[PATCH RFC bpf-next 1/7] bpf: Add bpf_btf_get_struct_member_info() helper

[Siddharth Nayyar]

Add a new BPF helper bpf_btf_get_struct_member_info() for
BPF_PROG_TYPE_SYSCALL programs to query struct member offset and index
(or the struct size when the member name is NULL) from vmlinux or
module BTF.

Given a struct's BTF type ID and an optional BTF object FD (0 for
vmlinux BTF), the helper returns a 64-bit value with the member's byte
offset in the lower 32 bits and the member's index in the upper 32 bits
when a member name is provided, or the byte size of the struct when the
member name is NULL.

This helper will be used by BPF light skeletons (gen_loader) to
initialize struct_ops maps and programs against kernel and module BTF
at load time.

Signed-off-by: Siddharth Nayyar <sidnayyar@xxxxxxxxxx>
---
 include/linux/bpf.h            |  1 +
 include/uapi/linux/bpf.h       | 13 ++++++++
 kernel/bpf/btf.c               | 73 ++++++++++++++++++++++++++++++++++++++++++
 kernel/bpf/syscall.c           |  2 ++
 tools/include/uapi/linux/bpf.h | 13 ++++++++
 5 files changed, 102 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index b011acc22c50..ab4d995b529c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -4206,6 +4206,7 @@ extern const struct bpf_func_proto bpf_task_storage_delete_recur_proto;
 extern const struct bpf_func_proto bpf_task_storage_delete_proto;
 extern const struct bpf_func_proto bpf_for_each_map_elem_proto;
 extern const struct bpf_func_proto bpf_btf_find_by_name_kind_proto;
+extern const struct bpf_func_proto bpf_btf_get_struct_member_info_proto;
 extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
 extern const struct bpf_func_proto bpf_sk_getsockopt_proto;
 extern const struct bpf_func_proto bpf_sk_setsockopt_nodelay_proto;
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 62de0919db72..3a630411b920 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -6065,6 +6065,18 @@ union bpf_attr {
  *		0 on success.
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
+ *
+ * long bpf_btf_get_struct_member_info(u32 btf_id, int btf_obj_fd, const char *name, int name_sz)
+ *	Description
+ *		Find a struct member with given *name* in the struct type identified
+ *		by *btf_id* in the BTF object referenced by *btf_obj_fd* (or in
+ *		vmlinux BTF if *btf_obj_fd* is 0). If *name* is **NULL** (and
+ *		*name_sz* is 0), return the size of the struct instead.
+ *	Return
+ *		Returns the member's byte offset in the lower 32 bits and the
+ *		member's index in the upper 32 bits when *name* is non-**NULL**,
+ *		or the byte size of the struct when *name* is **NULL**. On error, a
+ *		negative error code is returned.
  */
 #define ___BPF_FUNC_MAPPER(FN, ctx...)			\
 	FN(unspec, 0, ##ctx)				\
@@ -6279,6 +6291,7 @@ union bpf_attr {
 	FN(user_ringbuf_drain, 209, ##ctx)		\
 	FN(cgrp_storage_get, 210, ##ctx)		\
 	FN(cgrp_storage_delete, 211, ##ctx)		\
+	FN(btf_get_struct_member_info, 212, ##ctx)	\
 	/* This helper list is effectively frozen. If you are trying to	\
 	 * add a new helper, you should add a kfunc instead which has	\
 	 * less stability guarantees. See Documentation/bpf/kfuncs.rst	\
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 228b93d4f8c9..8d9188dda34e 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -9682,6 +9682,79 @@ const struct bpf_func_proto bpf_btf_find_by_name_kind_proto = {
 	.arg4_type	= ARG_ANYTHING,
 };
 
+BPF_CALL_4(bpf_btf_get_struct_member_info, u32, btf_id, int, btf_obj_fd,
+	   const char *, name, int, name_sz)
+{
+	const struct btf_member *member;
+	const struct btf_type *t;
+	struct btf *btf;
+	long ret = -EINVAL;
+	u32 i;
+
+	if (!name) {
+		if (name_sz)
+			return -EINVAL;
+	} else if (name_sz <= 1 || !*name || name[name_sz - 1]) {
+		return -EINVAL;
+	}
+
+	if (btf_obj_fd) {
+		btf = btf_get_by_fd(btf_obj_fd);
+		if (IS_ERR(btf))
+			return PTR_ERR(btf);
+	} else {
+		btf = bpf_get_btf_vmlinux();
+		if (IS_ERR(btf))
+			return PTR_ERR(btf);
+		if (!btf)
+			return -EINVAL;
+		btf_get(btf);
+	}
+
+	t = btf_type_by_id(btf, btf_id);
+	if (!t || !__btf_type_is_struct(t))
+		goto out;
+
+	if (!name) {
+		ret = t->size;
+		goto out;
+	}
+
+	ret = -ENOENT;
+	for_each_member(i, t, member) {
+		const char *mname = btf_name_by_offset(btf, member->name_off);
+
+		if (!mname || !*mname)
+			continue;
+		if (!strcmp(mname, name)) {
+			u32 moff;
+
+			if (__btf_member_bitfield_size(t, member)) {
+				ret = -EOPNOTSUPP;
+				goto out;
+			}
+			moff = __btf_member_bit_offset(t, member) / 8;
+			ret = moff | (((u64)i) << 32);
+			goto out;
+		}
+	}
+
+out:
+	btf_put(btf);
+	return ret;
+}
+
+const struct bpf_func_proto bpf_btf_get_struct_member_info_proto = {
+	.func		= bpf_btf_get_struct_member_info,
+	.gpl_only	= false,
+	.might_sleep	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_ANYTHING,
+	.arg2_type	= ARG_ANYTHING,
+	.arg3_type	= ARG_PTR_TO_MEM | MEM_RDONLY | PTR_MAYBE_NULL,
+	.arg4_type	= ARG_MEM_SIZE_OR_ZERO,
+};
+
 BTF_ID_LIST_GLOBAL(btf_tracing_ids, MAX_BTF_TRACING_TYPE)
 #define BTF_TRACING_TYPE(name, type) BTF_ID(struct, type)
 BTF_TRACING_TYPE_xxx
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 9004f2673ccf..6e7b6b10603e 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -6756,6 +6756,8 @@ syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		       ? NULL : &bpf_sys_bpf_proto;
 	case BPF_FUNC_btf_find_by_name_kind:
 		return &bpf_btf_find_by_name_kind_proto;
+	case BPF_FUNC_btf_get_struct_member_info:
+		return &bpf_btf_get_struct_member_info_proto;
 	case BPF_FUNC_sys_close:
 		return &bpf_sys_close_proto;
 	case BPF_FUNC_kallsyms_lookup_name:
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 62de0919db72..3a630411b920 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -6065,6 +6065,18 @@ union bpf_attr {
  *		0 on success.
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
+ *
+ * long bpf_btf_get_struct_member_info(u32 btf_id, int btf_obj_fd, const char *name, int name_sz)
+ *	Description
+ *		Find a struct member with given *name* in the struct type identified
+ *		by *btf_id* in the BTF object referenced by *btf_obj_fd* (or in
+ *		vmlinux BTF if *btf_obj_fd* is 0). If *name* is **NULL** (and
+ *		*name_sz* is 0), return the size of the struct instead.
+ *	Return
+ *		Returns the member's byte offset in the lower 32 bits and the
+ *		member's index in the upper 32 bits when *name* is non-**NULL**,
+ *		or the byte size of the struct when *name* is **NULL**. On error, a
+ *		negative error code is returned.
  */
 #define ___BPF_FUNC_MAPPER(FN, ctx...)			\
 	FN(unspec, 0, ##ctx)				\
@@ -6279,6 +6291,7 @@ union bpf_attr {
 	FN(user_ringbuf_drain, 209, ##ctx)		\
 	FN(cgrp_storage_get, 210, ##ctx)		\
 	FN(cgrp_storage_delete, 211, ##ctx)		\
+	FN(btf_get_struct_member_info, 212, ##ctx)	\
 	/* This helper list is effectively frozen. If you are trying to	\
 	 * add a new helper, you should add a kfunc instead which has	\
 	 * less stability guarantees. See Documentation/bpf/kfuncs.rst	\

-- 
2.56.0.455.g6c5a503b38-goog