[RFC,bpf-next,5/8] bpf: add support for BPF_OBJ_GET_INFO_BY_FD for bpf_link

Message ID 20200404000948.3980903-6-andriin@fb.com
State New
Headers show
Series
  • bpf_link observability APIs
Related show

Commit Message

Andrii Nakryiko April 4, 2020, 12:09 a.m.
Add ability to fetch bpf_link details through BPF_OBJ_GET_INFO_BY_FD command.
Also enhance show_fdinfo to potentially include bpf_link type-specific
information (similarly to obj_info).

Also introduce enum bpf_link_type stored in bpf_link itself and expose it in
UAPI. bpf_link_tracing also now will store and return bpf_attach_type.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 include/linux/bpf-cgroup.h     |   2 -
 include/linux/bpf.h            |  10 +-
 include/linux/bpf_types.h      |   6 ++
 include/uapi/linux/bpf.h       |  28 ++++++
 kernel/bpf/btf.c               |   2 +
 kernel/bpf/cgroup.c            |  45 ++++++++-
 kernel/bpf/syscall.c           | 164 +++++++++++++++++++++++++++++----
 kernel/bpf/verifier.c          |   2 +
 tools/include/uapi/linux/bpf.h |  31 +++++++
 9 files changed, 266 insertions(+), 24 deletions(-)

Patch

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index d2d969669564..ab95824a1d99 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -57,8 +57,6 @@  struct bpf_cgroup_link {
 	enum bpf_attach_type type;
 };
 
-extern const struct bpf_link_ops bpf_cgroup_link_lops;
-
 struct bpf_prog_list {
 	struct list_head node;
 	struct bpf_prog *prog;
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 67ce74890911..8cf182d256d4 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1026,9 +1026,11 @@  extern const struct file_operations bpf_prog_fops;
 	extern const struct bpf_verifier_ops _name ## _verifier_ops;
 #define BPF_MAP_TYPE(_id, _ops) \
 	extern const struct bpf_map_ops _ops;
+#define BPF_LINK_TYPE(_id, _name)
 #include <linux/bpf_types.h>
 #undef BPF_PROG_TYPE
 #undef BPF_MAP_TYPE
+#undef BPF_LINK_TYPE
 
 extern const struct bpf_prog_ops bpf_offload_prog_ops;
 extern const struct bpf_verifier_ops tc_cls_act_analyzer_ops;
@@ -1086,6 +1088,7 @@  int bpf_prog_new_fd(struct bpf_prog *prog);
 struct bpf_link {
 	atomic64_t refcnt;
 	u32 id;
+	enum bpf_link_type type;
 	const struct bpf_link_ops *ops;
 	struct bpf_prog *prog;
 	struct work_struct work;
@@ -1103,9 +1106,14 @@  struct bpf_link_ops {
 	void (*dealloc)(struct bpf_link *link);
 	int (*update_prog)(struct bpf_link *link, struct bpf_prog *new_prog,
 			   struct bpf_prog *old_prog);
+	void (*show_fdinfo)(const struct bpf_link *link, struct seq_file *seq);
+	int (*fill_link_info)(const struct bpf_link *link,
+			      struct bpf_link_info *info,
+			      const struct bpf_link_info *uinfo,
+			      u32 info_len);
 };
 
-void bpf_link_init(struct bpf_link *link,
+void bpf_link_init(struct bpf_link *link, enum bpf_link_type type,
 		   const struct bpf_link_ops *ops, struct bpf_prog *prog);
 int bpf_link_prime(struct bpf_link *link, struct bpf_link_primer *primer);
 int bpf_link_settle(struct bpf_link_primer *primer);
diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
index ba0c2d56f8a3..8345cdf553b8 100644
--- a/include/linux/bpf_types.h
+++ b/include/linux/bpf_types.h
@@ -118,3 +118,9 @@  BPF_MAP_TYPE(BPF_MAP_TYPE_STACK, stack_map_ops)
 #if defined(CONFIG_BPF_JIT)
 BPF_MAP_TYPE(BPF_MAP_TYPE_STRUCT_OPS, bpf_struct_ops_map_ops)
 #endif
+
+BPF_LINK_TYPE(BPF_LINK_TYPE_RAW_TRACEPOINT, raw_tracepoint)
+BPF_LINK_TYPE(BPF_LINK_TYPE_TRACING, tracing)
+#ifdef CONFIG_CGROUP_BPF
+BPF_LINK_TYPE(BPF_LINK_TYPE_CGROUP, cgroup)
+#endif
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 407c086bc9e4..d2f269082a33 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -222,6 +222,15 @@  enum bpf_attach_type {
 
 #define MAX_BPF_ATTACH_TYPE __MAX_BPF_ATTACH_TYPE
 
+enum bpf_link_type {
+	BPF_LINK_TYPE_UNSPEC = 0,
+	BPF_LINK_TYPE_RAW_TRACEPOINT = 1,
+	BPF_LINK_TYPE_TRACING = 2,
+	BPF_LINK_TYPE_CGROUP = 3,
+
+	MAX_BPF_LINK_TYPE,
+};
+
 /* cgroup-bpf attach flags used in BPF_PROG_ATTACH command
  *
  * NONE(default): No further bpf programs allowed in the subtree.
@@ -3601,6 +3610,25 @@  struct bpf_btf_info {
 	__u32 id;
 } __attribute__((aligned(8)));
 
+struct bpf_link_info {
+	__u32 type;
+	__u32 id;
+	__u32 prog_id;
+	union {
+		struct {
+			__aligned_u64 tp_name; /* in/out: tp_name buffer ptr */
+			__u32 tp_name_len;     /* in/out: tp_name buffer len */
+		} raw_tracepoint;
+		struct {
+			__u32 attach_type;
+		} tracing;
+		struct {
+			__u64 cgroup_id;
+			__u32 attach_type;
+		} cgroup;
+	};
+} __attribute__((aligned(8)));
+
 /* User bpf_sock_addr struct to access socket fields and sockaddr struct passed
  * by user and intended to be used by socket (e.g. to bind to, depends on
  * attach attach type).
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index d65c6912bdaf..a2cfba89a8e1 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3482,6 +3482,7 @@  extern char __weak __stop_BTF[];
 extern struct btf *btf_vmlinux;
 
 #define BPF_MAP_TYPE(_id, _ops)
+#define BPF_LINK_TYPE(_id, _name)
 static union {
 	struct bpf_ctx_convert {
 #define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type) \
@@ -3508,6 +3509,7 @@  static u8 bpf_ctx_convert_map[] = {
 	0, /* avoid empty array */
 };
 #undef BPF_MAP_TYPE
+#undef BPF_LINK_TYPE
 
 static const struct btf_member *
 btf_get_prog_ctx_type(struct bpf_verifier_log *log, struct btf *btf,
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index ae84c5c90631..25ca4c937595 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -833,10 +833,50 @@  static void bpf_cgroup_link_dealloc(struct bpf_link *link)
 	kfree(cg_link);
 }
 
-const struct bpf_link_ops bpf_cgroup_link_lops = {
+static void bpf_cgroup_link_show_fdinfo(const struct bpf_link *link,
+					struct seq_file *seq)
+{
+	struct bpf_cgroup_link *cg_link =
+		container_of(link, struct bpf_cgroup_link, link);
+	u64 cg_id = 0;
+
+	mutex_lock(&cgroup_mutex);
+	if (cg_link->cgroup)
+		cg_id = cgroup_id(cg_link->cgroup);
+	mutex_unlock(&cgroup_mutex);
+
+	seq_printf(seq,
+		   "cgroup_id:\t%llu\n"
+		   "attach_type:\t%d\n",
+		   cg_id,
+		   cg_link->type);
+}
+
+static int bpf_cgroup_link_fill_link_info(const struct bpf_link *link,
+					  struct bpf_link_info *info,
+					  const struct bpf_link_info *uinfo,
+					  u32 info_len)
+{
+	struct bpf_cgroup_link *cg_link =
+		container_of(link, struct bpf_cgroup_link, link);
+	u64 cg_id = 0;
+
+	mutex_lock(&cgroup_mutex);
+	if (cg_link->cgroup)
+		cg_id = cgroup_id(cg_link->cgroup);
+	mutex_unlock(&cgroup_mutex);
+
+	info->cgroup.cgroup_id = cg_id;
+	info->cgroup.attach_type = cg_link->type;
+	return 0;
+}
+
+static const struct bpf_link_ops bpf_cgroup_link_lops = {
 	.release = bpf_cgroup_link_release,
 	.dealloc = bpf_cgroup_link_dealloc,
 	.update_prog = cgroup_bpf_replace,
+	.show_fdinfo = bpf_cgroup_link_show_fdinfo,
+	.fill_link_info = bpf_cgroup_link_fill_link_info,
 };
 
 int cgroup_bpf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
@@ -858,7 +898,8 @@  int cgroup_bpf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 		err = -ENOMEM;
 		goto out_put_cgroup;
 	}
-	bpf_link_init(&link->link, &bpf_cgroup_link_lops, prog);
+	bpf_link_init(&link->link, BPF_LINK_TYPE_CGROUP, &bpf_cgroup_link_lops,
+		      prog);
 	link->cgroup = cgrp;
 	link->type = attr->link_create.attach_type;
 
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 527ec16702be..ed94b322c757 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -51,9 +51,11 @@  static const struct bpf_map_ops * const bpf_map_types[] = {
 #define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type)
 #define BPF_MAP_TYPE(_id, _ops) \
 	[_id] = &_ops,
+#define BPF_LINK_TYPE(_id, _name)
 #include <linux/bpf_types.h>
 #undef BPF_PROG_TYPE
 #undef BPF_MAP_TYPE
+#undef BPF_LINK_TYPE
 };
 
 /*
@@ -1550,9 +1552,11 @@  static const struct bpf_prog_ops * const bpf_prog_types[] = {
 #define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type) \
 	[_id] = & _name ## _prog_ops,
 #define BPF_MAP_TYPE(_id, _ops)
+#define BPF_LINK_TYPE(_id, _name)
 #include <linux/bpf_types.h>
 #undef BPF_PROG_TYPE
 #undef BPF_MAP_TYPE
+#undef BPF_LINK_TYPE
 };
 
 static int find_prog_type(enum bpf_prog_type type, struct bpf_prog *prog)
@@ -2186,10 +2190,11 @@  static int bpf_obj_get(const union bpf_attr *attr)
 				attr->file_flags);
 }
 
-void bpf_link_init(struct bpf_link *link,
+void bpf_link_init(struct bpf_link *link, enum bpf_link_type type,
 		   const struct bpf_link_ops *ops, struct bpf_prog *prog)
 {
 	atomic64_set(&link->refcnt, 1);
+	link->type = type;
 	link->id = 0;
 	link->ops = ops;
 	link->prog = prog;
@@ -2270,27 +2275,23 @@  static int bpf_link_release(struct inode *inode, struct file *filp)
 	return 0;
 }
 
-#ifdef CONFIG_PROC_FS
-static const struct bpf_link_ops bpf_raw_tp_lops;
-static const struct bpf_link_ops bpf_tracing_link_lops;
+#define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type)
+#define BPF_MAP_TYPE(_id, _ops)
+#define BPF_LINK_TYPE(_id, _name) [_id] = #_name,
+static const char *bpf_link_type_strs[] = {
+	[BPF_LINK_TYPE_UNSPEC] = "<invalid>",
+#include <linux/bpf_types.h>
+};
+#undef BPF_PROG_TYPE
+#undef BPF_MAP_TYPE
+#undef BPF_LINK_TYPE
 
+#ifdef CONFIG_PROC_FS
 static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 {
 	const struct bpf_link *link = filp->private_data;
 	const struct bpf_prog *prog = link->prog;
 	char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
-	const char *link_type;
-
-	if (link->ops == &bpf_raw_tp_lops)
-		link_type = "raw_tracepoint";
-	else if (link->ops == &bpf_tracing_link_lops)
-		link_type = "tracing";
-#ifdef CONFIG_CGROUP_BPF
-	else if (link->ops == &bpf_cgroup_link_lops)
-		link_type = "cgroup";
-#endif
-	else
-		link_type = "unknown";
 
 	bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
 	seq_printf(m,
@@ -2298,10 +2299,12 @@  static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 		   "link_id:\t%u\n"
 		   "prog_tag:\t%s\n"
 		   "prog_id:\t%u\n",
-		   link_type,
+		   bpf_link_type_strs[link->type],
 		   link->id,
 		   prog_tag,
 		   prog->aux->id);
+	if (link->ops->show_fdinfo)
+		link->ops->show_fdinfo(link, m);
 }
 #endif
 
@@ -2409,6 +2412,7 @@  struct bpf_link *bpf_link_get_from_fd(u32 ufd, fmode_t *link_mode)
 
 struct bpf_tracing_link {
 	struct bpf_link link;
+	enum bpf_attach_type attach_type;
 };
 
 static void bpf_tracing_link_release(struct bpf_link *link)
@@ -2424,9 +2428,35 @@  static void bpf_tracing_link_dealloc(struct bpf_link *link)
 	kfree(tr_link);
 }
 
+static void bpf_tracing_link_show_fdinfo(const struct bpf_link *link,
+					 struct seq_file *seq)
+{
+	struct bpf_tracing_link *tr_link =
+		container_of(link, struct bpf_tracing_link, link);
+
+	seq_printf(seq,
+		   "attach_type:\t%d\n",
+		   tr_link->attach_type);
+}
+
+static int bpf_tracing_link_fill_link_info(const struct bpf_link *link,
+					   struct bpf_link_info *info,
+					   const struct bpf_link_info *uinfo,
+					   u32 info_len)
+{
+	struct bpf_tracing_link *tr_link =
+		container_of(link, struct bpf_tracing_link, link);
+
+	info->tracing.attach_type = tr_link->attach_type;
+
+	return 0;
+}
+
 static const struct bpf_link_ops bpf_tracing_link_lops = {
 	.release = bpf_tracing_link_release,
 	.dealloc = bpf_tracing_link_dealloc,
+	.show_fdinfo = bpf_tracing_link_show_fdinfo,
+	.fill_link_info = bpf_tracing_link_fill_link_info,
 };
 
 static int bpf_tracing_prog_attach(struct bpf_prog *prog)
@@ -2466,7 +2496,9 @@  static int bpf_tracing_prog_attach(struct bpf_prog *prog)
 		err = -ENOMEM;
 		goto out_put_prog;
 	}
-	bpf_link_init(&link->link, &bpf_tracing_link_lops, prog);
+	bpf_link_init(&link->link, BPF_LINK_TYPE_TRACING,
+		      &bpf_tracing_link_lops, prog);
+	link->attach_type = prog->expected_attach_type;
 
 	err = bpf_link_prime(&link->link, &link_primer);
 	if (err) {
@@ -2508,9 +2540,66 @@  static void bpf_raw_tp_link_dealloc(struct bpf_link *link)
 	kfree(raw_tp);
 }
 
+static void bpf_raw_tp_link_show_fdinfo(const struct bpf_link *link,
+					struct seq_file *seq)
+{
+	struct bpf_raw_tp_link *raw_tp_link =
+		container_of(link, struct bpf_raw_tp_link, link);
+
+	seq_printf(seq,
+		   "tp_name:\t%s\n",
+		   raw_tp_link->btp->tp->name);
+}
+
+static int bpf_raw_tp_link_fill_link_info(const struct bpf_link *link,
+					  struct bpf_link_info *info,
+					  const struct bpf_link_info *uinfo,
+					  u32 info_len)
+{
+	struct bpf_raw_tp_link *raw_tp_link =
+		container_of(link, struct bpf_raw_tp_link, link);
+	u64 ubuf_ptr;
+	char __user *ubuf = u64_to_user_ptr(uinfo->raw_tracepoint.tp_name);
+	const char *tp_name = raw_tp_link->btp->tp->name;
+	size_t tp_len;
+	u32 ulen;
+
+	if (get_user(ulen, &uinfo->raw_tracepoint.tp_name_len))
+		return -EFAULT;
+	if (get_user(ubuf_ptr, &uinfo->raw_tracepoint.tp_name))
+		return -EFAULT;
+	ubuf = u64_to_user_ptr(ubuf_ptr);
+
+	if (ulen && !ubuf)
+		return -EINVAL;
+	if (!ubuf)
+		return 0;
+
+	tp_len = strlen(raw_tp_link->btp->tp->name);
+	info->raw_tracepoint.tp_name_len = tp_len + 1;
+	info->raw_tracepoint.tp_name = (u64)(unsigned long)ubuf;
+
+	if (ulen >= tp_len + 1) {
+		if (copy_to_user(ubuf, tp_name, tp_len + 1))
+			return -EFAULT;
+	} else {
+		char zero = '\0';
+
+		if (copy_to_user(ubuf, tp_name, ulen - 1))
+			return -EFAULT;
+		if (put_user(zero, ubuf + ulen - 1))
+			return -EFAULT;
+		return -ENOSPC;
+	}
+
+	return 0;
+}
+
 static const struct bpf_link_ops bpf_raw_tp_link_lops = {
 	.release = bpf_raw_tp_link_release,
 	.dealloc = bpf_raw_tp_link_dealloc,
+	.show_fdinfo = bpf_raw_tp_link_show_fdinfo,
+	.fill_link_info = bpf_raw_tp_link_fill_link_info,
 };
 
 #define BPF_RAW_TRACEPOINT_OPEN_LAST_FIELD raw_tracepoint.prog_fd
@@ -2576,7 +2665,8 @@  static int bpf_raw_tracepoint_open(const union bpf_attr *attr)
 		err = -ENOMEM;
 		goto out_put_btp;
 	}
-	bpf_link_init(&link->link, &bpf_raw_tp_link_lops, prog);
+	bpf_link_init(&link->link, BPF_LINK_TYPE_RAW_TRACEPOINT,
+		      &bpf_raw_tp_link_lops, prog);
 	link->btp = btp;
 
 	err = bpf_link_prime(&link->link, &link_primer);
@@ -3372,6 +3462,39 @@  static int bpf_btf_get_info_by_fd(struct btf *btf,
 	return btf_get_info_by_fd(btf, attr, uattr);
 }
 
+static int bpf_link_get_info_by_fd(struct bpf_link *link,
+				  const union bpf_attr *attr,
+				  union bpf_attr __user *uattr)
+{
+	struct bpf_link_info __user *uinfo = u64_to_user_ptr(attr->info.info);
+	struct bpf_link_info info;
+	u32 info_len = attr->info.info_len;
+	int err;
+
+	err = bpf_check_uarg_tail_zero(uinfo, sizeof(info), info_len);
+	if (err)
+		return err;
+	info_len = min_t(u32, sizeof(info), info_len);
+
+	memset(&info, 0, sizeof(info));
+	info.type = link->type;
+	info.id = link->id;
+	info.prog_id = link->prog->aux->id;
+
+	if (link->ops->fill_link_info) {
+		err = link->ops->fill_link_info(link, &info, uinfo, info_len);
+		if (err)
+			return err;
+	}
+
+	if (copy_to_user(uinfo, &info, info_len) ||
+	    put_user(info_len, &uattr->info.info_len))
+		return -EFAULT;
+
+	return 0;
+}
+
+
 #define BPF_OBJ_GET_INFO_BY_FD_LAST_FIELD info.info
 
 static int bpf_obj_get_info_by_fd(const union bpf_attr *attr,
@@ -3396,6 +3519,9 @@  static int bpf_obj_get_info_by_fd(const union bpf_attr *attr,
 					     uattr);
 	else if (f.file->f_op == &btf_fops)
 		err = bpf_btf_get_info_by_fd(f.file->private_data, attr, uattr);
+	else if (f.file->f_op == &bpf_link_fops)
+		err = bpf_link_get_info_by_fd(f.file->private_data,
+					      attr, uattr);
 	else
 		err = -EINVAL;
 
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 04c6630cc18f..48dd5fc7c269 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -28,9 +28,11 @@  static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
 #define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type) \
 	[_id] = & _name ## _verifier_ops,
 #define BPF_MAP_TYPE(_id, _ops)
+#define BPF_LINK_TYPE(_id, _name)
 #include <linux/bpf_types.h>
 #undef BPF_PROG_TYPE
 #undef BPF_MAP_TYPE
+#undef BPF_LINK_TYPE
 };
 
 /* bpf_check() is a static code analyzer that walks eBPF program
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 2e29a671d67e..d2f269082a33 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -113,6 +113,8 @@  enum bpf_cmd {
 	BPF_MAP_DELETE_BATCH,
 	BPF_LINK_CREATE,
 	BPF_LINK_UPDATE,
+	BPF_LINK_GET_FD_BY_ID,
+	BPF_LINK_GET_NEXT_ID,
 };
 
 enum bpf_map_type {
@@ -220,6 +222,15 @@  enum bpf_attach_type {
 
 #define MAX_BPF_ATTACH_TYPE __MAX_BPF_ATTACH_TYPE
 
+enum bpf_link_type {
+	BPF_LINK_TYPE_UNSPEC = 0,
+	BPF_LINK_TYPE_RAW_TRACEPOINT = 1,
+	BPF_LINK_TYPE_TRACING = 2,
+	BPF_LINK_TYPE_CGROUP = 3,
+
+	MAX_BPF_LINK_TYPE,
+};
+
 /* cgroup-bpf attach flags used in BPF_PROG_ATTACH command
  *
  * NONE(default): No further bpf programs allowed in the subtree.
@@ -523,6 +534,7 @@  union bpf_attr {
 			__u32		prog_id;
 			__u32		map_id;
 			__u32		btf_id;
+			__u32		link_id;
 		};
 		__u32		next_id;
 		__u32		open_flags;
@@ -3598,6 +3610,25 @@  struct bpf_btf_info {
 	__u32 id;
 } __attribute__((aligned(8)));
 
+struct bpf_link_info {
+	__u32 type;
+	__u32 id;
+	__u32 prog_id;
+	union {
+		struct {
+			__aligned_u64 tp_name; /* in/out: tp_name buffer ptr */
+			__u32 tp_name_len;     /* in/out: tp_name buffer len */
+		} raw_tracepoint;
+		struct {
+			__u32 attach_type;
+		} tracing;
+		struct {
+			__u64 cgroup_id;
+			__u32 attach_type;
+		} cgroup;
+	};
+} __attribute__((aligned(8)));
+
 /* User bpf_sock_addr struct to access socket fields and sockaddr struct passed
  * by user and intended to be used by socket (e.g. to bind to, depends on
  * attach attach type).