[PATCH bpf-next 1/3] bpf: Disallow freplace on XDP with mismatched xdp_has_frags values

[Leon Hwang <leon.hwang@linux.dev>]

xdp_has_frags was introduced by the commit
c2f2cdbeffda ("bpf: introduce BPF_F_XDP_HAS_FRAGS flag in prog_flags loading the ebpf program").

The commit f45d5b6ce2e8 ("bpf: generalise tail call map compatibility check")
was to ensure backwards compatibility against tail calls. However, it
missed that XDP progs can be extended by freplace progs, which could break
the backwards compatibility, e.g. xdp_has_frags=true freplace progs are
allowed to attach to xdp_has_frags=false XDP progs.

To avoid breaking the backwards compatibility via freplace, disallow
freplace on XDP programs with different xdp_has_frags values.

Cc: Toke Hoiland-Jorgensen <toke@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Lorenzo Bianconi <lorenzo@kernel.org>
Fixes: c2f2cdbeffda ("bpf: introduce BPF_F_XDP_HAS_FRAGS flag in prog_flags loading the ebpf program")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index cd008b146ee5..12330466d58b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -25519,6 +25519,10 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 		 */
 		env->ops = bpf_verifier_ops[tgt_prog->type];
 		prog->expected_attach_type = tgt_prog->expected_attach_type;
+		if (prog->aux->xdp_has_frags != tgt_prog->aux->xdp_has_frags) {
+			verbose(env, "Extension program cannot have different xdp_has_frags value with target prog\n");
+			return -EINVAL;
+		}
 	}
 
 	/* store info about the attachment target that will be used later */
-- 
2.53.0