JuliaLawall - Linux Kernel Newbies

About Me

I'm a researcher at Inria, in Paris France. I develop the tool [http://coccinelle.lip6.fr Coccinelle], which allows easy matching and transformation of C code. Coccinelle has been designed with the goal of contributing to Linux development, but it can also be used on other C code.

Please write to me directly if you would like to apply to the Coccinelle Outreachy project.

Overview

It would be a good idea to start with the first challenge problem, to check that you know how to use the tool properly. The remaining challenge problems can be done in any order. It is not obligatory to do all of them. You may find other things that can be done with Coccinelle. Sources of inspiration may be the results of checkpatch and patches that have been applied to the kernel in the past. Any kind of problem that occurs over and over might be amenable to being solved with Coccinelle.

These challenge problems may apply to many files in the kernel. Pick a few files, and send patches for those. Once they have been accepted, consider moving on to another challenge problem. You will get a better understanding of Coccinelle if you use it for many different things than if you use it do one thing over and over.

There are many examples of uses of Coccinelle, in previous patches, in the kernel source tree in the scripts/coccinelle directory, and at [https://github.com/coccinelle/coccinellery coccinellery]. If you use a script that is already in the Linux kernel, you don't need to include the script in your commit log, but rather something like Generate-by: sripts/coccinelle/misc/badty.cocci

Tutorial

A tutorial for Coccinelle is available [http://pagesperso-systeme.lip6.fr/Julia.Lawall/tutorial.pdf here]. These are slides that are intended to be presented, but they may be understandable independently of the presentation. Please note that the tutorial focuses on the source code of Linux 3.2, and so the patches created in doing the exercises of the tutorial are not suitable for submission to the opw-kernel mailing list. Doing the tutorial also does not count as a contribution to the project.

Coccinelle challenge problem 1

Consider the following function, from drivers/staging/most/hdm-dim2/dim2_sysfs.c

static ssize_t bus_kobj_attr_store(struct kobject *kobj, struct attribute *attr,
                                   const char *buf, size_t count)
{
        ssize_t ret;
        struct medialb_bus *bus =
                container_of(kobj, struct medialb_bus, kobj_group);
        struct bus_attr *xattr = container_of(attr, struct bus_attr, attr);

        if (!xattr->store)
                return -EIO;

        ret = xattr->store(bus, buf, count);
        return ret;
}

In this function, the last two lines could be compressed into one, as:

static ssize_t bus_kobj_attr_store(struct kobject *kobj, struct attribute *attr,
                                   const char *buf, size_t count)
{
        ssize_t ret;
        struct medialb_bus *bus =
                container_of(kobj, struct medialb_bus, kobj_group);
        struct bus_attr *xattr = container_of(attr, struct bus_attr, attr);

        if (!xattr->store)
                return -EIO;

        return xattr->store(bus, buf, count);
}

The following semantic patch makes this change:

@@
expression e, ret;
@@

-ret =
+return
     e;
-return ret;

Do the following:

Download and install Coccinelle. If you are using Linux, it should be available in your package manager. Any recent version is fine to start

with, but you may need to get the most recent version, which is 1.0.4. This is available on the Coccinelle webpage (coccinelle.lip6.fr) and on github.

Download staging-testing
Save the above semantic patch in a file ret.cocci
Run Coccinelle on ret.cocci and staging-testing, ie spatch --sp-file ret.cocci --no-includes --dir {your staging-testing path}/drivers/staging > ret.out. This may take some time.

Do you find the result satisfactory? If so, submit some patches. If not, let us know!

Your code may now declare some variables that are never used. Remove them before submitting your patch.

If you do submit a patch based on the use of Coccinelle, please mention Coccinelle in your patch, and the semantic patch that you used.

Coccinelle challenge problem 2

Parentheses are not needed around the right hand side of an assignment, like in value = (FLASH_CMD_STATUS_REG_READ << 24);. Write a semantic patch to remove these parentheses.

One could consider that parentheses might be useful in the case of eg rising = (dir == IIO_EV_DIR_RISING); because there could be a confusion between the different kinds of =. Extend your semantic patch using a disjunction so that it does not report on such cases.

Coccinelle challenge problem 3

In the following code, when x has any pointer type

 kfree((u8 *)x);

the cast to u8 *, or to any other pointer type is not needed. Write a semantic patch to remove such casts. Consider generalizing your semantic patch to functions other than kfree.

Currently, the only occurrence of this problem for kfree is in the file wilc1000/host_interface.c. There is something else quite bizarre about this code. Consider how this other bizarre thing could be eliminated using Coccinelle.

Coccinelle challenge problem 4

Assignments in if conditions slightly complicate program analyses and are frowned upon by checkpatch.

        if ((rc = pci_enable_device(pdev))) {
                printk(KERN_WARNING "i2o: couldn't enable device %s\n",
                       pci_name(pdev));
                return rc;
        }

Write a semantic patch to move such assignments out before the if. In the general case, it may be necessary to take into account the possibility of operators such as && and ||. Your semantic patch should not change the order in which expressions are evaluated. In the case of very complex conditions, the transformation may also not be desirable, if it requires duplicating code or introducing many layers of ifs.

Note that all of the instances of this problem seem to have already been fixed in the staging drivers

Coccinelle challenge problem 5

Assignments in function call arguments are also undesirable. Write a semantic patch to pull such assignments out before the function call.

Coccinelle challenge problem 6

Some functions return NULL as a return value on failure. NULL can be tested for as !x, NULL == x, or x == NULL. When NULL represents failure, !x is commonly used. The following are some functions that commonly follow this strategy:

kmalloc
devm_kzalloc
kmalloc_array
devm_ioremap
usb_alloc_urb
alloc_netdev

Write a semantic patch to clean up the tests on the results of one or more of these functions.

As a much harder problem, use Coccinelle to find other functions for which tests for NULL use !x at least 70% of the time.

Coccinelle challenge problem 7

The function devm_kzalloc has a first argument of type struct device *. This is the type of argument required by printing functions such as dev_dbg, dev_err, etc. Thus, functions like pr_err should not normally be used after a call to devm_kzalloc. Replace calls to the pr_* functions in such positions by their dev_* (or netdev_*) counterparts. Consider whether there are other functions like devm_kzalloc from which you can easily obtain the information required to do the conversion and extend your semantic patch to take advantage of these cases.

Contact info

Email: MailTo(Julia.Lawall AT lip6 DOT fr)

My IRC handle is jlawall.

Questions about using Coccinelle should go to the Coccinelle mailing list: MailTo(cocci AT systeme DOT lip6 DOT fr)

CategoryHomepage