[bpf-next,2/2] bpf, verifier: Check trampoline target is tail_call_reachable subprog

Commit Message

Leon Hwang Oct. 21, 2024, 1:39 p.m. UTC

In the x86_64 JIT, tailcall info is propagated through the trampoline when
the target program is tail_call_reachable. However, this propagation is
unnecessary if the target is a main prog, or a subprog that is not
tail_call_reachable.

Since the verifier can determine if a subprog is tail_call_reachable, it
should only propagate tailcall info when the target is subprog and the
subprog is actually tail_call_reachable.

Signed-off-by: Leon Hwang <[e-mail]>
---
 include/linux/bpf.h   | 1 +
 kernel/bpf/verifier.c | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

Comments

Yonghong Song Oct. 24, 2024, 2:46 a.m. UTC | #1

On 10/21/24 6:39 AM, Leon Hwang wrote:
> In the x86_64 JIT, tailcall info is propagated through the trampoline when
> the target program is tail_call_reachable. However, this propagation is
> unnecessary if the target is a main prog, or a subprog that is not
> tail_call_reachable.
>
> Since the verifier can determine if a subprog is tail_call_reachable, it
> should only propagate tailcall info when the target is subprog and the
> subprog is actually tail_call_reachable.
>
> Signed-off-by: Leon Hwang <[e-mail]>

Looks correct to me.

Acked-by: Yonghong Song <[e-mail]>

Patch

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 0c216e71cec76..32b56a759d788 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1220,6 +1220,7 @@  struct bpf_attach_target_info {
 	struct module *tgt_mod;
 	const char *tgt_name;
 	const struct btf_type *tgt_type;
+	bool tgt_tail_call_reachable;
 };
 
 #define BPF_DISPATCHER_MAX 48 /* Fits in 2048B */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6e7e42c7bc7b1..9a09621a8832c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -21916,6 +21916,8 @@  int bpf_check_attach_target(struct bpf_verifier_log *log,
 			bpf_log(log, "Subprog %s doesn't exist\n", tname);
 			return -EINVAL;
 		}
+		tgt_info->tgt_tail_call_reachable = subprog &&
+						    aux->func[subprog]->aux->tail_call_reachable;
 		if (aux->func && aux->func[subprog]->aux->exception_cb) {
 			bpf_log(log,
 				"%s programs cannot attach to exception callback\n",
@@ -22285,7 +22287,7 @@  static int check_attach_btf_id(struct bpf_verifier_env *env)
 	if (!tr)
 		return -ENOMEM;
 
-	if (tgt_prog && tgt_prog->aux->tail_call_reachable)
+	if (tgt_prog && tgt_info.tgt_tail_call_reachable)
 		tr->flags = BPF_TRAMP_F_TAIL_CALL_CTX;
 
 	prog->aux->dst_trampoline = tr;