diff --git a/init/README.md b/init/README.md
index 99522b9..709c667 100644
--- a/init/README.md
+++ b/init/README.md
@@ -282,6 +282,11 @@
 `copy <src> <dst>`
 > Copies a file. Similar to write, but useful for binary/large
   amounts of data.
+  Regarding to the src file, copying from symbol link file and world-writable
+  or group-writable files are not allowed.
+  Regarding to the dst file, the default mode created is 0600 if it does not
+  exist. And it will be truncated if dst file is a normal regular file and
+  already exists.
 
 `domainname <name>`
 > Set the domain name.
diff --git a/init/builtins.cpp b/init/builtins.cpp
index 43eb378..c7dffb1 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -839,61 +839,11 @@
 }
 
 static int do_copy(const std::vector<std::string>& args) {
-    char *buffer = NULL;
-    int rc = 0;
-    int fd1 = -1, fd2 = -1;
-    struct stat info;
-    int brtw, brtr;
-    char *p;
-
-    if (stat(args[1].c_str(), &info) < 0)
-        return -1;
-
-    if ((fd1 = open(args[1].c_str(), O_RDONLY|O_CLOEXEC)) < 0)
-        goto out_err;
-
-    if ((fd2 = open(args[2].c_str(), O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, 0660)) < 0)
-        goto out_err;
-
-    if (!(buffer = (char*) malloc(info.st_size)))
-        goto out_err;
-
-    p = buffer;
-    brtr = info.st_size;
-    while(brtr) {
-        rc = read(fd1, p, brtr);
-        if (rc < 0)
-            goto out_err;
-        if (rc == 0)
-            break;
-        p += rc;
-        brtr -= rc;
+    std::string data;
+    if (read_file(args[1].c_str(), &data)) {
+        return write_file(args[2].c_str(), data.data()) ? 0 : 1;
     }
-
-    p = buffer;
-    brtw = info.st_size;
-    while(brtw) {
-        rc = write(fd2, p, brtw);
-        if (rc < 0)
-            goto out_err;
-        if (rc == 0)
-            break;
-        p += rc;
-        brtw -= rc;
-    }
-
-    rc = 0;
-    goto out;
-out_err:
-    rc = -1;
-out:
-    if (buffer)
-        free(buffer);
-    if (fd1 >= 0)
-        close(fd1);
-    if (fd2 >= 0)
-        close(fd2);
-    return rc;
+    return 1;
 }
 
 static int do_chown(const std::vector<std::string>& args) {
diff --git a/init/util.cpp b/init/util.cpp
index c987181..9b02de2 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -184,8 +184,8 @@
 }
 
 bool write_file(const char* path, const char* content) {
-    android::base::unique_fd fd(
-        TEMP_FAILURE_RETRY(open(path, O_WRONLY | O_CREAT | O_NOFOLLOW | O_CLOEXEC, 0600)));
+    android::base::unique_fd fd(TEMP_FAILURE_RETRY(
+        open(path, O_WRONLY | O_CREAT | O_NOFOLLOW | O_TRUNC | O_CLOEXEC, 0600)));
     if (fd == -1) {
         PLOG(ERROR) << "write_file: Unable to open '" << path << "'";
         return false;
diff --git a/init/util_test.cpp b/init/util_test.cpp
index 24c75c4..4e82e76 100644
--- a/init/util_test.cpp
+++ b/init/util_test.cpp
@@ -17,9 +17,15 @@
 #include "util.h"
 
 #include <errno.h>
+#include <fcntl.h>
+
+#include <sys/stat.h>
 
 #include <gtest/gtest.h>
 
+#include <android-base/stringprintf.h>
+#include <android-base/test_utils.h>
+
 TEST(util, read_file_ENOENT) {
   std::string s("hello");
   errno = 0;
@@ -28,6 +34,35 @@
   EXPECT_EQ("", s); // s was cleared.
 }
 
+TEST(util, read_file_group_writeable) {
+    std::string s("hello");
+    TemporaryFile tf;
+    ASSERT_TRUE(tf.fd != -1);
+    EXPECT_TRUE(write_file(tf.path, s.c_str())) << strerror(errno);
+    EXPECT_NE(-1, fchmodat(AT_FDCWD, tf.path, 0620, AT_SYMLINK_NOFOLLOW)) << strerror(errno);
+    EXPECT_FALSE(read_file(tf.path, &s)) << strerror(errno);
+    EXPECT_EQ("", s);  // s was cleared.
+}
+
+TEST(util, read_file_world_writeable) {
+    std::string s("hello");
+    TemporaryFile tf;
+    ASSERT_TRUE(tf.fd != -1);
+    EXPECT_TRUE(write_file(tf.path, s.c_str())) << strerror(errno);
+    EXPECT_NE(-1, fchmodat(AT_FDCWD, tf.path, 0602, AT_SYMLINK_NOFOLLOW)) << strerror(errno);
+    EXPECT_FALSE(read_file(tf.path, &s)) << strerror(errno);
+    EXPECT_EQ("", s);  // s was cleared.
+}
+
+TEST(util, read_file_symbol_link) {
+    std::string s("hello");
+    errno = 0;
+    // lrwxrwxrwx 1 root root 13 1970-01-01 00:00 charger -> /sbin/healthd
+    EXPECT_FALSE(read_file("/charger", &s));
+    EXPECT_EQ(ELOOP, errno);
+    EXPECT_EQ("", s);  // s was cleared.
+}
+
 TEST(util, read_file_success) {
   std::string s("hello");
   EXPECT_TRUE(read_file("/proc/version", &s));
@@ -37,6 +72,42 @@
   EXPECT_STREQ("Linux", s.c_str());
 }
 
+TEST(util, write_file_not_exist) {
+    std::string s("hello");
+    std::string s2("hello");
+    TemporaryDir test_dir;
+    std::string path = android::base::StringPrintf("%s/does-not-exist", test_dir.path);
+    EXPECT_TRUE(write_file(path.c_str(), s.c_str()));
+    EXPECT_TRUE(read_file(path.c_str(), &s2));
+    EXPECT_EQ(s, s2);
+    struct stat sb;
+    int fd = open(path.c_str(), O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
+    EXPECT_NE(-1, fd);
+    EXPECT_EQ(0, fstat(fd, &sb));
+    EXPECT_NE(0u, sb.st_mode & S_IRUSR);
+    EXPECT_NE(0u, sb.st_mode & S_IWUSR);
+    EXPECT_EQ(0u, sb.st_mode & S_IXUSR);
+    EXPECT_EQ(0u, sb.st_mode & S_IRGRP);
+    EXPECT_EQ(0u, sb.st_mode & S_IWGRP);
+    EXPECT_EQ(0u, sb.st_mode & S_IXGRP);
+    EXPECT_EQ(0u, sb.st_mode & S_IROTH);
+    EXPECT_EQ(0u, sb.st_mode & S_IWOTH);
+    EXPECT_EQ(0u, sb.st_mode & S_IXOTH);
+    EXPECT_EQ(0, unlink(path.c_str()));
+}
+
+TEST(util, write_file_exist) {
+    std::string s2("");
+    TemporaryFile tf;
+    ASSERT_TRUE(tf.fd != -1);
+    EXPECT_TRUE(write_file(tf.path, "1hello1")) << strerror(errno);
+    EXPECT_TRUE(read_file(tf.path, &s2));
+    EXPECT_STREQ("1hello1", s2.c_str());
+    EXPECT_TRUE(write_file(tf.path, "2hello2"));
+    EXPECT_TRUE(read_file(tf.path, &s2));
+    EXPECT_STREQ("2hello2", s2.c_str());
+}
+
 TEST(util, decode_uid) {
   EXPECT_EQ(0U, decode_uid("root"));
   EXPECT_EQ(UINT_MAX, decode_uid("toot"));