Existing Infrastructure and Analysis¶

This document is about how Static Analysis occurs at Mozilla: the Firefox-specific and general llvm clang-tidy checks that are run on submissions in Phabricator and how to run them locally. For information about how to develop your own static analysis checks, please see Writing New Firefox-Specific Checks.

For linting, please see the linting documentation.

For reviews, use the #static-analysis-reviewers review group. Ask questions on #static-analysis:mozilla.org.

Clang-Tidy static analysis¶

As explained earlier, our current static-analysis infrastructure is based on clang-tidy. The checkers that we use are split into 3 categories:

Firefox specific checkers. They detect incorrect Gecko programming patterns which could lead to bugs or security issues.
Clang-tidy checkers. They aim to suggest better programming practices and to improve memory efficiency and performance.
Clang-analyzer checkers. These checks are more advanced, for example some of them can detect dead code or memory leaks, but as a typical side effect they have false positives. Because of that, we have disabled them for now, but will enable some of them in the near future.

In order to simplify the process of static-analysis we have focused on integrating this process with Phabricator and mach. A list of some checkers that are used during automated scan can be found here.

Static analysis at review phase¶

We created a TaskCluster bot that runs clang static analysis on every patch submitted to Phabricator. It then quickly reports any code defects directly on the review platform, thus preventing bad patches from landing until all their defects are fixed. Currently, its feedback is posted in about 10 minutes after a patch series is published on the review platform.

As part of the process, the various linting jobs are also executed using try. This can be also used to add new jobs, see: Profiler symbols for try builds. An example of automated review can be found on phabricator.

./mach static-analysis¶

The ./mach static-analysis command is supported on all Firefox built platforms. During the first run it automatically installs all of its dependencies, such as the clang-tidy executable, in the .mozbuild folder thus making it very easy to use. The resources that are used are provided by toolchain artifacts clang-tidy target.

This is used through mach static-analysis command that has the following parameters:

check - Runs the checks using the installed helper tool from ~/.mozbuild.
--checks, -c - Checks to enabled during the scan. The checks enabled in the yaml file are used by default.
--fix, -f - Try to autofix errors detected by the checkers. Depending on the checker, this option might not do anything. The list of checkers with autofix can be found on the clang-tidy website.
--header-filter, -h-f - Regular expression matching the names of the headers to output diagnostic from. Diagnostic from the main file of each translation unit are always displayed.

As an example we run static-analysis through mach on dom/presentation/Presentation.cpp with google-readability-braces-around-statements check and autofix we would have:

./mach static-analysis check --checks="-*, google-readability-braces-around-statements" --fix dom/presentation/Presentation.cpp

If you want to use a custom clang-tidy binary this can be done by using the install subcommand of mach static-analysis, but please note that the archive that is going to be used must be compatible with the directory structure clang-tidy from toolchain artifacts.

./mach static-analysis install clang.tar.gz

Regression Testing¶

In order to prevent regressions in our clang-tidy based static analysis, we have created a task on automation. This task runs on each commit and launches a test suite that is integrated into mach.

The test suite implements the following:

Downloads the necessary clang-tidy artifacts.
Reads the configuration file.
For each checker reads the test file plus the expected result. A sample of test and expected result can be found in the test file and the json file.

This testing suit can be run locally by doing the following:

./mach static-analysis autotest

If we want to test only a specific checker, let’s say modernize-raw-string-literal, we can run:

./mach static-analysis autotest modernize-raw-string-literal

If we want to add a new checker we need to generate the expected result file, by doing:

./mach static-analysis autotest modernize-raw-string-literal -d

Build-time static-analysis¶

If you want to build with the Firefox Clang plug-in (located in /build/clang-plugin and associated with MOZ_CLANG_PLUGIN and the attributes in /mfbt/Attributes.h) just add --enable-clang-plugin to your mozconfig! If you want to also have our experimental checkers that will produce warnings as diagnostic messages also add --enable-clang-plugin-alpha. This requires to build Firefox using Clang.

Configuring the build environment¶

Once you have your Clang build in place, you will need to set up tools to use it. A full working .mozconfig for the desktop browser is:

. $topsrcdir/browser/config/mozconfig
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-ff-dbg

ac_add_options --enable-debug

Attempts to use ccache will likely result in failure to compile. It is also necessary to avoid optimized builds, as these will modify macros which will result in many false positives.

At this point, your Firefox build environment should be configured to compile via the Clang static analyzer!

Performing scanning builds¶

It is not enough to simply start the build like normal. Instead, you need to run the build through a Clang utility script which will keep track of all produced analysis and consolidate it automatically.

Reports are published daily on https://sylvestre.ledru.info/reports/fx-scan-build/ Many of the defects reported as sources for Good First Bug.

That script is scan-build. You can find it in $clang_source/tools/scan-build/scan-build.

Try running your build through scan-build:

$ cd /path/to/mozilla/source

# Blow away your object directory because incremental builds don't make sense
$ rm -rf obj-dir

# To start the build:
scan-build --show-description ./mach build -v

# The above should execute without any errors. However, it should take longer than
# normal because all compilation will be executing through Clang's static analyzer,
# which adds overhead.

If things are working properly, you should see a bunch of console spew, just like any build.

The first time you run scan-build, CTRL+C after a few files are compiled. You should see output like:

scan-build: 3 bugs found.
scan-build: Run 'scan-view /Users/gps/tmp/mcsb/2011-12-15-3' to examine bug reports.

If you see a message like:

scan-build: Removing directory '/var/folders/s2/zc78dpsx2rz6cpc_21r9g5hr0000gn/T/scan-build-2011-12-15-1' because it contains no reports.

Either no static analysis results were available yet or your environment is not configured properly.

By default, scan-build writes results to a folder in a pseudo-temporary location. You can control where results go by passing the -o /path/to/output arguments to scan-build.

You may also want to run scan-build --help to see all the options available. For example, it is possible to selectively enable and disable individual analyzers.

Analyzing the output¶

Once the build has completed, scan-build will produce a report summarizing all the findings. This is called index.html in the output directory. You can run scan-view (from $clang_source/tools/scan-view/scan-view) as scan-build's output suggests; this merely fires up a local HTTP server. Or you should be able to open the index.html directly with your browser.

False positives¶

By definition, there are currently false positives in the static analyzer. A lot of these are due to the analyzer having difficulties following the relatively complicated error handling in various preprocessor macros.