bpf: btf: Add BPF_OBJ_GET_INFO_BY_FD support to BTF fd

This patch adds BPF_OBJ_GET_INFO_BY_FD support to BTF fd.
The original BTF data, which was used to create the BTF fd during
the earlier BPF_BTF_LOAD call, will be returned.

The userspace is expected to allocate buffer
to info.info and the buffer size is set to info.info_len before
calling BPF_OBJ_GET_INFO_BY_FD.

The original BTF data is copied to the userspace buffer (info.info).
Only upto the user's specified info.info_len will be copied.

The original BTF data size is set to info.info_len.  The userspace
needs to check if it is bigger than its allocated buffer size.
If it is, the userspace should realloc with the kernel-returned
info.info_len and call the BPF_OBJ_GET_INFO_BY_FD again.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/btf.h

@@ -10,9 +10,14 @@ struct btf;
 struct btf_type;
 union bpf_attr;
 
+extern const struct file_operations btf_fops;
+
 void btf_put(struct btf *btf);
 int btf_new_fd(const union bpf_attr *attr);
 struct btf *btf_get_by_fd(int fd);
+int btf_get_info_by_fd(const struct btf *btf,
+		       const union bpf_attr *attr,
+		       union bpf_attr __user *uattr);
 /* Figure out the size of a type_id.  If type_id is a modifier
  * (e.g. const), it will be resolved to find out the type with size.
  *

kernel/bpf/btf.c

@@ -2002,7 +2002,7 @@ static int btf_release(struct inode *inode, struct file *filp)
 	return 0;
 }
 
-static const struct file_operations btf_fops = {
+const struct file_operations btf_fops = {
 	.release	= btf_release,
 };
 
@@ -2047,3 +2047,18 @@ struct btf *btf_get_by_fd(int fd)
 
 	return btf;
 }
+
+int btf_get_info_by_fd(const struct btf *btf,
+		       const union bpf_attr *attr,
+		       union bpf_attr __user *uattr)
+{
+	void __user *udata = u64_to_user_ptr(attr->info.info);
+	u32 copy_len = min_t(u32, btf->data_size,
+			     attr->info.info_len);
+
+	if (copy_to_user(udata, btf->data, copy_len) ||
+	    put_user(btf->data_size, &uattr->info.info_len))
+		return -EFAULT;
+
+	return 0;
+}

kernel/bpf/syscall.c

@@ -2017,6 +2017,8 @@ static int bpf_obj_get_info_by_fd(const union bpf_attr *attr,
 	else if (f.file->f_op == &bpf_map_fops)
 		err = bpf_map_get_info_by_fd(f.file->private_data, attr,
 					     uattr);
+	else if (f.file->f_op == &btf_fops)
+		err = btf_get_info_by_fd(f.file->private_data, attr, uattr);
 	else
 		err = -EINVAL;