	}
 	return err;

this seems confusing, how about we just add abolish bpf_fsession_link

It was more confusing in V1. I adopted Andrii's suggestion in
this version to make the logic here more clear. But it seems
still confusing :/

Maybe we need more document here to help the understanding.

In this way, it indeed can make the update of the hlist more clear. However,
I think that you missed the reading of the hlist as I mentioned above.
You can't add both the "tramp_hlist" and "extra_hlist" to the same hlist. If
so, how do we iterate the hlist? Do I miss something?

Thanks!
Menglong Dong

[......]

 void test_tracing_failure(void)