libcutils: add android_get_control_file()

Solve one more issue where privilege is required to open a file and
we do not want to grant such to the service. This is the client side
of the picture, init is the server. The file's descriptor was placed
into the environment as "ANDROID_FILE_<path>" where non-alpha and
non-numeric characters in the <path> are replaced with _ and this
function picks the file descriptor up.

Added definition ANDROID_FILE_ENV_PREFIX ("ANDROID_FILE_") and
android_get_control_file() prototype in a new include <cutils/files.h>

android_get_control_file() checks if the resulting file descriptor is
valid, open and matches the name reference, which on purpose will fail
if a symbolic link is in the path rather than using a fully qualified
path.  Add gTest unit test for both.

Test: gTest libcutils_test --gtest_filter=FileTest.android_get_control_file
Bug: 32450474
Change-Id: I2d0310a1727f1e393a00d9fc7e6cf5d028f27905

include/cutils/files.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef __CUTILS_FILES_H
+#define __CUTILS_FILES_H
+
+#define ANDROID_FILE_ENV_PREFIX "ANDROID_FILE_"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * android_get_control_file - simple helper function to get the file
+ * descriptor of our init-managed file. `path' is the filename path as
+ * given in init.rc. Returns -1 on error.
+ */
+int android_get_control_file(const char* path);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* __CUTILS_FILES_H */

libcutils/Android.bp

@@ -34,6 +34,7 @@ cc_library {
     host_supported: true,
     srcs: [
         "config_utils.c",
+        "files.cpp",
         "fs_config.c",
         "canned_fs_config.c",
         "hashmap.c",

libcutils/files.cpp

@@ -0,0 +1,111 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+// This file contains files implementation that can be shared between
+// platforms as long as the correct headers are included.
+#define _GNU_SOURCE 1 // for asprintf
+
+#include <ctype.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <cutils/files.h>
+
+#ifndef TEMP_FAILURE_RETRY // _WIN32 does not define
+#define TEMP_FAILURE_RETRY(exp) (exp)
+#endif
+
+int android_get_control_file(const char* path) {
+    if (!path) return -1;
+
+    char *key = NULL;
+    if (asprintf(&key, ANDROID_FILE_ENV_PREFIX "%s", path) < 0) return -1;
+    if (!key) return -1;
+
+    char *cp = key;
+    while (*cp) {
+        if (!isalnum(*cp)) *cp = '_';
+        ++cp;
+    }
+
+    const char* val = getenv(key);
+    free(key);
+    if (!val) return -1;
+
+    errno = 0;
+    long fd = strtol(val, NULL, 10);
+    if (errno) return -1;
+
+    // validity checking
+    if ((fd < 0) || (fd > INT_MAX)) return -1;
+#if defined(_SC_OPEN_MAX)
+    if (fd >= sysconf(_SC_OPEN_MAX)) return -1;
+#elif defined(OPEN_MAX)
+    if (fd >= OPEN_MAX) return -1;
+#elif defined(_POSIX_OPEN_MAX)
+    if (fd >= _POSIX_OPEN_MAX) return -1;
+#endif
+
+#if defined(F_GETFD)
+    if (TEMP_FAILURE_RETRY(fcntl(fd, F_GETFD)) < 0) return -1;
+#elif defined(F_GETFL)
+    if (TEMP_FAILURE_RETRY(fcntl(fd, F_GETFL)) < 0) return -1;
+#else
+    struct stat s;
+    if (TEMP_FAILURE_RETRY(fstat(fd, &s)) < 0) return -1;
+#endif
+
+#if defined(__linux__)
+    char *proc = NULL;
+    if (asprintf(&proc, "/proc/self/fd/%ld", fd) < 0) return -1;
+    if (!proc) return -1;
+
+    size_t len = strlen(path);
+    char *buf = static_cast<char *>(calloc(1, len + 2));
+    if (!buf) {
+        free(proc);
+        return -1;
+    }
+    ssize_t ret = TEMP_FAILURE_RETRY(readlink(proc, buf, len + 1));
+    free(proc);
+    int cmp = (len != static_cast<size_t>(ret)) || strcmp(buf, path);
+    free(buf);
+    if (ret < 0) return -1;
+    if (cmp != 0) return -1;
+#endif
+
+    // It is what we think it is
+    return static_cast<int>(fd);
+}

libcutils/tests/Android.bp

@@ -14,7 +14,7 @@
 
 cc_defaults {
     name: "libcutils_test_default",
-    srcs: ["sockets_test.cpp"],
+    srcs: ["sockets_test.cpp", "files_test.cpp"],
 
     target: {
         android: {

libcutils/tests/files_test.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <time.h>
+
+#include <cutils/files.h>
+#include <gtest/gtest.h>
+
+TEST(FilesTest, android_get_control_file) {
+    static const char key[] = ANDROID_FILE_ENV_PREFIX "_dev_kmsg";
+    static const char name[] = "/dev/kmsg";
+
+    EXPECT_EQ(unsetenv(key), 0);
+    EXPECT_EQ(android_get_control_file(name), -1);
+
+    int fd;
+    ASSERT_GE(fd = open(name, O_RDONLY | O_CLOEXEC), 0);
+    EXPECT_EQ(android_get_control_file(name), -1);
+
+    char val[32];
+    snprintf(val, sizeof(val), "%d", fd);
+    EXPECT_EQ(setenv(key, val, true), 0);
+
+    EXPECT_EQ(android_get_control_file(name), fd);
+    close(fd);
+    EXPECT_EQ(android_get_control_file(name), -1);
+    EXPECT_EQ(unsetenv(key), 0);
+    EXPECT_EQ(android_get_control_file(name), -1);
+}