Android Operating System Requirements

Android Operating System Requirements

Android Debug Bridge, commonly known as adb, is the central Android development tool, both for application and operating system development. adb remount command is used for remounting Android system partition from read-only to writable. This is very important for operating system development, since it enables modifying and updating only one library until the issue is resolved, instead of flashing the whole partition, which lasts significantly longer and slows down the development. When Android Nougat was brought up using UBIFS for the first time, it has been noticed that adb remount was not functional. Investigation determined that the origin of the issue came from a security update provided by Google [3]. The update fixed a security breach where files under read-only file system could have been altered by writing raw bytes to block devices. As a final result, hacker would be obtaining Android root access, which provides the highest possible access and modification permissions [4]. Google resolved this issue by using BLKROSET ioctl(). Each time a read-only file system is mounted, its corresponding block device also is being set to read-only state and vice versa. This ioctl() behaves as a simple lock which forbids writing. However, Google’s solution assumed that the file system is mounted on a block device. BLKROSET is only intended for block devices and supported by those. This ioctl() would fail when used on a MTD based file system, like UBIFS. And the failure would result with adb remount command failing. BLKROSET is even being called during the initial file system mounting, but the failure does not stop or alter the boot sequence. Further investigation showed that this double lock mechanism is completely unnecessary for UBIFS. Regardless of whether a partition has been mounted as read-only or not, MTD and UBI devices could not be modified by a simple write operation. The results of the access test are shown in Table 1. First column specifies the file system device. Column named “BLKROSET” shows if this ioctl() has been enabled or not. Columns “write” shows if the corresponding file operation was available under the specified device. Last column specifies if the file system associated to the device has been mounted as read-only or not. Symbol “X” is used if the value is not important or if it does not make sense. code shoppy

Single UNIX Specification version 3 [6] specifies a function, posix_fallocate(fd, offset, len), that ensures the space is allocated on disk for the byte range specified by offset and len for the disk file referred to by the descriptor fd. This allows an application to be sure that a later write()to the file won’t fail because disk space is exhausted (which could otherwise occur if a hole in the file was filled in, or some other application consumed space on the disk). Historically, the glibc library implementation of this function achieved the desired result by writing a 0 byte into each block in the specified range. Since version 2.6.23Linux provides an fallocate() system call, which provides a more efficient way of ensuring that the necessary space is allocated, and that glibc posix_fallocate() implementation makes use of this system call when it is available [7]. Kernel side of fallocate() did not only implement the POSIX version of fallocate(). It expanded fallocate()capabilities by requiring its mode to be set when issuing the system call. Upon testing Android Nougat with UBIFS file system, it has been detected that installation of new applications is not functional. It has been determined that the root cause was the fact that fallocate() system call was not supported for UBIFS. Android source code inspection showed that there were several ways of using fallocate() mode throughout the entire operating system. The main issue here is that fallocate() system call has to be implemented for each of the file systems separately. Not even EXT4 and F2FS file systems support all possible fallocate() modes. But Google has decided to make mandatory some of those which are supported. Mandatory modes are: default (0) mode, keepsize mode and keep size punch hole mode. In order to provide Android UBIFS support, this missing functionality has been implemented. Figures in the paper will be used to explain previously mentioned modes. Fig. 1 is showing the existing file, which contains 14 characters. Each of the orange rectangles represents one of the previously written characters. This figure will be used as a base file for modifications achieved by using fallocate() system call. https://codeshoppy.com/android-app-ideas-for-students-college-project.html