C Test Case Tutorial ==================== NOTE: See also https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines[Test Writing Guidelines], https://github.com/linux-test-project/ltp/wiki/C-Test-API[C Test API]. This is a step-by-step tutorial on writing a simple C LTP test, where topics of the LTP and Linux kernel testing will be introduced gradually using a concrete example. Most sections will include exercises, some trivial and others not so much. If you find an exercise is leading you off at too much of a tangent, just leave it for later and move on. LTP tests can be written in C or Shell script. This tutorial is only for tests written in C using the new LTP test API. Note that while we go into some detail on using Git, this is not intended as a canonical or complete guide for Git. 0. Assumptions & Feedback ------------------------- We assume the reader is familiar with C, Git and common Unix/Linux/GNU tools and has some general knowledge of Operating Systems. Experienced Linux developers may find it too verbose while people new to system level Linux development may find it overwhelming. Comments and feedback are welcome, please direct them to the mailing list (see +README+). 1. Getting Started ------------------ Git-clone the main LTP repository as described in the +README+ and change directory to the checked-out Git repository. We recommend installing the LTP and running one of the tests mentioned in the Quick guide (in the +README+) to ensure you are starting from a good state. We also recommended cloning the Linux kernel repository for reference, this guide will refer to files and directories within the mainline kernel 4.12. [source,shell] ------------------------------------------------------------------------------ $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git ------------------------------------------------------------------------------ There are a number of other repositories which are useful for reference as well, including the GNU C library +glibc+ and the alternative C library +musl+. Some system calls are partially or even entirely implemented in user land as part of the standard C library. So in these cases, the C library is an important reference. +glibc+ is the most common C library for Linux, however +musl+ is generally easier to understand. How system calls are implemented varies from one architecture to another and across kernel and C library versions. To find out whether a system call is actually accessing the kernel (whether it is actually a system call) on any given machine you can use the +strace+ utility. This intercepts system calls made by an executable and prints them. We will use this later in the tutorial. 2. Choose a System Call to test ------------------------------- We will use the +statx()+ system call, to provide a concrete example of a test. At the time of writing there is no test for this call which was introduced in Linux kernel version 4.11. Linux system call specific tests are primarily contained in +testcases/kernel/syscalls+, but you should also +git grep+ the entire LTP repository to check for any existing usages of a system call. One way to find a system call which is not currently tested by the LTP is to look at +include/linux/syscalls.h+ in the kernel tree. Something the LTP excels at is ensuring bug-fixes are back ported to maintenance releases, so targeting a specific regression is another option. 2.1. Find an untested System call ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Try to find an untested system call which has a manual page (i.e. +man syscall+ produces a result). It is a good idea to Git-clone the latest kernel man-pages repository. [source,shell] ------------------------------------------------------------------------------ $ git clone git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git ------------------------------------------------------------------------------ At the time of writing the difference between the latest man-pages release and the +HEAD+ of the repository (usually the latest commit) is well over 100 commits. This represents about 9 weeks of changes. If you are using a stable Linux distribution, your man-pages package may well be years old. So as with the kernel, it is best to have the Git repository as a reference. You could also find a system call with untested parameters or use whatever it is you are planning to use the LTP for. 3. Create the test skeleton --------------------------- I shall call my test +statx01.c+, by the time you read this that file name will probably be taken, so increment the number in the file name as appropriate or replace +statx+ with the system call chosen in exercise 2.1. [source,shell] ------------------------------------------------------------------------------ $ mkdir testcases/kernel/syscalls/statx $ cd testcases/kernel/syscalls/statx $ echo statx >> .gitignore ------------------------------------------------------------------------------ Next open +statx01.c+ and add the following boilerplate. Make sure to change the copy right notice to your name/company, correct the test name and minimum kernel version if necessary. I will explain what the code does below. [source,c] ------------------------------------------------------------------------------ // SPDX-License-Identifier: GPL-2.0-or-later /* * Copyright (c) 2017 Instruction Ignorer <"can't"@be.bothered.com> */ /* * Test statx * * All tests should start with a description of _what_ we are testing. * Non-trivial explanations of _how_ the code works should also go here. * Include relevant links, Git commit hashes and CVE numbers. * Inline comments should be avoided. */ #include "tst_test.h" static void run(void) { tst_res(TPASS, "Doing hardly anything is easy"); } static struct tst_test test = { .test_all = run, .min_kver = "4.11", }; ------------------------------------------------------------------------------ Starting with the +#include+ statement we copy in the main LTP test library headers. This includes the most common test API functions and the test harness initialisation code. It is important to note that this is a completely ordinary, independent C program, however +main()+ is missing because it is implemented in +tst_test.h+. We specify what code we want to run as part of the test using the +tst_test test+ structure. Various callbacks can be set by the test writer, including +test.test_all+, which we have set to +run()+. The test harness will execute this callback in a separate process (using +fork()+), forcibly terminating it if it does not return after +test.timeout+ seconds. We have also set +test.min_kver+ to the kernel version where +statx+ was introduced. The test library will determine the kernel version at runtime. If the version is less than 4.11 then the test harness will return +TCONF+, indicating that this test is not suitable for the current system configuration. Occasionally features are back ported to older kernel versions, so +statx+ may exist on kernels with a lower version. However we don't need to worry about that unless there is evidence of it happening. As mentioned in the code itself, you should specify what you are testing and the expected outcome, even if it is relatively simple. If your program flow is necessarily complex and difficult to understand (which is often the case when trying to manipulate the kernel into doing something bad), then a detailed explanation of how the code works is welcome. What you should not do, is use inline comments or include the same level of explanation which is written here. As a general rule, if something is easy to document, then the code should also be easy to read. So don't document the easy stuff (except for the basic test specification). Before continuing we should compile this and check that the basics work. In order to compile the test we need a +Makefile+ in the same subdirectory. If one already exists, then nothing needs to be done, otherwise add one with the following contents. [source,make] ------------------------------------------------------------------------------ # SPDX-License-Identifier: GPL-2.0-or-later # Copyright (c) 2019 Linux Test Project top_srcdir ?= ../../../.. include $(top_srcdir)/include/mk/testcases.mk include $(top_srcdir)/include/mk/generic_leaf_target.mk ------------------------------------------------------------------------------ This will automatically add +statx01.c+ as a build target producing a +statx01+ executable. Unless you have heavily deviated from the tutorial, and probably need to change +top_srcdir+, nothing else needs to be done. Normally, if you were starting a Makefile from scratch, then you would need to add +statx01+ as a build target. Specifying that you would like to run some program (e.g. +gcc+ or +clang+) to transform +statx01.c+ into +statx01+. Here we don't need to do that, but sometimes it is still necessary. For example, if we needed to link to the POSIX threading library, then we could add the following line after +testcases.mk+. [source,make] -------------------------------------------------------------------------------- statx01: CFLAGS += -pthread -------------------------------------------------------------------------------- Assuming you are in the test's subdirectory +testcases/kernel/syscalls/statx+, do [source,shell] -------------------------------------------------------------------------------- $ make $ ./statx01 -------------------------------------------------------------------------------- This should build the test and then run it. However, even though the test is in the +syscalls+ directory it won't be automatically ran as part of the _syscalls_ test group (remember +./runltp -f syscalls+ from the +README+?). For this we need to add it to the +runtest+ file. So open +runtest/statx+ and add the lines starting with a +++. [source,diff] -------------------------------------------------------------------------------- statvfs01 statvfs01 statvfs02 statvfs02 +statx01 statx01 + stime01 stime01 stime02 stime02 -------------------------------------------------------------------------------- The +runtest+ files are in a two column format. The first column is the test name, which is mainly used by test runners for reporting and filtering. It is just a single string of text with no spaces. The second column, which can contain spaces, is passed to the shell in order to execute the test. Often it is just the executable name, but some tests also take arguments (the LTP has a library for argument parsing, by the way). If you haven't done so already, we should add all these new files to Git. It is vitally important that you do not make changes to the master branch. If you do then pulling changes from upstream becomes a major issue. So first of all create a new branch. [source,shell] -------------------------------------------------------------------------------- $ git checkout -b statx01 master -------------------------------------------------------------------------------- Now we want to add the files we have created or modified, but before doing a commit make sure you have configured Git correctly. You need to at least set your Name and e-mail address in +~/.gitconfig+, but there are some other settings which come in handy too. My relatively simple configuration is similar to the below [source,conf] -------------------------------------------------------------------------------- [user] name = Sarah Jane email = sjane@e-mail.address [core] editor = emacs [sendemail] smtpServer = smtp.server.address -------------------------------------------------------------------------------- Obviously you need to at least change your name and e-mail. The SMTP server is useful for +git send-mail+, which we will discuss later. The editor value is used for things like writing commits (without the +-m+ option). [source,shell] -------------------------------------------------------------------------------- $ git add -v :/testcases/kernel/syscalls/statx :/runtest/syscalls $ git commit -m "statx01: Add new test for statx syscall" -------------------------------------------------------------------------------- This should add all the new files in the +statx+ directory and the +runtest+ file. It is good practice to commit early and often. Later on we will do a Git-rebase, which allows us to clean up the commit history. So don't worry about how presentable your commit log is for now. Also don't hesitate to create a new branch when doing the exercises or experimenting. This will allow you to diverge from the tutorial and then easily come back again. I can't emphasize enough that Git makes things easy through branching and that things quickly get complicated if you don't do it. However if you do get into a mess, Git-reflog and Git-reset, will usually get you out of it. If you also mess that up then it may be possible to cherry pick 'dangling' commits out of the database into a branch. 3.1 Report TCONF instead of TPASS ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Maybe the test should report "TCONF: Not implemented" instead or perhaps +TBROK+. Try changing it do so (see +doc/test-writing-guidelines.txt+ or https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines[the Wiki]). 3.2 Check Git ignores the executable ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Is your +.gitignore+ correct? 3.3 Run make check ~~~~~~~~~~~~~~~~~~ Check coding style with `make check` (more in https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines#21-c-coding-style[C coding style]) 3.4 Install the LTP and run the test with runtest ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Run +statx01+ on its own; similar to the +madvise+ tests in the +README+. 4. Call the system call ----------------------- At the time of writing +statx+ has no +glibc+ wrapper. It is also fairly common for a distribution's C library version to be older than its kernel or it may use a cut down C library in comparison to the GNU one. So we must call +statx()+ using the general +syscall()+ interface. The LTP contains a library for dealing with the +syscall+ interface, which is located in +include/lapi+. System call numbers are listed against the relevant call in the +*.in+ files (e.g. +x86_64.in+) which are used to generate +syscalls.h+, which is the header you should include. On rare occasions you may find the system call number is missing from the +*.in+ files and will need to add it (see +include/lapi/syscalls/strip_syscall.awk+). System call numbers vary between architectures, hence why there are multiple +*.in+ files for each architecture. You can find the various values for the +statx+ system call across a number of +uinstd.h+ files in the Linux kernel. Note that we don't use the system-call-identifier value available in +/usr/include/linux/uinstd.h+ because the kernel might be much newer than the user land development packages. For +statx+ we had to add +statx 332+ to +testcases/kernel/include/x86_64.in+, +statx 383+ to +testcases/kernel/include/powerpc.in+, etc. Now lets look at the code, which I will explain in more detail further down. [source,c] -------------------------------------------------------------------------------- /* * Test statx * * Check if statx exists and what error code it returns when we give it dodgy * data. */ #include #include "tst_test.h" #include "lapi/syscalls.h" struct statx_timestamp { int64_t tv_sec; uint32_t tv_nsec; int32_t __reserved; }; struct statx { uint32_t stx_mask; uint32_t stx_blksize; uint64_t stx_attributes; uint32_t stx_nlink; uint32_t stx_uid; uint32_t stx_gid; uint16_t stx_mode; uint16_t __spare0[1]; uint64_t stx_ino; uint64_t stx_size; uint64_t stx_blocks; uint64_t stx_attributes_mask; struct statx_timestamp stx_atime; struct statx_timestamp stx_btime; struct statx_timestamp stx_ctime; struct statx_timestamp stx_mtime; uint32_t stx_rdev_major; uint32_t stx_rdev_minor; uint32_t stx_dev_major; uint32_t stx_dev_minor; uint64_t __spare2[14]; }; static int sys_statx(int dirfd, const char *pathname, int flags, unsigned int mask, struct statx *statxbuf) { return tst_syscall(__NR_statx, dirfd, pathname, flags, mask, statxbuf); } ... -------------------------------------------------------------------------------- So the top part of the code is now boiler plate for calling +statx+. It is common for the kernel to be newer than the user land libraries and headers. So for new system calls like +statx+, we copy, with a few modifications, the relevant definitions into the LTP. This is somewhat like 'vendoring', although we are usually just copying headers required for interacting with the Kernel's ABI (Application Binary Interface), rather than internalising actual functionality. So from the top we include the +stdint.h+ library which gives us the standard +(u)int*_t+ type definitions. We use these in place of the Kernel type definitions such as +__u64+ in +linux/types.h+. We then have a couple of structure definitions which form part of the +statx+ API. These were copied from +include/uapi/linux/stat.h+ in the Kernel tree. After that, there is a wrapper function, which saves us from writing +tst_syscall(__NR_statx, ...+, every time we want to make a call to +statx+. This also provides a stub for when +statx+ is eventually integrated into the LTP library and also implemented by the C library. At that point we can switch to using the C library implementation if available or fallback to our own. The advantage of using the C library implementation is that it will often be better supported across multiple architectures. It will also mean we are using the system call in the same way most real programs would. Sometimes there are advantages to bypassing the C library, but in general it should not be our first choice. The final test should do a check during configuration (i.e. when we run +./configure+ before building) which checks if the +statx+ system call and associated structures exists. This requires writing an +m4+ file for use with +configure.ac+ which is processed during +make autotools+ and produces the configure script. For the time being though we shall just ignore this. All you need to know for now is that this is a problem which eventually needs to be dealt with and that there is a system in place to handle it. [source,c] -------------------------------------------------------------------------------- ... static void run(void) { struct statx statxbuf = { 0 }; TEST(sys_statx(0, NULL, 0, 0, &statxbuf)); if (TST_RET == 0) tst_res(TFAIL, "statx thinks it can stat NULL"); else if (TST_ERR == EFAULT) tst_res(TPASS, "statx set errno to EFAULT as expected"); else tst_res(TFAIL | TERRNO, "statx set errno to some unexpected value"); } static struct tst_test test = { .test_all = run, .min_kver = "4.11", }; -------------------------------------------------------------------------------- The +TEST+ macro sets +TST_RET+ to the return value of +tst_statx()+ and +TST_ERR+ to the value of +errno+ immediately after the functions return. This is mainly just for convenience, although it potentially could have other uses. We check whether the return value indicates success and if it doesn't also check the value of +errno+. The last call to +tst_res+ includes +TERRNO+, which will print the current error number and associated description in addition to the message we have provided. Note that it uses the current value of +errno+ not +TST_ERR+. What we should have done in the example above is use +TTERRNO+ which takes the value of +TST_ERR+. If we try to run the test on a kernel where +statx+ does not exist, then +tst_syscall+ will cause it to fail gracefully with +TCONF+. Where +TCONF+ indicates the test is not applicable to our configuration. The function +tst_syscall+ calls +tst_brk(TCONF,...)+ on failure. +tst_brk+ causes the test to exit immediately, which prevents any further test code from being run. 4.1 What are the differences between tst_brk and tst_res? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ See +include/tst_test.h+ and the https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines[test writing guide]. Also what do they have in common? 4.2 What happens if you call tst_res(TINFO, ...) after sys_statx? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Does the test still function correctly? 4.3 Extend the test to handle other basic error conditions. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For example, see if you can trigger +ENOENT+ instead. You shouldn't have to create any files, which is discussed in the next section. 5. Setup, Cleanup and files --------------------------- Some tests require resources to be allocated, or system settings to be changed, before the test begins. This 'setup' only has to be done once at the beginning and at the end of the test needs to be removed or reverted. The 'cleanup' also has to be done regardless of whether the test breaks. Fortunately, like most test libraries, we have setup and cleanup (teardown) callbacks. +setup+ is called once before +run+ and +cleanup+ is called once afterwards. Note that +run+ itself can be called multiple times by the test harness, but that +setup+ and +cleanup+ are only called once. If either your code, a +SAFE_*+ macro or a library function such as +tst_syscall+ call +tst_brk+, then +run+ will exit immediately and the +cleanup+ function is then called. Once 'cleanup' is completed, the test executable will then exit altogether abandoning any remaining iterations of +run+. For +statx+ we would like to create some files or file like objects which we have control over. Deciding where to create the files is easy, we just create it in the current working directory and let the LTP test harness handle where that should be by setting +.needs_tmpdir = 1+. [source,c] -------------------------------------------------------------------------------- /* * Test statx * * Check if statx exists and what error code it returns when we give it dodgy * data. Then stat a file and check it returns success. */ #include #include "tst_test.h" #include "lapi/syscalls.h" #include "lapi/fcntl.h" #define FNAME "file_to_stat" #define STATX_BASIC_STATS 0x000007ffU /*************** statx structure and wrapper goes here ! ***************/ ... -------------------------------------------------------------------------------- We have added an extra include +lapi/fcntl.h+ which wraps the system header by the same name (+#include +). This header ensures we have definitions for recently added macros such as +AT_FDCWD+ by providing fall backs if the system header does not have them. The +lapi+ directory contains a number of headers like this. At some point we may wish to add +lapi/stat.h+ to provide a fall back for macros such as +STATX_BASIC_STATS+. However for the time being we have just defined it in the test. [source,c] -------------------------------------------------------------------------------- ... static void setup(void) { SAFE_TOUCH(FNAME, 0777, NULL); } static void run(void) { struct statx statxbuf = { 0 }; TEST(sys_statx(0, NULL, 0, 0, &statxbuf)); if (TST_RET == 0) tst_res(TFAIL, "statx thinks it can stat NULL"); else if (TST_ERR == EFAULT) tst_res(TPASS, "statx set errno to EFAULT as expected"); else tst_res(TFAIL | TERRNO, "statx set errno to some unexpected value"); TEST(sys_statx(AT_FDCWD, FNAME, 0, STATX_BASIC_STATS, &statxbuf)); if (TST_RET == 0) tst_res(TPASS, "It returned zero so it must have worked!"); else tst_res(TFAIL | TERRNO, "statx can not stat a basic file"); } static struct tst_test test = { .setup = setup, .test_all = run, .min_kver = "4.11", .needs_tmpdir = 1 }; -------------------------------------------------------------------------------- The +setup+ callback uses one of the LTP's +SAFE+ functions to create an empty file +file_to_stat+. Because we have set +.needs_tmpdir+, we can just create this file in the present working directory. We don't need to create a +cleanup+ callback yet because the LTP test harness will recursively delete the temporary directory and its contents. The +run+ function can be called multiple times by the test harness, however +setup+ and +cleanup+ callbacks will only be ran once. [WARNING] By this point you may have begun to explore the LTP library headers or older tests. In which case you will have come across functions from the old API such as +tst_brkm+. The old API is being phased out, so you should not use these functions. So far we haven't had to do any clean up. So our example doesn't answer the question "what happens if part of the clean up fails?". To answer this we are going to modify the test to ask the (highly contrived) question "What happens if I create and open a file, then create a hard-link to it, then call open again on the hard-link, then 'stat' the file". [source,c] -------------------------------------------------------------------------------- #define LNAME "file_to_stat_link" ... static void setup(void) { fd = SAFE_OPEN(FNAME, O_CREAT, 0777); SAFE_LINK(FNAME, LNAME); lfd = SAFE_OPEN(LNAME, 0); } static void cleanup(void) { if (lfd != 0) SAFE_CLOSE(lfd); if (fd != 0) SAFE_CLOSE(fd); } static void run(void) { ... TEST(sys_statx(AT_FDCWD, LNAME, 0, STATX_BASIC_STATS, &statxbuf)); if (TST_RET == 0) tst_res(TPASS, "It returned zero so it must have worked!"); else tst_res(TFAIL | TERRNO, "statx can not stat a basic file"); } static struct tst_test test = { .setup = setup, .cleanup = cleanup, .test_all = run, .tcnt = 2, .min_kver = "4.11", .needs_tmpdir = 1 }; -------------------------------------------------------------------------------- Because we are now opening a file, we need a +cleanup+ function to close the file descriptors. We have to manually close the files to ensure the temporary directory is deleted by the test harness (see the https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines[test writing guidelines] for details). As a matter of good practice, the file descriptors are closed in reverse order. In some circumstances the order in which clean up is performed is significant. In that case resources created towards the end of 'setup' are dependent on ones near the beginning. So during 'cleanup' we remove the dependants before their dependencies. If, for some reason, the file descriptor +lfd+ became invalid during the test, but +fd+ was still open, we do not want +SAFE_CLOSE(lfd)+ to cause the +cleanup+ function to exit prematurely. If it did, then +fd+ would remain open which would cause problems on some file systems. Nor do we want to call +cleanup+ recursively. So during 'cleanup' +tst_brk+, and consequently the +SAFE+ functions, do not cause the test to exit with +TBROK+. Instead they just print an error message with +TWARN+. It is not entirely necessary to check if the file descriptors have a none zero value before attempting to close them. However it avoids a bunch of spurious warning messages if we fail to open +file_to_stat+. Test case failures can be difficult to interpret at the best of times, so avoid filling the log with noise. 5.1 Check statx returns the correct number of hard links ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The field +statx.stx_nlink+ should be equal to 2, right? 5.2 Git-branch ~~~~~~~~~~~~~~ We are about to make some organisational changes to the test, so now would be a good time to branch. Then we can switch between the old and new versions, to check the behavior has not been changed by accident. 6. Split the test ----------------- In our current test, we have essentially rolled two different test cases into one. Firstly we check if an error is returned when bad arguments are provided and secondly we check what happens when we stat an actual file. Quite often it makes sense to call +tst_res+ multiple times in a single test case because we are checking different properties of the same result, but here we are clearly testing two different scenarios. So we should split the test in two. One obvious way to do this is to create +statx02.c+, but that seems like overkill in order to separate two simple test cases. So, for now at least, we are going to do it a different way. [source,c] -------------------------------------------------------------------------------- ... static void run_stat_null(void) { struct statx statxbuf = { 0 }; TEST(sys_statx(0, NULL, 0, 0, &statxbuf)); if (TST_RET == 0) tst_res(TFAIL, "statx thinks it can stat NULL"); else if (TST_ERR == EFAULT) tst_res(TPASS, "statx set errno to EFAULT as expected"); else tst_res(TFAIL | TERRNO, "statx set errno to some unexpected value"); } static void run_stat_symlink(void) { struct statx statxbuf = { 0 }; TEST(sys_statx(AT_FDCWD, LNAME, 0, STATX_BASIC_STATS, &statxbuf)); if (TST_RET == 0) tst_res(TPASS, "It returned zero so it must have worked!"); else tst_res(TFAIL | TERRNO, "statx can not stat a basic file"); } static void run(unsigned int i) { switch(i) { case 0: run_stat_null(); case 1: run_stat_symlink(); } } static struct tst_test test = { .setup = setup, .cleanup = cleanup, .test = run, .tcnt = 2, .min_kver = "4.11", .needs_tmpdir = 1 }; -------------------------------------------------------------------------------- So we have used an alternative form of the +test+ or +run+ callback which accepts an index. Some tests use this index with an array of parameters and expected return values. Others do something similar to the above. The index can be used how you want so long as each iteration calls +tst_res+ in a meaningful way. If an iteration fails to return a result (i.e. call +tst_res+ with a value other than +TINFO+) then the test harness will report +TBROK+ and print the iteration which failed. This prevents a scenario in your test from silently failing due to some faulty logic. 6.1 What is wrong with the switch statement? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Were you paying attention? Also see the output of +make check+. 6.2 Test a feature unique to statx ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ So far we have not tested anything which is unique to +statx+. So, for example, you could check stx_btime is correct (possibly only to within a margin of error) and that it differs from +stx_mtime+ after writing to the file. Alternatively you could check that +stx_dev_major+ and +stx_dev_minor+ are set correctly. Note that the LTP has helper functions for creating devices and file systems (see https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines#2214-testing-with-a-block-device[section 2.2.14] of the Test Writing Guidelines). This could be quite a challenging exercise. You may wish to tackle an altogether different test scenario instead. If you get stuck just move onto the next section and come back later. 7. Submitting the test for review --------------------------------- Ignoring the fact we should probably create +lapi/stat.h+ along with a bunch of fallback logic in the build system. We can now get our test ready for submission. The first thing you need to do before considering submitting your test is run +make check-statx01+ or + make check+ in the test's directory. Again, we use the kernel style guidelines where possible. Next you should create a new branch, this will allow you to reshape your commit history without fear. After that we have the pleasure of doing an interactive 'rebase' to clean up our commit history. In its current form the test only really needs a single commit, but if you have been using Git correctly then you should have many. The main reason we want to compress it to a single commit, is to make the LTP's Git-log readable. It also allows us to write a coherent description of the work as a whole in retrospective. Although, when adding a new test, the test description in the code will probably make the commit message redundant. Anyway, as an example, we shall look at my personal commit history from this tutorial and 'rebase' it. You should try following along with your own repository. First lets look at the commit history since we branched from master. [source,shell] -------------------------------------------------------------------------------- $ git log -oneline master..HEAD 152d39fe7 (HEAD -> tutorial-rebase2, tutorial-rebase) tutorial: Start Submitting patch section 70f7ce7ce statx01: Stop checkpatch from complaining bb0332bd7 tutorial: Fix review problems 6a87a084a statx01: Fix review problems d784b1e85 test-writing-guidelines: Remove old API argument c26e1be7a fixup! tutorial 1e24a5fb5 (me/tutorial-rebase) fixup! tutorial 568a3f7be fixup! tutorial 09dd2c829 statx: stage 6 bfeef7902 statx: stage 5b 76e03d714 statx: stage 5a 98f5bc7ac statx: stage 4 6f8c16438 statx: stage 3 (Add statx01) 5d93b84d8 Add statx and other syscall numbers 5ca627b78 tutorial: Add a step-by-step C test tutorial -------------------------------------------------------------------------------- So we have told git to show all the commits which don't exist in 'master', but are in +HEAD+, where +HEAD+ is the top of the current branch. The current branch is +tutorial-rebase2+ which I just created. I have already done one 'rebase' and submitted a patch for review, so my original branch was just called +tutorial+. As usual my commit history is starting to look like a bit of mess! There is even a commit in there which should not be in the this branch (Remove old API argument), however it can be ignored for now and 'cherry picked' into a new branch later. For my patch I actually need at least two commits, one which contains the tutorial text and one which contains the test and associated files. So first of all I want to 'squash' (amalgamate) all the commits appended with +tutorial:+ into the bottom commit. [source,shell] -------------------------------------------------------------------------------- $ git rebase -i 5ca627b78\^ ... -------------------------------------------------------------------------------- This begins an interactive 'rebase' where commit 5ca6427b78 is the earliest commit we want to edit. The +^+ symbol after the commit hash, specifies the commit before this one. The interactive 'rebase' command takes the last commit we want to keep unaltered as it's argument (in other words it takes a non-inclusive range). Upon entering a similar command you will be presented with a text file similar to the following. The file should be displayed in your text editor of choice, if it doesn't, then you may change the editor variable in +.gitconfig+ which was shown in section 3. [source,rebase] -------------------------------------------------------------------------------- pick 5ca627b78 tutorial: Add a step-by-step C test tutorial pick 5d93b84d8 Add statx and other syscall numbers pick 6f8c16438 statx: stage 3 (Add statx01) pick 98f5bc7ac statx: stage 4 pick 76e03d714 statx: stage 5a pick bfeef7902 statx: stage 5b pick 09dd2c829 statx: stage 6 pick 568a3f7be fixup! tutorial pick 1e24a5fb5 fixup! tutorial pick c26e1be7a fixup! tutorial pick d784b1e85 test-writing-guidelines: Remove old API argument pick 6a87a084a statx01: Fix review problems pick bb0332bd7 tutorial: Fix review problems pick 70f7ce7ce statx01: Stop checkpatch from complaining pick 152d39fe7 tutorial: Start Submitting patch section -------------------------------------------------------------------------------- The last commit from Git-log is shown at the top. The left hand column contains the commands we want to run on each commit. +pick+ just means we re-apply the commit as-is. We can reorder the lines to apply the commits in a different order, but we need to be careful when reordering commits to the same file. If your 'rebase' results in a merge conflict, then you have probably reordered some commits which contained changes to the same piece of code. Perhaps a better name for the interactive 'rebase' command would be 'replay'. As we pick a point in the commit history, undo all those commits before that point, then reapply them one at a time. During the replay we can reorder the commits, drop, merge, split and edit them, creating a new history. The commands I am going to use are +reword+ and +fixup+. The +reword+ command allows you to edit a single commit's message. The 'fixup' command 'squashes' a commit into the commit above/preceding it, merging the two commits into one. The commit which has +fixup+ applied has its commit message deleted. If you think a commit might have something useful in its message then you can use +squash+ instead. [source,rebase] -------------------------------------------------------------------------------- reword 5ca627b78 tutorial: Add a step-by-step C test tutorial fixup 568a3f7be fixup! tutorial fixup 1e24a5fb5 fixup! tutorial fixup c26e1be7a fixup! tutorial fixup bb0332bd7 tutorial: Fix review problems fixup 152d39fe7 tutorial: Start Submitting patch section fixup 276edecab tutorial: Save changes before rebase pick 5d93b84d8 Add statx and other syscall numbers pick 6f8c16438 statx: stage 3 (Add statx01) pick 98f5bc7ac statx: stage 4 pick 76e03d714 statx: stage 5a pick bfeef7902 statx: stage 5b pick 09dd2c829 statx: stage 6 pick d784b1e85 test-writing-guidelines: Remove old API argument pick 6a87a084a statx01: Fix review problems -------------------------------------------------------------------------------- So all the commits marked with +fixup+ will be re-played by Git immediately after 5ca62 at the top. A new commit will then be created with the amalgamated changes of all the commits and 5ca62's log message. It turns out that I didn't need to reword anything, but there is no harm in checking. It is easy to forget the +Signed-off-by:+ line. I could now do the same for the commits to the +statx+ test, making the commit message prefixes consistent. However I am not actually going to submit the test (yet). I won't attempt to show you this, but if you need to do the opposite and split apart a commit. It is also possible using Git-rebase by marking a line with +edit+. This will pause Git just after replaying the marked commit. You can then use a 'soft' Git-reset to bring the selected commit's changes back into the 'index' where you are then able to un-stage some parts before re-committing. You can also use +edit+ and +git commit --amend+ together to change a commit deep in your history, but without reseting the 'index'. The 'index' contains changes which you have staged with +git add+, but not yet committed. So now that the commit history has been cleaned up, we need to submit a patch to the mailing list or make a pull request on GitHub. The mailing list is the preferred place to make submissions and is more difficult for most people, so I will only cover that method. Just before we create the patch, we need to check that our changes will still apply to the master branch without problems. To do this we can use another type of 'rebase' and then try rebuilding and running the test. [source,shell] -------------------------------------------------------------------------------- $ git checkout master $ git pull origin $ git checkout tutorial-rebase2 $ git rebase master -------------------------------------------------------------------------------- Above, I update the master branch and then replay our changes onto it using +git rebase master+. You may find that after the rebase there is a merge conflict. This will result in something which looks like the following (taken from a Makefile conflict which was caused by reordering commits in a 'rebase'). [source,diff] -------------------------------------------------------------------------------- <<<<<<< HEAD cve-2016-7117: LDFLAGS += -lpthread ======= cve-2014-0196: LDFLAGS += -lpthread -lutil -lrt cve-2016-7117: LDFLAGS += -lpthread -lrt >>>>>>> 4dbfb8e79... Add -lrt -------------------------------------------------------------------------------- The first line tells us this is the beginning of a conflict. The third line separates the two conflicting pieces of content and the last line is the end of the conflict. Usually, all you need to do is remove the lines you don't want, stage the changes and continue the 'rebase' with +git rebase --continue+. In order to create a patch e-mail we use +git format-patch+, we can then send that e-mail using +git send-email+. It is also possible to import the patch (+mbox+) file into a number of e-mail programs. [source,shell] -------------------------------------------------------------------------------- $ git format-patch -1 -v 2 -o output --to ltp@lists.linux.it fd3cc8596 output/v2-0001-tutorial-Add-a-step-by-step-C-test-tutorial.patch -------------------------------------------------------------------------------- The first argument +-1+ specifies we want one commit from fd3cc8596 onwards. If we wanted this commit and the one after it we could specify +-2+ instead. This is my second patch submission so I have used +-v 2+, which indicates this is the second version of a patch set. The +-o+ option specifies the output directory (literally called +output+). The +--to+ option adds the +To:+ e-mail header, which I have set to the LTP mailing list. We can then send this patch with the following command sans +--dry-run+. [source,shell] -------------------------------------------------------------------------------- $ git send-email --dry-run output/v2-0001-tutorial-Add-a-step-by-step-C-test-tutorial.patch -------------------------------------------------------------------------------- Git will ask some questions (which you an ignore) and then tell you what it would do if this weren't a dry-run. In order for this to work you have to have a valid SMTP server set in +.gitconfig+ and also be signed up to the LTP mailing list under the same e-mail address you have configured in Git. You can sign up at https://lists.linux.it/listinfo/ltp. 8. Doing code review -------------------- While waiting for your test to be reviewed, you are invited and encouraged to review other contributors' code. This may seem bizarre when you are completely new to the project, but there are two important ways in which you can contribute here: A. Point out logical errors in the code. B. Improve your own understanding It doesn't matter whether you know the canonical way of writing an LTP test in C. An error of logic, when properly explained, is usually indisputable. These are the most important errors to find as they always result in false test results. Once someone points out such an error it is usually obvious to everyone that it is a bug and needs to be fixed. Obviously testing the patch is one way of finding errors. You can apply patches using +git am+. Then it is just a case of compiling and running the tests. Finally, reading and attempting to comment on other peoples patches, gives you a better understanding of the reviewers perspective. This is better for the project and for you. Style and organisational issues are best left to after you have found logical errors. 9. Final notes -------------- Hopefully you can now grasp the structure of an LTP test and have some idea of what is available in the LTP test library. There are a vast number of library functions available (mainly located in include and lib), some of which are documented in the test writing guidelines and many of which are not. We have only scratched the surface of the immense technical complexity of systems programming across multiple Kernel and C lib versions as well as different hardware architectures. The important thing to take away from this is that you have to be conscientious of what will happen on systems different from yours. The LTP has a huge and varied user base, so situations you may thing are unlikely can and do happen to somebody. Of course you don't want to spend time allowing for situations which may never arise either, so you have to do your research and think about each situation critically. The more systems you can test on before submitting your changes, the better, although we understand not everyone has access to a lab. One important topic which has not been covered by this tutorial, is multi-process or multi-threaded testing. The LTP library functions work inside child processes and threads, but their semantics change slightly. There are also various helper functions for synchronising and forking processes. For more information see https://github.com/linux-test-project/ltp/wiki/C-Test-API[C Test API], in particular sections https://github.com/linux-test-project/ltp/wiki/C-Test-API#17-fork-ing[1.7 Fork()-ing] to https://github.com/linux-test-project/ltp/wiki/C-Test-API#110-signals-and-signal-handlers[1.10 Signals and signal handlers] and https://github.com/linux-test-project/ltp/wiki/C-Test-API#114-thread-safety-in-the-ltp-library[1.14 Thread-safety in the LTP library]. When it comes time to submit a test, the preferred way to do it is on the mailing list although you can also use GitHub. The LTP follows similar rules to the kernel for formatting and submitting patches. Generally speaking the review cycle is easier for small patches, so try to make small changes or additions where possible.