[RFC PATCH bpf-next 1/4] bpf: Allow trampoline re-attach

[Jiri Olsa <jolsa@kernel.org>]

Currently we don't allow re-attaching of trampolines. Once
it's detached, it can't be re-attach even when the program
is still loaded.

Adding the possibility to re-attach the loaded tracing
kernel program.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/bpf/syscall.c    | 25 +++++++++++++++++++------
 kernel/bpf/trampoline.c |  2 +-
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 9603de81811a..e14926b2e95a 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2645,14 +2645,27 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 	 *   target_btf_id using the link_create API.
 	 *
 	 * - if tgt_prog == NULL when this function was called using the old
-         *   raw_tracepoint_open API, and we need a target from prog->aux
-         *
-         * The combination of no saved target in prog->aux, and no target
-         * specified on load is illegal, and we reject that here.
+	 *   raw_tracepoint_open API, and we need a target from prog->aux
+	 *
+	 * The combination of no saved target in prog->aux, and no target
+	 * specified on is legal only for tracing programs re-attach, rest
+	 * is illegal, and we reject that here.
 	 */
 	if (!prog->aux->dst_trampoline && !tgt_prog) {
-		err = -ENOENT;
-		goto out_unlock;
+		/*
+		 * Allow re-attach for tracing programs, if it's currently
+		 * linked, bpf_trampoline_link_prog will fail.
+		 */
+		if (prog->type != BPF_PROG_TYPE_TRACING) {
+			err = -ENOENT;
+			goto out_unlock;
+		}
+		if (!prog->aux->attach_btf) {
+			err = -EINVAL;
+			goto out_unlock;
+		}
+		btf_id = prog->aux->attach_btf_id;
+		key = bpf_trampoline_compute_key(NULL, prog->aux->attach_btf, btf_id);
 	}
 
 	if (!prog->aux->dst_trampoline ||
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 4aa8b52adf25..0d937c63fc22 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -467,7 +467,7 @@ int bpf_trampoline_unlink_prog(struct bpf_prog *prog, struct bpf_trampoline *tr)
 		tr->extension_prog = NULL;
 		goto out;
 	}
-	hlist_del(&prog->aux->tramp_hlist);
+	hlist_del_init(&prog->aux->tramp_hlist);
 	tr->progs_cnt[kind]--;
 	err = bpf_trampoline_update(tr);
 out:
-- 
2.30.2