[RFC PATCH bpf-next 3/5] bpf: Report freplace attach failure reason via extended syscall

[Leon Hwang <leon.hwang@xxxxxxxxx>]

This patch enables detailed error reporting when a freplace program fails
to attach to its target.

By leveraging the extended 'bpf()' syscall with common attributes, users
can now retrieve the failure reason through the provided log buffer.

Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
---
 kernel/bpf/syscall.c | 39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ca7ce8474812..4d1f58b14a0a 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3446,7 +3446,8 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 				   int tgt_prog_fd,
 				   u32 btf_id,
 				   u64 bpf_cookie,
-				   enum bpf_attach_type attach_type)
+				   enum bpf_attach_type attach_type,
+				   struct bpf_verifier_log *log)
 {
 	struct bpf_link_primer link_primer;
 	struct bpf_prog *tgt_prog = NULL;
@@ -3571,7 +3572,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 		 */
 		struct bpf_attach_target_info tgt_info = {};
 
-		err = bpf_check_attach_target(NULL, prog, tgt_prog, btf_id,
+		err = bpf_check_attach_target(log, prog, tgt_prog, btf_id,
 					      &tgt_info);
 		if (err)
 			goto out_unlock;
@@ -4109,7 +4110,7 @@ static int bpf_raw_tp_link_attach(struct bpf_prog *prog,
 			tp_name = prog->aux->attach_func_name;
 			break;
 		}
-		return bpf_tracing_prog_attach(prog, 0, 0, 0, attach_type);
+		return bpf_tracing_prog_attach(prog, 0, 0, 0, attach_type, NULL);
 	case BPF_PROG_TYPE_RAW_TRACEPOINT:
 	case BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE:
 		if (strncpy_from_user(buf, user_tp_name, sizeof(buf) - 1) < 0)
@@ -5514,9 +5515,11 @@ static int bpf_map_do_batch(const union bpf_attr *attr,
 }
 
 #define BPF_LINK_CREATE_LAST_FIELD link_create.uprobe_multi.pid
-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 *common_attrs)
 {
+	struct bpf_verifier_log *log = NULL;
 	struct bpf_prog *prog;
+	u32 log_true_size;
 	int ret;
 
 	if (CHECK_ATTR(BPF_LINK_CREATE))
@@ -5529,10 +5532,23 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 	if (IS_ERR(prog))
 		return PTR_ERR(prog);
 
+	if (common_attrs->log_buf) {
+		log = kvzalloc(sizeof(*log), GFP_KERNEL);
+		if (!log) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		ret = bpf_vlog_init(log, BPF_LOG_FIXED,
+				    (char __user *) (unsigned long) common_attrs->log_buf,
+				    common_attrs->log_size);
+		if (ret)
+			goto out_free_log;
+	}
+
 	ret = bpf_prog_attach_check_attach_type(prog,
 						attr->link_create.attach_type);
 	if (ret)
-		goto out;
+		goto out_free_log;
 
 	switch (prog->type) {
 	case BPF_PROG_TYPE_CGROUP_SKB:
@@ -5549,7 +5565,8 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 					      attr->link_create.target_fd,
 					      attr->link_create.target_btf_id,
 					      attr->link_create.tracing.cookie,
-					      attr->link_create.attach_type);
+					      attr->link_create.attach_type,
+					      log);
 		break;
 	case BPF_PROG_TYPE_LSM:
 	case BPF_PROG_TYPE_TRACING:
@@ -5569,7 +5586,8 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 						      attr->link_create.target_fd,
 						      attr->link_create.target_btf_id,
 						      attr->link_create.tracing.cookie,
-						      attr->link_create.attach_type);
+						      attr->link_create.attach_type,
+						      log);
 		break;
 	case BPF_PROG_TYPE_FLOW_DISSECTOR:
 	case BPF_PROG_TYPE_SK_LOOKUP:
@@ -5612,6 +5630,11 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 		ret = -EINVAL;
 	}
 
+	if (ret < 0 && log)
+		(void) bpf_vlog_finalize(log, &log_true_size);
+out_free_log:
+	if (log)
+		kvfree(log);
 out:
 	if (ret < 0)
 		bpf_prog_put(prog);
@@ -6099,7 +6122,7 @@ 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, &common_attrs);
 		break;
 	case BPF_LINK_UPDATE:
 		err = link_update(&attr);
-- 
2.50.1