Re: [PATCH bpf-next v4 2/6] bpf: Add syscall common attrs support for xdp link_create

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Sat, Oct 24, 2026 at 08:36 AM Leon Hwang <leon.hwang@linux.dev> wrote:
> Instead of passing bpf log or extack to bpf_xdp_link_attach(), pass an
> errmsg pointer to receive the kstrdup() error msg, because

[...]

> 2) passing extack will access underlying driver string out of dev_put()
>    guard, which will crash kernel when the device is unloaded
>    concurrently.

Netlink does exactly that.
rtnl_setlink() looks up the dev with __dev_get_by_index(), so no refcnt,
do_setlink() -> dev_change_xdp_fd(dev, extack, ...) calls into the same
drivers that set the same strings, rtnl is unlocked, and only then
netlink_rcv_skb() -> netlink_ack() does strlen(extack->_msg).
All extack users work this way. kstrdup() in one of them fixes nothing.

Instead of patches 1 and 2:

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index c0ae442..6f7fec2 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4440,7 +4440,8 @@ struct sk_buff *validate_xmit_skb_list(struct sk_buff *skb, struct net_device *d
 struct sk_buff *dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 				    struct netdev_queue *txq, int *ret);

-int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
+int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog,
+			struct netlink_ext_ack *extack);
 u8 dev_xdp_prog_count(struct net_device *dev);
 int netif_xdp_propagate(struct net_device *dev, struct netdev_bpf *bpf);
 int dev_xdp_propagate(struct net_device *dev, struct netdev_bpf *bpf);
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 90bc723..0ce75ee 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5862,8 +5862,12 @@ err_put:
 }

 #define BPF_LINK_CREATE_LAST_FIELD link_create.uprobe_multi.path_fd
-static int link_create(union bpf_attr *attr, bpfptr_t uattr)
+static int link_create(union bpf_attr *attr, bpfptr_t uattr, struct bpf_common_attr *attr_common,
+		       bpfptr_t uattr_common, u32 size_common)
 {
+	struct netlink_ext_ack extack = {};
+	struct bpf_verifier_log *log;
+	struct bpf_log_attr attr_log;
 	struct bpf_prog *prog;
 	int ret;

@@ -5873,14 +5877,24 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 	if (attr->link_create.attach_type == BPF_STRUCT_OPS)
 		return bpf_struct_ops_link_create(attr);

+	log = bpf_log_attr_create_vlog(&attr_log, attr_common, uattr_common, size_common);
+	if (IS_ERR(log))
+		return PTR_ERR(log);
+
 	prog = bpf_prog_get(attr->link_create.prog_fd);
-	if (IS_ERR(prog))
-		return PTR_ERR(prog);
+	if (IS_ERR(prog)) {
+		bpf_log(log, "Invalid prog_fd.\n");
+		ret = PTR_ERR(prog);
+		prog = NULL;
+		goto out;
+	}

 	ret = bpf_prog_attach_check_attach_type(prog,
 						attr->link_create.attach_type);
-	if (ret)
+	if (ret) {
+		bpf_log(log, "Invalid attach_type.\n");
 		goto out;
+	}

 	switch (prog->type) {
 	case BPF_PROG_TYPE_CGROUP_SKB:
@@ -5931,7 +5945,7 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 		break;
 #ifdef CONFIG_NET
 	case BPF_PROG_TYPE_XDP:
-		ret = bpf_xdp_link_attach(attr, prog);
+		ret = bpf_xdp_link_attach(attr, prog, &extack);
 		break;
 	case BPF_PROG_TYPE_SCHED_CLS:
 		if (attr->link_create.attach_type == BPF_TCX_INGRESS ||
@@ -5959,12 +5973,22 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 			ret = bpf_uprobe_multi_link_attach(attr, prog);
 		break;
 	default:
+		bpf_log(log, "Invalid prog type.\n");
 		ret = -EINVAL;
 	}

 out:
-	if (ret < 0)
-		bpf_prog_put(prog);
+	if (ret < 0) {
+		int err;
+
+		if (extack._msg)
+			bpf_log(log, "%s\n", extack._msg);
+		err = bpf_log_attr_finalize(&attr_log, log);
+		ret = err ? err : ret;
+		if (prog)
+			bpf_prog_put(prog);
+	}
+	kfree(log);
 	return ret;
 }

@@ -6536,7 +6560,7 @@ static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size,
 		err = bpf_map_do_batch(&attr, uattr.user, BPF_MAP_DELETE_BATCH);
 		break;
 	case BPF_LINK_CREATE:
-		err = link_create(&attr, uattr);
+		err = link_create(&attr, uattr, &attr_common, uattr_common, size_common);
 		break;
 	case BPF_LINK_UPDATE:
 		err = link_update(&attr);
diff --git a/net/core/dev.c b/net/core/dev.c
index 1a09e78..4aa5525 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10882,11 +10882,11 @@ static const struct bpf_link_ops bpf_xdp_link_lops = {
 	.update_prog = bpf_xdp_link_update,
 };

-int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
+int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog,
+			struct netlink_ext_ack *extack)
 {
 	struct net *net = current->nsproxy->net_ns;
 	struct bpf_link_primer link_primer;
-	struct netlink_ext_ack extack = {};
 	struct bpf_xdp_link *link;
 	struct net_device *dev;
 	int err, fd;
@@ -10895,6 +10895,7 @@ int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 	dev = dev_get_by_index(net, attr->link_create.target_ifindex);
 	if (!dev) {
 		rtnl_unlock();
+		NL_SET_ERR_MSG(extack, "Invalid target_ifindex.");
 		return -EINVAL;
 	}

@@ -10916,14 +10917,14 @@ int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 	}

 	netdev_lock_ops(dev);
-	err = dev_xdp_attach_link(dev, &extack, link);
+	err = dev_xdp_attach_link(dev, extack, link);
 	netdev_unlock_ops(dev);
 	rtnl_unlock();

 	if (err) {
 		link->dev = NULL;
 		bpf_link_cleanup(&link_primer);
-		trace_bpf_xdp_link_attach_failed(extack._msg);
+		trace_bpf_xdp_link_attach_failed(extack->_msg);
 		goto out_put_dev;
 	}

I'm think I gave this suggestion earlier. Why did you go with strdup()?

pw-bot: cr