Re: [PATCH bpf-next v2 1/6] bpf: Add syscall common attrs support for link_create

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

On Mon, Oct 19, 2026 at 08:36 AM Leon Hwang <leon.hwang@linux.dev> wrote:
> +	ret = bpf_log_attr_report_size(&attr_log, 0);
> +	if (ret)
> +		goto put_prog;

can we drop bpf_log_attr_report_size() ?
Nothing is logged when the attach succeeds, so there is no size to
report, and libbpf zeroes attr_common, so log_true_size reads 0 anyway.
Finalizing only when ret < 0 already keeps -EFAULT away from a live link.
Then no need for the 2nd write, the log.c refactoring and the put_prog
label.

Something like this (compile tested only):

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 849bc91..61d100e 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5856,8 +5856,11 @@ 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 bpf_verifier_log *log;
+	struct bpf_log_attr attr_log;
 	struct bpf_prog *prog;
 	int ret;

@@ -5867,14 +5870,24 @@ 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:
@@ -5953,12 +5966,19 @@ 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 = bpf_log_attr_finalize(&attr_log, log);
+
+		ret = err ? err : ret;
+		if (prog)
+			bpf_prog_put(prog);
+	}
+	kfree(log);
 	return ret;
 }

@@ -6530,7 +6550,7 @@ 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);

pw-bot: cr