325 lines
15 KiB
ReStructuredText
325 lines
15 KiB
ReStructuredText
Understanding Crash Reports
|
|
===========================
|
|
|
|
+--------------------------------------------------------------------+
|
|
| This page is an import from MDN and the contents might be outdated |
|
|
+--------------------------------------------------------------------+
|
|
|
|
If a user experiences a crash they will be prompted to submit a raw
|
|
crash report, which is generated by Breakpad. The raw crash report is
|
|
received by `Socorro <https://github.com/mozilla/socorro>`__ which
|
|
`creates <https://github.com/mozilla/socorro/blob/master/socorro/processor/mozilla_processor_2015.py>`__
|
|
a processed crash report. The processed crash report is based on the raw
|
|
crash report but also has a signature, classifications, and a number of
|
|
improved fields (e.g. OS, product, version). Many of the fields in both
|
|
the raw crash report and the processed crash report are viewable and
|
|
searchable on `crash-stats <https://crash-stats.mozilla.org/>`__.
|
|
Although there are two distinct crash reports, the raw and the
|
|
processed, people typically talk about a single "crash report" because
|
|
crash-stats mostly presents them in a combined way.
|
|
|
|
Each crash report contains a wealth of data about the crash
|
|
circumstances. Despite this, many crash reports lack sufficient data for
|
|
a developer to understand why the crash occurred. As well as providing a
|
|
general overview, this page aims to highlight parts of a crash report
|
|
that may provide non-obvious insights.
|
|
|
|
Note that most crash report fields are visible, but a few
|
|
privacy-sensitive parts of it are only available to users who are logged
|
|
in and have "minidump access". A relatively small number of users have
|
|
minidump access, and they are required to follow certain rules. For
|
|
access, see the `Protected Data Access docs on Crash Stats
|
|
<https://crash-stats.mozilla.org/documentation/protected_data_access/>`__.
|
|
|
|
Each crash report has the following tabs: Details, Metadata, Modules,
|
|
Raw Dump, Extensions, and (optional) Correlations.
|
|
|
|
Details tab
|
|
-----------
|
|
|
|
The Details tab is the first place to look because it contains the most
|
|
important pieces of information.
|
|
|
|
Primary fields
|
|
~~~~~~~~~~~~~~
|
|
|
|
| The first part of the Details tab shows a table containing the most
|
|
important crash report fields. It includes such things as when the
|
|
crash occurred, in which product and version, the crash kind, and
|
|
various details about the OS and configuration of the machine on which
|
|
the crash occurred. The following screenshot shows some of these
|
|
fields.
|
|
| |Example fields in the "Details" tab of a crash report|
|
|
|
|
All fields have a tool-tip. For many fields, the tool-tip describes its
|
|
meaning. For all fields, the tool-tip indicates the key to use when you
|
|
want to do searches involving this field. (The field name is usually but
|
|
not always similar to the search key. E.g. the field "Adapter Device ID"
|
|
has the search key "adapter_device_id".) These descriptions are shown in
|
|
the `SuperSearchFields
|
|
API <https://crash-stats.mozilla.org/api/SuperSearchFields/>`__ and can be
|
|
`modified in super_search_fields.py <https://github.com/mozilla-services/socorro/blob/main/socorro/external/es/super_search_fields.py>`__
|
|
or by writing up a `bug in Socorro <https://bugzilla.mozilla.org/enter_bug.cgi?format=__standard__&product=Socorro>`__.
|
|
|
|
The fields present in this tab vary depending on the crash kind. Not all
|
|
fields are always present.
|
|
|
|
The "Signature" field is the main identifier or label for a crash report.
|
|
Rather than considering each crash report in isolation, we want to put
|
|
crash reports into clusters so we can deal with groups of them at once.
|
|
An ideal clustering algorithm would put all crash reports with the same
|
|
root cause into a single cluster, and all crash reports with different
|
|
root causes into different clusters. The crash signature is our
|
|
imperfect but still useful attempt at such an algorithm. Most crash
|
|
signatures are based on the crashing stack trace, but some
|
|
special-purpose annotations are used to indicate particular kinds of
|
|
crashes.
|
|
|
|
- ``Abort``: A controlled abort, e.g. via ``NS_RUNTIMEABORT``.
|
|
(Controlled aborts that occur via ``MOZ_CRASH`` or
|
|
``MOZ_RELEASE_ASSERT`` currently don't get an ``Abort`` annotation,
|
|
but they do get a "MOZ_CRASH Reason" field.)
|
|
- ``OOM | <size>``, where ``<size>`` is one of ``large``, ``small``,
|
|
``unknown``: an out-of-memory (OOM) abort. The ``<size>`` annotation
|
|
is determined by the "OOM Allocation Size" field; if that field is
|
|
missing ``<size>`` will be ``unknown``.
|
|
- ``hang``: a hang prior to shutdown.
|
|
- ``shutdownhang``: a hang during shutdown.
|
|
- ``IPCError-browser``: a problem involving IPC. If the parent Firefox
|
|
process detects that the child process has sent broken or
|
|
unprocessable IPDL data, or is not shutting down in a timely manner,
|
|
it kills the child process with a crash report. These crashes will
|
|
now have a signature that indicates why the process was killed,
|
|
rather than the child stack at the moment.
|
|
|
|
When no special-purpose annotation is present and the signature begins
|
|
with a stack frame, it's usually a vanilla uncontrolled crash. The crash
|
|
cause can be determined from the "Crash Reason" field. Most commonly
|
|
it's a bad memory access. In that case, on Windows you can tell from the
|
|
reason field if the crash occurred while reading, writing or executing
|
|
memory (e.g. ``EXCEPTION_VIOLATION_ACCESS_READ`` indicates a bad memory
|
|
read). On Mac and Linux the reason will be SIGSEGV or SIGBUS and you
|
|
cannot tell from this field what kind of memory access it was.
|
|
|
|
See `this
|
|
file <https://github.com/mozilla-services/socorro/blob/master/socorro/signature/README.rst>`__
|
|
for a detailed explanation of the crash report signature generation
|
|
procedure, and for information on how modify this procedure.
|
|
|
|
There are no fields that uniquely identify the user that a crash report
|
|
came from, but if you want to know if multiple crashes come from a
|
|
single user the "Install Time" field is a good choice. Use it in
|
|
conjunction with other fields that don't change, such as those
|
|
describing the OS or graphics card, for additional confidence.
|
|
|
|
For bad memory accesses, the "Crash Address" field can give additional
|
|
indications what went wrong.
|
|
|
|
- 0x0 is probably a null pointer deference[*].
|
|
- Small addresses like 0x8 can indicate an object access (e.g.
|
|
``this->mFoo``) via a null ``this`` pointer.
|
|
- Addresses like 0xfffffffffd8 might be stack accesses, depending on
|
|
the platform[*].
|
|
- Addresses like 0x80cdefd3 might be heap accesses, depending on the
|
|
platform.
|
|
- Addresses may be poisoned: 0xe4 indicates the address comes from
|
|
memory that has been allocated by jemalloc but not yet initialized;
|
|
0xe5 indicates the address comes from memory freed by jemalloc. The
|
|
JS engine also has multiple poison values defined in
|
|
``js/src/jsutil.h``.
|
|
|
|
[*] Note that due to the way addressing works on x86-64, if the crash
|
|
address is 0x0 for a Linux/macOS crash report, or 0xffffffffffffffff for
|
|
a Windows crash report, it's highly likely that the value is incorrect.
|
|
(There is a `bug
|
|
report <https://bugzilla.mozilla.org/show_bug.cgi?id=1493342>`__ open
|
|
for this problem.) You can sanity-check these crashes by looking at the
|
|
raw dump or minidump in the Raw Dump tab (see below).
|
|
|
|
Note that for non-release builds the "Version" field represents multiple
|
|
different builds since nightly and beta version numbers are reused for
|
|
builds created over a series of days until the version number is bumped.
|
|
(The "Build ID" field can disambiguate.) It's not currently possible to
|
|
`restrict searches to a given version or
|
|
later <https://bugzilla.mozilla.org/show_bug.cgi?id=1401517>`__ (using
|
|
>= with a build ID and a given release channel may work around this).
|
|
|
|
Some fields, such as "URL" and "Email Address", are privacy-sensitive
|
|
and are only visible to users with minidump access.
|
|
|
|
The Windows-only "Total Virtual Memory" field indicates if the Firefox
|
|
build and OS are 32-bit or 64-bit.
|
|
|
|
- A value of 2 GiB indicates 32-bit Firefox on 32-bit Windows.
|
|
- A value of 3 or 4 GiB indicates 32-bit Firefox on 64-bit Windows
|
|
(a.k.a. "WoW64"). Such a user could switch to 64-bit Firefox.
|
|
- A value much larger than 4 GiB (e.g. 128 TiB) indicates 64-bit
|
|
Firefox. (The "Build Architecture" field should be "amd64" in this
|
|
case.)
|
|
|
|
Some crash reports might contain a memory report. This memory report will
|
|
have been made some time before the crash, at a time when available
|
|
memory was low. In this case, a number of key measurements from the
|
|
memory report are shown in the Details tab, each one having a field name
|
|
starting with "MR:", short for "memory report". The full memory report
|
|
can be obtained in the Raw Dump tab (see below).
|
|
|
|
Bug-related information
|
|
~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The second part of the Details tab shows bug-related information, as the
|
|
following screenshot shows.
|
|
|
|
|Information relating to bug reports in the "Details" tab of a crash
|
|
report|
|
|
|
|
The "Report this bug in" links can be used to easily file bug reports.
|
|
Each one links to a Bugzilla bug report creation page that has various
|
|
fields pre-filled, such as the crash signature.
|
|
|
|
The "Related Bugs" section shows related bug reports, as determined by
|
|
the crash signature.
|
|
|
|
Stack traces
|
|
~~~~~~~~~~~~
|
|
|
|
The third part of the Details tab shows the stack trace and thread
|
|
number of the crashing thread, as the following screenshot shows.
|
|
|
|
|Information relating to threads in the "Details" tab of a crash report|
|
|
|
|
Each stack frame has a link to the source code, when possible. If a
|
|
crash is new, the regressing changeset can often be identified by
|
|
looking for recent changes in the blame annotations for one or more of
|
|
the top stack frames. Blame annotations are also good for identifying
|
|
who might know about the code in question.
|
|
|
|
Sometimes the highlighted source code is puzzling, e.g. the identified
|
|
line may not touch memory even though the crash is memory-related. This
|
|
can be caused by compiler optimizations. It's often better to look at
|
|
the disassembly (e.g. in a minidump) to understand exactly what code is
|
|
being executed.
|
|
|
|
Stack frame entries take on a variety of forms.
|
|
|
|
- The simplest are functions names, such as ``NS_InitXPCOM2``.
|
|
- Name/address pairs such as ``nss3.dll@0x1eb720`` are within system
|
|
libraries.
|
|
- Names such as ``F1398665248_____________________________`` ('F'
|
|
followed by many numbers then many underscores) are in Flash.
|
|
- Addresses such as ``@0xe1a850ac`` may indicate an address that wasn't
|
|
part of any legitimate code. If an address such as this occurs in the
|
|
first stack frame, the crash may be
|
|
`exploitable <https://developer.mozilla.org/en-US/docs/Mozilla/Security/Exploitable_crashes>`__.
|
|
|
|
Stack traces for other threads can be viewed by clicking on the small
|
|
"Show other threads" link.
|
|
|
|
If the crash report is for a hang, the crashing thread will be the
|
|
"watchdog" thread, which exists purely to detect hangs; its top stack
|
|
frame will be something
|
|
like\ :literal:`mozilla::`anonymous namespace'::RunWatchdog`. In that
|
|
case you should look at the other threads' stack traces to determine the
|
|
problem; many of them will be waiting on some kind of response, as shown
|
|
by a top stack frame containing a function like
|
|
``NtWaitForSingleObject`` or ``ZwWaitForMultipleObjects``.
|
|
|
|
Metadata tab
|
|
------------
|
|
|
|
The Metadata tab is similar to the first part of the Details tab,
|
|
containing a table with various fields. These are the fields from the
|
|
raw crash report, ordered alphabetically by field name, but with
|
|
privacy-sensitive fields shown only to users with minidump access. There
|
|
is some overlap with the fields shown in the Details tab.
|
|
|
|
Modules tab
|
|
-----------
|
|
|
|
The modules tab shows all the system libraries loaded at the time of the
|
|
crash, as the following screenshot shows.
|
|
|
|
|Table of modules in the "Modules" tab of a crash report|
|
|
|
|
On Windows these are mostly DLLs, on Mac they are mostly ``.dylib``
|
|
files, and on Linux they are mostly ``.so`` files.
|
|
|
|
This information is most useful for Windows crashes, because DLLs loaded
|
|
by antivirus software or malware often cause Firefox to crash.
|
|
Correlations between loaded modules and crash signatures can be seen in
|
|
the "Correlations" tab (see below).
|
|
|
|
`This page <https://support.mozilla.org/en-US/kb/helping-crashes>`__
|
|
says that files lacking version/debug identifier/debug filename are
|
|
likely to be malware.
|
|
|
|
Raw Dump tab
|
|
------------
|
|
|
|
The first part of the Raw Dump tab shows the raw crash report, in JSON
|
|
format. Once again, privacy-sensitive fields are shown only to users
|
|
with minidump access.
|
|
|
|
|JSON data in the "Raw Dump" tab of a crash report|
|
|
|
|
For users with minidump access, the second part of the Raw Dump tab has
|
|
some links, as the following screenshot shows.
|
|
|
|
|Links to downloadable files in the "Raw Dump" tab of a crash report|
|
|
|
|
These links are to the following items.
|
|
|
|
#. A minidump. Minidumps can be extremely useful in understanding a
|
|
crash report; see :ref:`this page <Debugging A Minidump>` for an
|
|
explanation how to use them.
|
|
#. The aforementioned JSON raw crash report.
|
|
#. The memory report contained within the crash report.
|
|
#. The unredacted crash report, which has additional information.
|
|
|
|
Extensions tab
|
|
--------------
|
|
|
|
The Extensions tab shows which extensions are installed and enabled.
|
|
|
|
|Table of extensions in the "Extensions" tab of a crash report|
|
|
|
|
Usually it just shows an ID rather than the proper extension name.
|
|
|
|
Note that several extensions ship by default with Firefox and so will be
|
|
present in almost all crash reports. (The exact set of default
|
|
extensions depends on the release channel.) The least obvious of these
|
|
has an Id of ``{972ce4c6-7e08-4474-a285-3208198ce6fd}``, which is the
|
|
default Firefox theme. Some (but not all) of the other extensions
|
|
shipped by default have the following Ids: ``webcompat@mozilla.org``,
|
|
``e10srollout@mozilla.org``, ``firefox@getpocket.com``,
|
|
``flyweb@mozilla.org``, ``loop@mozilla.org``.
|
|
|
|
If an extension only has a hexadecimal identifier, a Google search of
|
|
that identifier is usually enough to identify the extension's name.
|
|
|
|
This information is useful because some crashes are caused by
|
|
extensions. Correlations between extensions and crash signatures can be
|
|
seen in the "Correlations" tab (see below).
|
|
|
|
Correlations tab
|
|
----------------
|
|
|
|
This tab is only shown when crash-stats identifies correlations between
|
|
a crash and modules or extensions that are present, which happens
|
|
occasionally.
|
|
|
|
See also
|
|
--------
|
|
|
|
- `A talk about understanding crash
|
|
reports <https://air.mozilla.org/a-talk-about-understanding-crash-reports/>`__,
|
|
by David Baron, from March 2016.
|
|
- :ref:`A guide to searching crash reports`
|
|
|
|
.. |Example fields in the "Details" tab of a crash report| image:: https://mdn.mozillademos.org/files/13579/Details1.png
|
|
.. |Information relating to bug reports in the "Details" tab of a crash report| image:: https://mdn.mozillademos.org/files/13581/Details2.png
|
|
.. |Information relating to threads in the "Details" tab of a crash report| image:: https://mdn.mozillademos.org/files/13583/Details3.png
|
|
.. |Table of modules in the "Modules" tab of a crash report| image:: https://mdn.mozillademos.org/files/13593/Modules1.png
|
|
.. |JSON data in the "Raw Dump" tab of a crash report| image:: https://mdn.mozillademos.org/files/13595/RawDump1.png
|
|
.. |Links to downloadable files in the "Raw Dump" tab of a crash report| image:: https://mdn.mozillademos.org/files/14047/raw-dump-links.png
|
|
.. |Table of extensions in the "Extensions" tab of a crash report| image:: https://mdn.mozillademos.org/files/13599/Extensions1.png
|