Android M 外部存储剖析

// Create a private mount namespace and bind mount appropriate emulated
// storage for the given user.
static bool MountEmulatedStorage(uid_t uid, jint mount_mode,
        bool force_mount_namespace) {
    // See storage config details at http://source.android.com/tech/storage/

    // Create a second private mount namespace for our process
    if (unshare(CLONE_NEWNS) == -1) {
        ALOGW("Failed to unshare(): %s", strerror(errno));
        return false;
    }

    // Unmount storage provided by root namespace and mount requested view
    UnmountTree("/storage");

    String8 storageSource;
    if (mount_mode == MOUNT_EXTERNAL_DEFAULT) {
        storageSource = "/mnt/runtime/default";
    } else if (mount_mode == MOUNT_EXTERNAL_READ) {
        storageSource = "/mnt/runtime/read";
    } else if (mount_mode == MOUNT_EXTERNAL_WRITE) {
        storageSource = "/mnt/runtime/write";
    } else {
        // Sane default of no storage visible
        return true;
    }
    if (TEMP_FAILURE_RETRY(mount(storageSource.string(), "/storage",
            NULL, MS_BIND | MS_REC | MS_SLAVE, NULL)) == -1) {
        ALOGW("Failed to mount %s to /storage: %s", storageSource.string(), strerror(errno));
        return false;
    }

    // Mount user-specific symlink helper into place
    userid_t user_id = multiuser_get_user_id(uid);
    const String8 userSource(String8::format("/mnt/user/%d", user_id));
    if (fs_prepare_dir(userSource.string(), 0751, 0, 0) == -1) {
        return false;
    }
    if (TEMP_FAILURE_RETRY(mount(userSource.string(), "/storage/self",
            NULL, MS_BIND, NULL)) == -1) {
        ALOGW("Failed to mount %s to /storage/self: %s", userSource.string(), strerror(errno));
        return false;
    }

    return true;
}

grantRuntimePermission
  -> onExternalStoragePolicyChanged
    -->remountUidExternalStorage
    mConnector.execute("volume", "remount_uid", uid, modeName);

CommandListener::VolumeCmd::runCommand
  ->sendGenericOkFail(cli, vm->remountUid(uid, mode));

int VolumeManager::remountUid(uid_t uid, const std::string& mode) {
    LOG(DEBUG) << "Remounting " << uid << " as mode " << mode;

    DIR* dir;
    struct dirent* de;
    char rootName[PATH_MAX];
    char pidName[PATH_MAX];
    int pidFd;
    int nsFd;
    struct stat sb;
    pid_t child;

    if (!(dir = opendir("/proc"))) {
        PLOG(ERROR) << "Failed to opendir";
        return -1;
    }

    // Figure out root namespace to compare against below
    if (sane_readlinkat(dirfd(dir), "1/ns/mnt", rootName, PATH_MAX) == -1) {
        PLOG(ERROR) << "Failed to readlink";
        closedir(dir);
        return -1;
    }

    // Poke through all running PIDs look for apps running as UID
    while ((de = readdir(dir))) {
        pidFd = -1;
        nsFd = -1;

        pidFd = openat(dirfd(dir), de->d_name, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
        if (pidFd < 0) {
            goto next;
        }
        if (fstat(pidFd, &sb) != 0) {
            PLOG(WARNING) << "Failed to stat " << de->d_name;
            goto next;
        }
        if (sb.st_uid != uid) {
            goto next;
        }

        // Matches so far, but refuse to touch if in root namespace
        LOG(DEBUG) << "Found matching PID " << de->d_name;
        if (sane_readlinkat(pidFd, "ns/mnt", pidName, PATH_MAX) == -1) {
            PLOG(WARNING) << "Failed to read namespace for " << de->d_name;
            goto next;
        }
        if (!strcmp(rootName, pidName)) {
            LOG(WARNING) << "Skipping due to root namespace";
            goto next;
        }

        // We purposefully leave the namespace open across the fork
        nsFd = openat(pidFd, "ns/mnt", O_RDONLY);
        if (nsFd < 0) {
            PLOG(WARNING) << "Failed to open namespace for " << de->d_name;
            goto next;
        }

        if (!(child = fork())) {
            if (setns(nsFd, CLONE_NEWNS) != 0) {
                PLOG(ERROR) << "Failed to setns for " << de->d_name;
                _exit(1);
            }

            unmount_tree("/storage");

            std::string storageSource;
            if (mode == "default") {
                storageSource = "/mnt/runtime/default";
            } else if (mode == "read") {
                storageSource = "/mnt/runtime/read";
            } else if (mode == "write") {
                storageSource = "/mnt/runtime/write";
            } else {
                // Sane default of no storage visible
                _exit(0);
            }
            if (TEMP_FAILURE_RETRY(mount(storageSource.c_str(), "/storage",
                    NULL, MS_BIND | MS_REC | MS_SLAVE, NULL)) == -1) {
                PLOG(ERROR) << "Failed to mount " << storageSource << " for "
                        << de->d_name;
                _exit(1);
            }

            // Mount user-specific symlink helper into place
            userid_t user_id = multiuser_get_user_id(uid);
            std::string userSource(StringPrintf("/mnt/user/%d", user_id));
            if (TEMP_FAILURE_RETRY(mount(userSource.c_str(), "/storage/self",
                    NULL, MS_BIND, NULL)) == -1) {
                PLOG(ERROR) << "Failed to mount " << userSource << " for "
                        << de->d_name;
                _exit(1);
            }

            _exit(0);
        }

        if (child == -1) {
            PLOG(ERROR) << "Failed to fork";
            goto next;
        } else {
            TEMP_FAILURE_RETRY(waitpid(child, nullptr, 0));
        }

next:
        close(nsFd);
        close(pidFd)
    }
    closedir(dir);
    return 0;
}