bpf: Move prog->aux->linked_prog and trampoline into bpf_link on attach

In preparation for allowing multiple attachments of freplace programs, move
the references to the target program and trampoline into the
bpf_tracing_link structure when that is created. To do this atomically,
introduce a new mutex in prog->aux to protect writing to the two pointers
to target prog and trampoline, and rename the members to make it clear that
they are related.

With this change, it is no longer possible to attach the same tracing
program multiple times (detaching in-between), since the reference from the
tracing program to the target disappears on the first attach. However,
since the next patch will let the caller supply an attach target, that will
also make it possible to attach to the same place multiple times.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/160138355059.48470.2503076992210324984.stgit@toke.dk

1 files changed, 44 insertions, 12 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 3bc2ed2e171b..e6a0a948e30c 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2154,14 +2154,14 @@ static int bpf_prog_load(union bpf_attr *attr, union bpf_attr __user *uattr)
 	prog->expected_attach_type = attr->expected_attach_type;
 	prog->aux->attach_btf_id = attr->attach_btf_id;
 	if (attr->attach_prog_fd) {
-		struct bpf_prog *tgt_prog;
+		struct bpf_prog *dst_prog;
 
-		tgt_prog = bpf_prog_get(attr->attach_prog_fd);
-		if (IS_ERR(tgt_prog)) {
-			err = PTR_ERR(tgt_prog);
+		dst_prog = bpf_prog_get(attr->attach_prog_fd);
+		if (IS_ERR(dst_prog)) {
+			err = PTR_ERR(dst_prog);
 			goto free_prog_nouncharge;
 		}
-		prog->aux->linked_prog = tgt_prog;
+		prog->aux->dst_prog = dst_prog;
 	}
 
 	prog->aux->offload_requested = !!attr->prog_ifindex;
@@ -2498,11 +2498,23 @@ struct bpf_link *bpf_link_get_from_fd(u32 ufd)
 struct bpf_tracing_link {
 	struct bpf_link link;
 	enum bpf_attach_type attach_type;
+	struct bpf_trampoline *trampoline;
+	struct bpf_prog *tgt_prog;
 };
 
 static void bpf_tracing_link_release(struct bpf_link *link)
 {
-	WARN_ON_ONCE(bpf_trampoline_unlink_prog(link->prog));
+	struct bpf_tracing_link *tr_link =
+		container_of(link, struct bpf_tracing_link, link);
+
+	WARN_ON_ONCE(bpf_trampoline_unlink_prog(link->prog,
+						tr_link->trampoline));
+
+	bpf_trampoline_put(tr_link->trampoline);
+
+	/* tgt_prog is NULL if target is a kernel function */
+	if (tr_link->tgt_prog)
+		bpf_prog_put(tr_link->tgt_prog);
 }
 
 static void bpf_tracing_link_dealloc(struct bpf_link *link)
@@ -2545,7 +2557,9 @@ static const struct bpf_link_ops bpf_tracing_link_lops = {
 static int bpf_tracing_prog_attach(struct bpf_prog *prog)
 {
 	struct bpf_link_primer link_primer;
+	struct bpf_prog *tgt_prog = NULL;
 	struct bpf_tracing_link *link;
+	struct bpf_trampoline *tr;
 	int err;
 
 	switch (prog->type) {
@@ -2583,19 +2597,37 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog)
 		      &bpf_tracing_link_lops, prog);
 	link->attach_type = prog->expected_attach_type;
 
-	err = bpf_link_prime(&link->link, &link_primer);
-	if (err) {
-		kfree(link);
-		goto out_put_prog;
+	mutex_lock(&prog->aux->dst_mutex);
+
+	if (!prog->aux->dst_trampoline) {
+		err = -ENOENT;
+		goto out_unlock;
 	}
+	tr = prog->aux->dst_trampoline;
+	tgt_prog = prog->aux->dst_prog;
+
+	err = bpf_link_prime(&link->link, &link_primer);
+	if (err)
+		goto out_unlock;
 
-	err = bpf_trampoline_link_prog(prog);
+	err = bpf_trampoline_link_prog(prog, tr);
 	if (err) {
 		bpf_link_cleanup(&link_primer);
-		goto out_put_prog;
+		link = NULL;
+		goto out_unlock;
 	}
 
+	link->tgt_prog = tgt_prog;
+	link->trampoline = tr;
+
+	prog->aux->dst_prog = NULL;
+	prog->aux->dst_trampoline = NULL;
+	mutex_unlock(&prog->aux->dst_mutex);
+
 	return bpf_link_settle(&link_primer);
+out_unlock:
+	mutex_unlock(&prog->aux->dst_mutex);
+	kfree(link);
 out_put_prog:
 	bpf_prog_put(prog);
 	return err;