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177 lines
6.1 KiB
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Debugging Firefox with Valgrind
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===============================
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+--------------------------------------------------------------------+
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| This page is an import from MDN and the contents might be outdated |
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+--------------------------------------------------------------------+
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This page describes how to use Valgrind (specifically, its Memcheck
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tool) to find memory errors.
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Supported platforms
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-------------------
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Valgrind runs desktop Firefox fine on Linux, especially on x86 and
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x86-64. Firefox for Android and Firefox OS on ARMv7 should also run,
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though perhaps not as smoothly. The other architectures supported by
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Valgrind on Linux (AARCH64, PPC{32,64}, MIPS{32,64}, S390X) should also
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work, in theory.
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MacOS X 10.10 (Yosemite), 64-bit only, works, although it can be a bit
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of a rough ride.
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- Expect lower performance and a somewhat higher false positive error
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rate than on Linux.
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- Valgrind's handling of malloc zones on Yosemite is imperfect. Regard
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leak reports with caution.
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- Valgrind has been known to cause kernel panics, for unknown reasons.
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Where to get Valgrind
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---------------------
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Linux: Download `Valgrind <https://valgrind.org/>`__ directly, or use
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your distribution's package manager (if it has a recent enough version).
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MacOSX: `Get Valgrind trunk from
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SVN <https://valgrind.org/downloads/repository.html>`__ and build it.
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Don't use 3.10.x or any other tarball.
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Make sure you have Valgrind 3.14 or later, version 3.16.1 is known to work,
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3.13.0 did not. Newer versions tend to have better compatibility with both
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Firefox's JITs and newer toolchain components (compiler, libc and linker
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versions).
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Basics
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------
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Build
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~~~~~
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Build Firefox with the following options, which maximize speed and
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accuracy.
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.. code::
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ac_add_options --disable-jemalloc
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ac_add_options --disable-strip
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ac_add_options --enable-valgrind
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ac_add_options --enable-optimize="-g -O2"
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ac_add_options --disable-sandbox
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Run
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~~~
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Note that programs run *much* more slowly under Valgrind than they do
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natively. Slow-downs of 20x or 30x aren't unexpected, and it's slower on
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Mac than on Linux. Don't try this on an underpowered machine.
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Linux
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^^^^^
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On Linux, run Valgrind with the following options.
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.. code::
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--smc-check=all-non-file --vex-iropt-register-updates=allregs-at-mem-access --show-mismatched-frees=no --read-inline-info=yes
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The ``--smc-check`` and ``--vex-iropt-register-updates`` options are
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necessary to avoid crashes in JIT-generated code.
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The ``--show-mismatched-frees`` option is necessary due to inconsistent
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inlining of ``new`` and ``delete`` -- i.e. one gets inlined but the
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other doesn't -- which lead to false-positive mismatched-free errors.
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The ``--read-inline-info`` option improves stack trace readability in
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the presence of inlining.
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Also, run with the following environment variable set.
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.. code::
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G_SLICE=always-malloc
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This is necessary to get the Gnome system libraries to use plain
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``malloc`` instead of pool allocators.
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Mac
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^^^
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On Mac, run Valgrind with the following options.
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.. code::
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--smc-check=all-non-file --vex-iropt-register-updates=allregs-at-mem-access --show-mismatched-frees=no --dsymutil=yes
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The ``--dsymutil`` option ensures line number information is present in
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stack traces.
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Advanced usage
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--------------
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Shared suppression files
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~~~~~~~~~~~~~~~~~~~~~~~~
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`/build/valgrind/ <https://searchfox.org/mozilla-central/source/build/valgrind/>`__
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contains the suppression files used by the periodic Valgrind jobs on
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Treeherder. Some of these files are platform-specific.
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Running mochitests under Valgrind?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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To run a mochitest under Valgrind, use the following command.
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.. code:: bash
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$ ./mach mochitest-plain --debugger="valgrind" --debugger-args="$VALGRIND_OPTIONS" relative/path/to/tests
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Where ``$VALGRIND_OPTIONS`` are the options described
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:ref:`above <Debugging Firefox With Valgrind>`. You might also
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need ``--trace-children=yes`` to trace into child processes.
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As of December 2014 it is possible to do a complete run of
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mochitests-plain on Valgrind in about 8 CPU hours on a Core i4910
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(Haswell) machine. Maximum process size is 5.4G, of which about 80% is
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in memory. Runs of small subsets of mochitests take far less memory.
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Bits and pieces
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~~~~~~~~~~~~~~~
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For un-released Linux distros (Fedora Rawhide, etc.) you'll need to use
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a version of Valgrind trunk build, because fixes for the latest gcc and
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glibc versions appear there first. Without them you'll be flooded with
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false errors from Memcheck, and have debuginfo reading problems.
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On Linux, code compiled by LLVM at high optimisation levels can cause
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Memcheck to report false uninitialised value errors. See
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`here <https://bugs.kde.org/show_bug.cgi?id=242137#c3>`__ for an easy
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workaround. On Mac, Valgrind has this workaround built in.
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You can make stack traces easier to read by asking for source file names
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to be given relative to the root of your source tree. Do this by using
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``--fullpath-after=`` to specify the rightmost part of the absolute path
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that you don't want to see. For example, if your source tree is rooted
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at ``/home/sewardj/MC-20-12-2014``, use ``--fullpath-after=2014/`` to
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get path names relative to the source directory.
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The ``--track-origins=yes`` slows down Valgrind greatly, so don't use it
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unless you are hunting down a specific uninitialised value error. But if
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you are hunting down such an error, it's extremely helpful and worth
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waiting for.
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Additional help
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---------------
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The `Valgrind Quick Start
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Guide <http://www.valgrind.org/docs/manual/quick-start.html>`__ is short
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and worth reading. The `User
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Manual <https://valgrind.org/docs/manual/manual.html>`__ is also useful.
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If Valgrind asserts, crashes, doesn't do what you expect, or otherwise
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acts up, first of all read this page and make sure you have both Firefox
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and Valgrind correctly configured. If that's all OK, try using the
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`Valgrind trunk from
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SVN <http://www.valgrind.org/downloads/repository.html>`__. Oftentimes
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bugs are fixed in the trunk before most users fall across them. If that
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doesn't help, consider `filing a bug
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report <http://www.valgrind.org/support/bug_reports.html>`__, and/or
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mailing Julian Seward or Nick Nethercote.
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