[Sync] Move PassphraseType to org.chromium.sync.

[chromium-blink-merge.git] / docs / linux_debugging.md

#summary tips for debugging on Linux
#labels 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
**GDB-7.7 is required in order to debug Chrome on Linux.**

Any prior version will fail to resolve symbols or segfault.

### Basic browser process debugging

```
gdb -tui -ex=r --args out/Debug/chrome --disable-seccomp-sandbox http://google.com
```

### Allowing attaching to foreign processes
On distributions that use the [Yama LSM](https://www.kernel.org/doc/Documentation/security/Yama.txt) (that includes Ubuntu and Chrome OS), 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 Chromium itself spawns the renderers, it can be tricky to grab a particular with gdb.  This command does the trick:
```
chrome --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:
```
chrome --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
```
chrome "--renderer-cmd-prefix=gdb --args"
```

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 http://www.chromium.org/blink/getting-started-with-blink-debugging for more information on how this can be done.

#### 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 chrome --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 chrome` will not give very helpful output. Try `pstree -p | grep chrome` to get something like

```
        |                      |-bash(21969)---chrome(672)-+-chrome(694)
        |                      |                           |-chrome(695)---chrome(696)-+-{chrome}(697)
        |                      |                           |                           \-{chrome}(709)
        |                      |                           |-{chrome}(675)
        |                      |                           |-{chrome}(678)
        |                      |                           |-{chrome}(679)
        |                      |                           |-{chrome}(680)
        |                      |                           |-{chrome}(681)
        |                      |                           |-{chrome}(682)
        |                      |                           |-{chrome}(684)
        |                      |                           |-{chrome}(685)
        |                      |                           |-{chrome}(705)
        |                      |                           \-{chrome}(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 Chrome'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`:
```
chrome --plugin-launcher='xterm -e gdb --args'
```

_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 chrome --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 http://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.  The directory `tools/gdb/` contains a Python gdb scripts useful for Chromium code.  There are similar scripts [in WebKit](http://trac.webkit.org/wiki/GDB) (in fact, the Chromium script relies on using it with the WebKit one).

To include these pretty-printers with your gdb, put the following into `~/.gdbinit`:
```
python
import sys
sys.path.insert(0, "<path/to/chromium/src>/third_party/WebKit/Tools/gdb/")
import webkit
sys.path.insert(0, "<path/to/chromium/src>/tools/gdb/")
import gdb_chrome
```

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
```

[More STL GDB macros](http://www.yolinux.com/TUTORIALS/src/dbinit_stl_views-1.01.txt)

### Graphical Debugging Aid for Chromium Views

The following link describes a tool that can be used on Linux, Windows and Mac under GDB.

http://code.google.com/p/chromium/wiki/GraphicalDebuggingAidChromiumViews

### 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.

Alternatively, specify:
```
linux_use_debug_fission=0
```

in GYP\_DEFINES. This improves load time of gdb significantly at the cost of link time.

## Core files
`ulimit -c unlimited` should cause all Chrome 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

See LinuxMinidumpToCore

## 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 LayoutTestsLinux.

### 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 --gtest_filter=MyTestName
```
**note the underscore in single\_process** -- 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.

### Layout tests
See LayoutTestsLinux for some tips.  In particular, note that it's possible to debug a layout test via `ssh`ing 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 memeory, 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 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 Trybot Slowness

Trybots 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 trybot 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 chromium with VLOG may need --v=1 too. For more VLOG tips, see the chromium-dev thread: http://groups.google.com/a/chromium.org/group/chromium-dev/browse_thread/thread/dcd0cd7752b35de6?pli=1

### Seeing IPC debug messages
Run with CHROME\_IPC\_LOGGING=1 eg.
```
CHROME_IPC_LOGGING=1 out/Debug/chrome
```
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.

## Using valgrind

To run valgrind on the browser and renderer processes, with our suppression file and flags:
```
$ cd $CHROMIUM_ROOT/src
$ tools/valgrind/valgrind.sh out/Debug/chrome
```

You can use valgrind on chrome and/or on the renderers e.g
`valgrind --smc-check=all ../sconsbuild/Debug/chrome`
or by passing valgrind as the argument to `--render-cmd-prefix`.

Beware that there are several valgrind "false positives" e.g. pickle, sqlite and some instances in webkit that are ignorable. On systems with prelink and address space randomization (e.g. Fedora), you may also see valgrind errors in libstdc++ on startup and in gnome-breakpad.

Valgrind doesn't seem to play nice with tcmalloc. To disable tcmalloc run GYP
```
$ cd $CHROMIUM_ROOT/src
$ build/gyp_chromium -Duse_allocator=none
```
and rebuild.

## Profiling
See https://sites.google.com/a/chromium.org/dev/developers/profiling-chromium-and-webkit and http://code.google.com/p/chromium/wiki/LinuxProfiling

## i18n
We obey your system locale.  Try something like:
```
LANG=ja_JP.UTF-8 out/Debug/chrome
```
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/Debug/chrome
```

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, Chrome will use a mixture of your system language and the language you run Chrome 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 `gtk20.mo` files.  (Please update these docs with the appropriate instructions if you know what they are.)

## Breakpad
See the last section of LinuxCrashDumping; you need to set a gyp variable and an environment variable for the crash dump tests to work.

## Drag and Drop
If you break in a debugger during a drag, Chrome 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 LayoutTestsLinux page.

## Tracking Down Bugs

### Isolating Regressions
Old builds are archived here: http://build.chromium.org/buildbot/snapshots/chromium-rel-linux/

`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](http://wiki/Main/ChromeOfficialBuildLinux#The_Build_Archive).

### 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 'chrome' 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 `--sync` on the command line to make all X calls synchronous
  * run chrome via [xtrace](http://xtrace.alioth.debian.org/)
  * 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 LayoutTestsLinux 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