22 KiB
Tips for debugging Thorium on Linux
This page is for Chromium-specific debugging tips; learning how to run gdb is out of scope.
Symbolized stack trace
The sandbox can interfere with the internal symbolizer. Use --no-sandbox (but
keep this temporary) or an external symbolizer (see
tools/valgrind/asan/asan_symbolize.py).
Generally, do not use --no-sandbox on waterfall bots, sandbox testing is
needed. Talk to security@chromium.org.
GDB
*** promo GDB-7.7 is required in order to debug Thorium on Linux.
Any prior version will fail to resolve symbols or segfault.
Basic browser process debugging
gdb -tui -ex=r --args out/thorium/thorium --disable-seccomp-sandbox \
http://google.com
Allowing attaching to foreign processes
On distributions that use the Yama LSM (that includes Ubuntu and ChromiumOS/ThoriumOS), process A can attach to process B only if A is an ancestor of B.
You will probably want to disable this feature by using
echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
If you don't you'll get an error message such as "Could not attach to process".
Note that you'll also probably want to use --no-sandbox, as explained below.
Multiprocess Tricks
Getting renderer subprocesses into gdb
Since Thorium itself spawns the renderers, it can be tricky to grab a particular with gdb. This command does the trick:
thorium --no-sandbox --renderer-cmd-prefix='xterm -title renderer -e gdb --args'
The --no-sandbox flag is needed because otherwise the seccomp sandbox will
kill the renderer process on startup, or the setuid sandbox will prevent xterm's
execution. The "xterm" is necessary or gdb will run in the current terminal,
which can get particularly confusing since it's running in the background, and
if you're also running the main process in gdb, won't work at all (the two
instances will fight over the terminal). To auto-start the renderers in the
debugger, send the "run" command to the debugger:
thorium --no-sandbox --renderer-cmd-prefix='xterm -title renderer -e gdb \
-ex run --args'
If you're using Emacs and M-x gdb, you can do
thorium "--renderer-cmd-prefix=gdb --args"
*** note
Note: using the --renderer-cmd-prefix option bypasses the zygote launcher, so
the renderers won't be sandboxed. It is generally not an issue, except when you
are trying to debug interactions with the sandbox. If that's what you are doing,
you will need to attach your debugger to a running renderer process (see below).
You may also want to pass --disable-hang-monitor to suppress the hang monitor,
which is rather annoying.
You can also use --renderer-startup-dialog and attach to the process in order
to debug the renderer code. Go to
https://www.chromium.org/blink/getting-started-with-blink-debugging for more
information on how this can be done.
For utilities you can use --utility-startup-dialog to have all utilities
prompt, or --utility-startup-dialog=data_decoder.mojom.DataDecoderService
to debug only a particular service type.
Choosing which renderers to debug
If you are starting multiple renderers then the above means that multiple gdb's start and fight over the console. Instead, you can set the prefix to point to this shell script:
#!/bin/sh
echo "**** Child $$ starting: y to debug"
read input
if [ "$input" = "y" ] ; then
gdb --args $*
else
$*
fi
Selective breakpoints
When debugging both the browser and renderer process, you might want to have separate set of breakpoints to hit. You can use gdb's command files to accomplish this by putting breakpoints in separate files and instructing gdb to load them.
gdb -x ~/debug/browser --args thorium --no-sandbox --disable-hang-monitor \
--renderer-cmd-prefix='xterm -title renderer -e gdb -x ~/debug/renderer \
--args '
Also, instead of running gdb, you can use the script above, which let's you
select which renderer process to debug. Note: you might need to use the full
path to the script and avoid $HOME or ~/.
Connecting to a running renderer
Usually ps aux | grep thorium will not give very helpful output. Try
pstree -p | grep thorium to get something like
| |-bash(21969)---thorium(672)-+-thorium(694)
| | |-thorium(695)---thorium(696)-+-{thorium}(697)
| | | \-{thorium}(709)
| | |-{thorium}(675)
| | |-{thorium}(678)
| | |-{thorium}(679)
| | |-{thorium}(680)
| | |-{thorium}(681)
| | |-{thorium}(682)
| | |-{thorium}(684)
| | |-{thorium}(685)
| | |-{thorium}(705)
| | \-{thorium}(717)
Most of those are threads. In this case the browser process would be 672 and the
(sole) renderer process is 696. You can use gdb -p 696 to attach.
Alternatively, you might find out the process ID from thorium's built-in Task
Manager (under the Tools menu). Right-click on the Task Manager, and enable
"Process ID" in the list of columns.
Note: by default, sandboxed processes can't be attached by a debugger. To be
able to do so, you will need to pass the --allow-sandbox-debugging option.
If the problem only occurs with the seccomp sandbox enabled (and the previous tricks don't help), you could try enabling core-dumps (see the Core files section). That would allow you to get a backtrace and see some local variables, though you won't be able to step through the running program.
Note: If you're interested in debugging LinuxSandboxIPC process, you can attach
to 694 in the above diagram. The LinuxSandboxIPC process has the same command
line flag as the browser process so that it's easy to identify it if you run
pstree -pa.
Getting GPU subprocesses into gdb
Use --gpu-launcher flag instead of --renderer-cmd-prefix in the instructions
for renderer above.
Getting browser_tests launched browsers into gdb
Use environment variable BROWSER_WRAPPER instead of --renderer-cmd-prefix
switch in the instructions above.
Example:
BROWSER_WRAPPER='xterm -title renderer -e gdb --eval-command=run \
--eval-command=quit --args' out/Debug/browser_tests --gtest_filter=Print
Plugin Processes
Same strategies as renderers above, but the flag is called --plugin-launcher:
thorium --plugin-launcher='xterm -e gdb --args'
*** note Note: For now, this does not currently apply to PPAPI plugins because they currently run in the renderer process.
Single-Process mode
Depending on whether it's relevant to the problem, it's often easier to just run in "single process" mode where the renderer threads are in-process. Then you can just run gdb on the main process.
gdb --args thorium --single-process
Currently, the --disable-gpu flag is also required, as there are known crashes
that occur under TextureImageTransportSurface without it. The crash described in
https://crbug.com/361689 can also sometimes occur, but that crash can be
continued from without harm.
Note that for technical reasons plugins cannot be in-process, so
--single-process only puts the renderers in the browser process. The flag is
still useful for debugging plugins (since it's only two processes instead of
three) but you'll still need to use --plugin-launcher or another approach.
Printing Chromium types
gdb 7 lets us use Python to write pretty-printers for Chromium types. See gdbinit to enable pretty-printing of Chromium types. This will import Blink pretty-printers as well.
Pretty printers for std types shouldn't be necessary in gdb 7, but they're
provided here in case you're using an older gdb. Put the following into
~/.gdbinit:
# Print a C++ string.
define ps
print $arg0.c_str()
end
# Print a C++ wstring or wchar_t*.
define pws
printf "\""
set $c = (wchar_t*)$arg0
while ( *$c )
if ( *$c > 0x7f )
printf "[%x]", *$c
else
printf "%c", *$c
end
set $c++
end
printf "\"\n"
end
JsDbg -- visualize data structures in the browser
JsDbg is a debugger plugin to display various Chromium data structures in a browser window, such as the accessibility tree, layout object tree, DOM tree, and others. Installation instructions are here, and see here for screenshots and an introduction.
For Googlers, please see go/jsdbg for installation instructions.
Time travel debugging with rr
You can use rr for time travel debugging, so you can also step or execute backwards. This works by first recording a trace and then debugging based on that.
You need an up-to-date version of rr, since rr is frequently updated to support new parts of the Linux system call API surface that Chromium uses. If you have any issues with the latest release version, try compiling rr from source.
Once installed, you can use it like this:
rr record out/Debug/content_shell --single-process
rr replay
(rr) c
(rr) break blink::NGBlockNode::Layout
(rr) rc # reverse-continue to the last Layout call
(rr) jsdbg # run JsDbg as described above to find the interesting object
(rr) watch -l box_->frame_rect_.size_.width_.value_
(rr) rc # reverse-continue to the last time the width was changed
(rr) rn # reverse-next to the previous line
(rr) reverse-fin # run to where this function was called from
You can debug multi-process thorium using rr -f [PID]
for processes fork()ed from a zygote process without exec,
which includes renderer processes,
or rr -p [PID] for other processes.
To find the process id you can either run rr ps after recording, or for
renderer processes use --vmodule=render_frame_impl=1 which will log a
message on navigations. Example:
$ rr record out/Debug/content_shell --disable-hang-monitor --vmodule=render_frame_impl=1 https://www.google.com/
rr: Saving execution to trace directory `...'.
...
[128515:128515:0320/164124.768687:VERBOSE1:render_frame_impl.cc(4244)] Committed provisional load: https://www.google.com/
From the log message we can see that the site was loaded into process 128515 and can set a breakpoint for when that process is forked.
rr replay -f 128515
If you want to call debugging functions from gdb that use LOG(),
then those functions need to disable the printing of timestamps using
SetLogItems.
See LayoutObject::ShowLayoutObject() for an example of this, and
issue 2829 for why it is needed.
If rr doesn't work correctly, the rr developers are generally quite responsive to bug reports, especially ones that have enough information so that they don't have to build Chromium.
See Also:
- The Chromium Chronicle #13: Time-Travel Debugging with RR
- @davidbaron demo using rr
- @davidbaron demo using pernosco (Googlers: see go/pernosco)
Graphical Debugging Aid for Chromium Views
The following link describes a tool that can be used on Linux, Windows and Mac under GDB.
graphical_debugging_aid_chromium_views
Faster startup
Use the gdb-add-index script (e.g.
build/gdb-add-index out/Debug/browser_tests)
Only makes sense if you run the binary multiple times or maybe if you use the
component build since most .so files won't require reindexing on a rebuild.
See https://groups.google.com/a/chromium.org/forum/#!searchin/chromium-dev/gdb-add-index/chromium-dev/ELRuj1BDCL4/5Ki4LGx41CcJ for more info.
You can improve GDB load time significantly at the cost of link time by not
splitting symbols from the object files. In GN, set use_debug_fission=false in
your "gn args".
Source level debug with -fdebug-compilation-dir
When strip_absolute_paths_from_debug_symbols is enabled (which is the
default), gdb may not be able to find debug files, making source-level debugging
impossible. See
gdbinit
to configure gdb to be able to find debug files.
Core files
ulimit -c unlimited should cause all Thorium processes (run from that shell) to
dump cores, with the possible exception of some sandboxed processes.
Some sandboxed subprocesses might not dump cores unless you pass the
--allow-sandbox-debugging flag.
If the problem is a freeze rather than a crash, you may be able to trigger a core-dump by sending SIGABRT to the relevant process:
kill -6 [process id]
Breakpad minidump files
Running Tests
Many of our tests bring up windows on screen. This can be annoying (they steal
your focus) and hard to debug (they receive extra events as you mouse over them).
Instead, use Xvfb or Xephyr to run a nested X session to debug them, as
outlined on testing/web_tests_linux.md.
Browser tests
By default the browser_tests forks a new browser for each test. To debug the
browser side of a single test, use a command like
gdb --args out/Debug/browser_tests --single-process-tests --gtest_filter=MyTestName
note the use of single-process-tests -- this makes the test harness and
browser process share the outermost process.
To debug a renderer process in this case, use the tips above about renderers.
Web tests
See testing/web_tests_linux.md for some tips. In particular,
note that it's possible to debug a web test via sshing to a Linux box; you
don't need anything on screen if you use Xvfb.
UI tests
UI tests are run in forked browsers. Unlike browser tests, you cannot do any
single process tricks here to debug the browser. See below about
BROWSER_WRAPPER.
To pass flags to the browser, use a command line like
--extra-chrome-flags="--foo --bar".
Timeouts
UI tests have a confusing array of timeouts in place. (Pawel is working on reducing the number of timeouts.) To disable them while you debug, set the timeout flags to a large value:
--test-timeout=100000000--ui-test-action-timeout=100000000--ui-test-terminate-timeout=100000000
To replicate Window Manager setup on the bots
Chromium try bots and main waterfall's bots run tests under Xvfb&openbox combination. Xvfb is an X11 server that redirects the graphical output to the memory, and openbox is a simple window manager that is running on top of Xvfb. The behavior of openbox is markedly different when it comes to focus management and other window tasks, so test that runs fine locally may fail or be flaky on try bots. To run the tests on a local machine as on a bot, follow these steps:
Make sure you have openbox:
apt-get install openbox
Start Xvfb and openbox on a particular display:
Xvfb :6.0 -screen 0 1280x1024x24 & DISPLAY=:6.0 openbox &
Run your tests with graphics output redirected to that display:
DISPLAY=:6.0 out/Debug/browser_tests --gtest_filter="MyBrowserTest.MyActivateWindowTest"
You can look at a snapshot of the output by:
xwd -display :6.0 -root | xwud
Alternatively, you can use testing/xvfb.py to set up your environment for you:
testing/xvfb.py out/Debug/browser_tests \
--gtest_filter="MyBrowserTest.MyActivateWindowTest"
BROWSER_WRAPPER
You can also get the browser under a debugger by setting the BROWSER_WRAPPER
environment variable. (You can use this for browser_tests too, but see above
for discussion of a simpler way.)
BROWSER_WRAPPER='xterm -e gdb --args' out/Debug/browser_tests
Replicating try bot Slowness
Try bots are pretty stressed, and can sometimes expose timing issues you can't normally reproduce locally.
You can simulate this by shutting down all but one of the CPUs (http://www.cyberciti.biz/faq/debian-rhel-centos-redhat-suse-hotplug-cpu/) and running a CPU loading tool (e.g., http://www.devin.com/lookbusy/). Now run your test. It will run slowly, but any flakiness found by the try bot should replicate locally now - and often nearly 100% of the time.
Logging
Seeing all LOG(foo) messages
Default log level hides LOG(INFO). Run with --log-level=0 and
--enable-logging=stderr flags.
Newer versions of Thorium with VLOG may need --v=1 too. For more VLOG tips, see the chromium-dev thread.
Seeing IPC debug messages
Run with CHROME_IPC_LOGGING=1 eg.
CHROME_IPC_LOGGING=1 out/thorium/thorium
or within gdb:
set environment CHROME_IPC_LOGGING 1
If some messages show as unknown, check if the list of IPC message headers in
chrome/common/logging_chrome.cc is
up to date. In case this file reference goes out of date, try looking for usage
of macros like IPC_MESSAGE_LOG_ENABLED or IPC_MESSAGE_MACROS_LOG_ENABLED.
Profiling
See https://sites.google.com/a/chromium.org/dev/developers/profiling-chromium-and-webkit and Linux Profiling.
i18n
We obey your system locale. Try something like:
LANG=ja_JP.UTF-8 out/thorium/thorium
If this doesn't work, make sure that the LANGUAGE, LC_ALL and LC_MESSAGE
environment variables aren't set -- they have higher priority than LANG in the
order listed. Alternatively, just do this:
LANGUAGE=fr out/thorium/thorium
Note that because we use GTK, some locale data comes from the system -- for example, file save boxes and whether the current language is considered RTL. Without all the language data available, Thorium will use a mixture of your system language and the language you run Thorium in.
Here's how to install the Arabic (ar) and Hebrew (he) language packs:
sudo apt-get install language-pack-ar language-pack-he \
language-pack-gnome-ar language-pack-gnome-he
Note that the --lang flag does not work properly for this.
On non-Debian systems, you need the gtk30.mo files. (Please update these docs
with the appropriate instructions if you know what they are.)
Breakpad
See the last section of Linux Crash Dumping.
Drag and Drop
If you break in a debugger during a drag, Thorium will have grabbed your mouse
and keyboard so you won't be able to interact with the debugger! To work around
this, run via Xephyr. Instructions for how to use Xephyr are on the
Running web tests on Linux page.
Tracking Down Bugs
Isolating Regressions
Old builds are archived here: https://build.chromium.org/buildbot/snapshots/chromium-rel-linux/ (TODO: does not exist).
tools/bisect-builds.py in the tree automates bisecting through the archived
builds. Despite a computer science education, I am still amazed how quickly
binary search will find its target.
Screen recording for bug reports
sudo apt-get install gtk-recordmydesktop
Version-specific issues
Google Chrome
Google Chrome binaries don't include symbols. Googlers can read where to get symbols from the Google-internal wiki.
Ubuntu Chromium
Since we don't build the Ubuntu packages (Ubuntu does) we can't get useful backtraces from them. Direct users to https://wiki.ubuntu.com/Chromium/Debugging
Fedora's Chromium
Like Ubuntu, but direct users to https://fedoraproject.org/wiki/TomCallaway/Chromium_Debug
Xlib
If you're trying to track down X errors like:
The program 'thorium' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadDrawable (invalid Pixmap or Window parameter)'.
Some strategies are:
- pass
--syncon the command line to make all X calls synchronous - run thorium via xtrace
- turn on IPC debugging (see above section)
Window Managers
To test on various window managers, you can use a nested X server like Xephyr.
Instructions for how to use Xephyr are on the
Running web tests on Linux page.
If you need to test something with hardware accelerated compositing
(e.g., compiz), you can use Xgl (sudo apt-get install xserver-xgl). E.g.:
Xgl :1 -ac -accel glx:pbuffer -accel xv:pbuffer -screen 1024x768
Mozilla Tips
https://developer.mozilla.org/en/Debugging_Mozilla_on_Linux_FAQ
Google Chrome Symbol Files
Symbols for Google Chrome's official builds are available from
https://edgedl.me.gvt1.com/chrome/linux/symbols/google-chrome-debug-info-linux64-${VERSION}.zip
where ${VERSION} is any version of Google Chrome that has recently been served
to Stable, Beta, or Unstable (Dev) channels on Linux.