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

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

With the syscall common attrs support for link_create, add syscall common
attrs support for xdp link_create in order to report the invalid target
ifindex error and the underlying driver's error.

Instead of passing bpf log or extack to bpf_xdp_link_attach(), pass an
errmsg pointer to receive the kstrdup() error msg, because
1) passing bpf log will introduce bpf_verifier.h dependency for
   net/core/dev.c, which is not acceptable for maintainability.
2) passing extack will access underlying driver string out of dev_put()
   guard, which will crash kernel when the device is unloaded
   concurrently.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 include/linux/netdevice.h | 3 ++-
 kernel/bpf/syscall.c      | 8 +++++++-
 net/core/dev.c            | 6 +++++-
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index c0ae442c4344..e07b740604c7 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,
+			char **errmsg);
 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 db870e795d04..675bff4d7aab 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5868,6 +5868,7 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr, struct bpf_common_a
 	struct bpf_verifier_log *log;
 	struct bpf_log_attr attr_log;
 	struct bpf_prog *prog;
+	char *errmsg = NULL;
 	int ret;
 
 	if (CHECK_ATTR(BPF_LINK_CREATE))
@@ -5944,7 +5945,7 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr, struct bpf_common_a
 		break;
 #ifdef CONFIG_NET
 	case BPF_PROG_TYPE_XDP:
-		ret = bpf_xdp_link_attach(attr, prog);
+		ret = bpf_xdp_link_attach(attr, prog, &errmsg);
 		break;
 	case BPF_PROG_TYPE_SCHED_CLS:
 		if (attr->link_create.attach_type == BPF_TCX_INGRESS ||
@@ -5980,6 +5981,11 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr, struct bpf_common_a
 	if (ret < 0) {
 		int err;
 
+		if (errmsg) {
+			bpf_log(log, "%s\n", errmsg);
+			kfree(errmsg);
+		}
+
 		err = bpf_log_attr_finalize(&attr_log, log);
 		ret = err ? err : ret;
 		if (prog)
diff --git a/net/core/dev.c b/net/core/dev.c
index 1a09e7869118..da71c0d2120c 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10882,7 +10882,8 @@ 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,
+			char **errmsg)
 {
 	struct net *net = current->nsproxy->net_ns;
 	struct bpf_link_primer link_primer;
@@ -10895,6 +10896,7 @@ 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();
+		*errmsg = kstrdup("Invalid target_ifindex.", GFP_KERNEL);
 		return -EINVAL;
 	}
 
@@ -10923,6 +10925,8 @@ int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 	if (err) {
 		link->dev = NULL;
 		bpf_link_cleanup(&link_primer);
+		if (extack._msg)
+			*errmsg = kstrdup(extack._msg, GFP_KERNEL);
 		trace_bpf_xdp_link_attach_failed(extack._msg);
 		goto out_put_dev;
 	}
-- 
2.55.0