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+Licensed under the Apache License, Version 2.0 (the "License");
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+You may obtain a copy of the License at
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+    http://www.apache.org/licenses/LICENSE-2.0
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+
+# Crashpad Overview Design
+
+[TOC]
+
+## Objective
+
+Crashpad is a library for capturing, storing and transmitting postmortem crash
+reports from a client to an upstream collection server. Crashpad aims to make it
+possible for clients to capture process state at the time of crash with the best
+possible fidelity and coverage, with the minimum of fuss.
+
+Crashpad also provides a facility for clients to capture dumps of process state
+on-demand for diagnostic purposes.
+
+Crashpad additionally provides minimal facilities for clients to adorn their
+crashes with application-specific metadata in the form of per-process key/value
+pairs. More sophisticated clients are able to adorn crash reports further
+through extensibility points that allow the embedder to augment the crash report
+with application-specific metadata.
+
+## Background
+
+It’s an unfortunate truth that any large piece of software will contain bugs
+that will cause it to occasionally crash. Even in the absence of bugs, software
+incompatibilities can cause program instability.
+
+Fixing bugs and incompatibilities in client software that ships to millions of
+users around the world is a daunting task. User reports and manual reproduction
+of crashes can work, but even given a user report, often times the problem is
+not readily reproducible. This is for various reasons, such as e.g. system
+version or third-party software incompatibility, or the problem can happen due
+to a race of some sort. Users are also unlikely to report problems they
+encounter, and user reports are often of poor quality, as unfortunately most
+users don’t have experience with making good bug reports.
+
+Automatic crash telemetry has been the best solution to the problem so far, as
+this relieves the burden of manual reporting from users, while capturing the
+hardware and software state at the time of crash.
+
+TODO(siggi): examples of this?
+
+Crash telemetry involves capturing postmortem crash dumps and transmitting them
+to a backend collection server. On the server they can be stackwalked and
+symbolized, and evaluated and aggregated in various ways. Stackwalking and
+symbolizing the reports on an upstream server has several benefits over
+performing these tasks on the client. High-fidelity stackwalking requires access
+to bulky unwind data, and it may be desirable to not ship this to end users out
+of concern for the application size. The process of symbolization requires
+access to debugging symbols, which can be quite large, and the symbolization
+process can consume considerable other resources. Transmitting un-stackwalked
+and un-symbolized postmortem dumps to the collection server also allows deep
+analysis of individual dumps, which is often necessary to resolve the bug
+causing the crash.
+
+Transmitting reports to the collection server allows aggregating crashes by
+cause, which in turn allows assessing the importance of different crashes in
+terms of the occurrence rate and e.g. the potential security impact.
+
+A postmortem crash dump must contain the program state at the time of crash
+with sufficient fidelity to allow diagnosing and fixing the problem. As the full
+program state is usually too large to transmit to an upstream server, the
+postmortem dump captures a heuristic subset of the full state.
+
+The crashed program is in an indeterminate state and, in fact, has often crashed
+because of corrupt global state - such as heap. It’s therefore important to
+generate crash reports with as little execution in the crashed process as
+possible. Different operating systems vary in the facilities they provide for
+this.
+
+## Overview
+
+Crashpad is a client-side library that focuses on capturing machine and program
+state in a postmortem crash report, and transmitting this report to a backend
+server - a “collection server”. The Crashpad library is embedded by the client
+application. Conceptually, Crashpad breaks down into the handler and the client.
+The handler runs in a separate process from the client or clients. It is
+responsible for snapshotting the crashing client process’ state on a crash,
+saving it to a crash dump, and transmitting the crash dump to an upstream
+server. Clients register with the handler to allow it to capture and upload
+their crashes.
+
+### The Crashpad handler
+
+The Crashpad handler is instantiated in a process supplied by the embedding
+application. It provides means for clients to register themselves by some means
+of IPC, or where operating system support is available, by taking advantage of
+such support to cause crash notifications to be delivered to the handler. On
+crash, the handler snapshots the crashed client process’ state, writes it to a
+postmortem dump in a database, and may also transmit the dump to an upstream
+server if so configured.
+
+The Crashpad handler is able to handle cross-bitted requests and generate crash
+dumps across bitness, where e.g. the handler is a 64-bit process while the
+client is a 32-bit process or vice versa. In the case of Windows, this is
+limited by the OS such that a 32-bit handler can only generate crash dumps for
+32-bit clients, but a 64-bit handler can acquire nearly all of the detail for a
+32-bit process.
+
+### The Crashpad client
+
+The Crashpad client provides two main facilities.
+1. Registration with the Crashpad handler.
+2. Metadata communication to the Crashpad handler on crash.
+
+A Crashpad embedder links the Crashpad client library into one or more
+executables, whether a loadable library or a program file. The client process
+then registers with the Crashpad handler through some mode of IPC or other
+operating system-specific support.
+
+On crash, metadata is communicated to the Crashpad handler via the CrashpadInfo
+structure. Each client executable module linking the Crashpad client library
+embeds a CrashpadInfo structure, which can be updated by the client with
+whatever state the client wishes to record with a crash.
+
+![Overview image](overview.png)
+
+Here is an overview picture of the conceptual relationships between embedder (in
+light blue), client modules (darker blue), and Crashpad (in green). Note that
+multiple client modules can contain a CrashpadInfo structure, but only one
+registration is necessary.
+
+## Detailed Design
+
+### Requirements
+
+The purpose of Crashpad is to capture machine, OS and application state in
+sufficient detail and fidelity to allow developers to diagnose and, where
+possible, fix the issue causing the crash.
+
+Each distinct crash report is assigned a globally unique ID, in order to allow
+users to associate them with a user report, report in bug reports and so on.
+
+It’s critical to safeguard the user’s privacy by ensuring that no crash report
+is ever uploaded without user consent. Likewise it’s important to ensure that
+Crashpad never captures or uploads reports from non-client processes.
+
+### Concepts
+
+* **Client ID**. A UUID tied to a single instance of a Crashpad database. When
+  creating a crash report, the Crashpad handler includes the client ID stored
+  in the database. This provides a means to determine how many individual end
+  users are affected by a specific crash signature.
+
+* **Crash ID**. A UUID representing a single crash report. Uploaded crash
+  reports also receive a “server ID.” The Crashpad database indexes both the
+  locally-generated and server-generated IDs.
+
+* **Collection Server**. See [crash server documentation.](
+  https://goto.google.com/crash-server-overview)
+
+* **Client Process**. Any process that has registered with a Crashpad handler.
+
+* **Handler process**. A process hosting the Crashpad handler library. This may
+  be a dedicated executable, or it may be hosted within a client executable
+  with control passed to it based on special signaling under the client’s
+  control, such as a command-line parameter.
+
+* **CrashpadInfo**. A structure used by client modules to provide information to
+  the handler.
+
+* **Annotations**. Each CrashpadInfo structure points to a dictionary of
+  {string, string} annotations that the client can use to communicate
+  application state in the case of crash.
+
+* **Database**. The Crashpad database contains persistent client settings as
+  well as crash dumps pending upload.
+
+TODO(siggi): moar concepts?
+
+### Overview Picture
+
+Here is a rough overview picture of the various Crashpad constructs, their
+layering and intended use by clients.
+
+![Layering image](layering.png)
+
+Dark blue boxes are interfaces, light blue boxes are implementation. Gray is the
+embedding client application. Note that wherever possible, implementation that
+necessarily has to be OS-specific, exposes OS-agnostic interfaces to the rest of
+Crashpad and the client.
+
+### Registration
+
+The particulars of how a client registers with the handler varies across
+operating systems.
+
+#### macOS
+
+At registration time, the client designates a Mach port monitored by the
+Crashpad handler as the EXC_CRASH exception port for the client. The port may be
+acquired by launching a new handler process or by retrieving service already
+registered with the system. The registration is maintained by the kernel and is
+inherited by subprocesses at creation time by default, so only the topmost
+process of a process tree need register.
+
+Crashpad provides a facility for a process to disassociate (unregister) with an
+existing crash handler, which can be necessary when an older client spawns an
+updated version.
+
+#### Windows
+
+There are two modes of registration on Windows. In both cases the handler is
+advised of the address of a set of structures in the client process’ address
+space. These structures include a pair of ExceptionInformation structs, one for
+generating a postmortem dump for a crashing process, and another one for
+generating a dump for a non- crashing process.
+
+##### Normal registration
+
+In the normal registration mode, the client connects to a named pipe by a
+pre-arranged name. A registration request is written to the pipe. During
+registration, the handler creates a set of events, duplicates them to the
+registering client, then returns the handle values in the registration response.
+This is a blocking process.
+
+##### Initial Handler Creation
+
+In order to avoid blocking client startup for the creation and initialization of
+the handler, a different mode of registration can be used for the handler
+creation. In this mode, the client creates a set of event handles and inherits
+them into the newly created handler process. The handler process is advised of
+the handle values and the location of the ExceptionInformation structures by way
+of command line arguments in this mode.
+
+#### Linux/Android
+
+TODO(mmentovai): describe this. See this preliminary doc.
+
+### Capturing Exceptions
+
+The details of how Crashpad captures the exceptions leading to crashes varies
+between operating systems.
+
+#### macOS
+
+On macOS, the operating system will notify the handler of client crashes via the
+Mach port set as the client process’ exception port. As exceptions are
+dispatched to the Mach port by the kernel, on macOS, exceptions can be handled
+entirely from the Crashpad handler without the need to run any code in the crash
+process at the time of the exception.
+
+#### Windows
+
+On Windows, the OS dispatches exceptions in the context of the crashing thread.
+To notify the handler of exceptions, the Crashpad client registers an
+UnhandledExceptionFilter (UEF) in the client process. When an exception trickles
+up to the UEF, it stores the exception information and the crashing thread’s ID
+in the ExceptionInformation structure registered with the handler. It then sets
+an event handle to signal the handler to go ahead and process the exception.
+
+##### Caveats
+
+* If the crashing thread’s stack is smashed when an exception occurs, the
+  exception cannot be dispatched. In this case the OS will summarily terminate
+  the process, without the handler having an opportunity to generate a crash
+  report.
+* If an exception is handled in the crashing thread, it will never propagate
+  to the UEF, and thus a crash report won’t be generated. This happens a fair
+  bit in Windows as system libraries will often dispatch callbacks under a
+  structured exception handler. This occurs during Window message dispatching
+  on some system configurations, as well as during e.g. DLL entry point
+  notifications.
+* A growing number of conditions in the system and runtime exist where
+  detected corruption or illegal calls result in summary termination of the
+  process, in which case no crash report will be generated.
+
+###### Out-Of-Process Exception Handling
+
+There exists a mechanism in Windows Error Reporting (WER) that allows a client
+process to register for handling client exceptions out of the crashing process.
+Unfortunately this mechanism is difficult to use, and doesn’t provide coverage
+for many of the caveats above. [Details
+here.](https://crashpad.chromium.org/bug/133)
+
+#### Linux/Android
+
+TODO(mmentovai): describe this. See [this preliminary
+doc.](https://goto.google.com/crashpad-android-dd)
+
+### The CrashpadInfo structure
+
+The CrashpadInfo structure is used to communicate information from the client to
+the handler. Each executable module in a client process can contain a
+CrashpadInfo structure. On a crash, the handler crawls all modules in the
+crashing process to locate all CrashpadInfo structures present. The CrashpadInfo
+structures are linked into a special, named section of the executable, where the
+handler can readily find them.
+
+The CrashpadInfo structure has a magic signature, and contains a size and a
+version field. The intent is to allow backwards compatibility from older client
+modules to newer handler. It may also be necessary to provide forwards
+compatibility from newer clients to older handler, though this hasn’t occurred
+yet.
+
+The CrashpadInfo structure contains such properties as the cap for how much
+memory to include in the crash dump, some tristate flags for controlling the
+handler’s behavior, a pointer to an annotation dictionary and so on.
+
+### Snapshot
+
+Snapshot is a layer of interfaces that represent the machine and OS entities
+that Crashpad cares about. Different concrete implementations of snapshot can
+then be backed different ways, such as e.g. from the in-memory representation of
+a crashed process, or e.g. from the contents of a minidump.
+
+### Crash Dump Creation
+
+To create a crash dump, a subset of the machine, OS and application state is
+grabbed from the crashed process into an in-memory snapshot structure in the
+handler process. Since the full application state is typically too large for
+capturing to disk and transmitting to an upstream server, the snapshot contains
+a heuristically selected subset of the full state.
+
+The precise details of what’s captured varies between operating systems, but
+generally includes the following
+* The set of modules (executable, shared libraries) that are loaded into the
+  crashing process.
+* An enumeration of the threads running in the crashing process, including the
+  register contents and the contents of stack memory of each thread.
+* A selection of the OS-related state of the process, such as e.g. the command
+  line, environment and so on.
+* A selection of memory potentially referenced from registers and from stack.
+
+To capture a crash dump, the crashing process is first suspended, then a
+snapshot is created in the handler process. The snapshot includes the
+CrashpadInfo structures of the modules loaded into the process, and the contents
+of those is used to control the level of detail captured for the crash dump.
+
+Once the snapshot has been constructed, it is then written to a minidump file,
+which is added to the database. The process is un-suspended after the minidump
+file has been written. In the case of a crash (as opposed to a client request to
+produce a dump without crashing), it is then either killed by the operating
+system or the Crashpad handler.
+
+In general the snapshotting process has to be very intimate with the operating
+system it’s working with, so there will be a set of concrete implementation
+classes, many deriving from the snapshot interfaces, doing this for each
+operating system.
+
+### Minidump
+
+The minidump implementation is responsible for writing a snapshot to a
+serialized on-disk file in the minidump format. The minidump implementation is
+OS-agnostic, as it works on an OS-agnostic Snapshot interface.
+
+TODO(siggi): Talk about two-phase writes and contents ordering here.
+
+### Database
+
+The Crashpad database contains persistent client settings, including a unique
+crash client identifier and the upload-enabled bit. Note that the crash client
+identifier is assigned by Crashpad, and is distinct from any identifiers the
+client application uses to identify users, installs, machines or such - if any.
+The expectation is that the client application will manage the user’s upload
+consent, and inform Crashpad of changes in consent.
+
+The unique client identifier is set at the time of database creation. It is then
+recorded into every crash report collected by the handler and communicated to
+the upstream server.
+
+The database stores a configurable number of recorded crash dumps to a
+configurable maximum aggregate size. For each crash dump it stores annotations
+relating to whether the crash dumps have been uploaded. For successfully
+uploaded crash dumps it also stores their server-assigned ID.
+
+The database consists of a settings file, named "settings.dat" with binary
+contents (see crashpad::Settings::Data for the file format), as well as
+directory containing the crash dumps. Additionally each crash dump is adorned
+with properties relating to the state of the dump for upload and such. The
+details of how these properties are stored vary between platforms.
+
+#### macOS
+
+The macOS implementation simply stores database properties on the minidump files
+in filesystem extended attributes.
+
+#### Windows
+
+The Windows implementation stores database properties in a binary file named
+“metadata” at the top level of the database directory.
+
+### Report Format
+
+Crash reports are recorded in the Windows minidump format with
+extensions to support Crashpad additions, such as e.g. Annotations.
+
+### Upload to collection server
+
+#### Wire Format
+
+For the time being, Crashpad uses the Breakpad wire protocol, which is
+essentially a MIME multipart message communicated over HTTP(S). To support this,
+the annotations from all the CrashpadInfo structures found in the crashing
+process are merged to create the Breakpad “crash keys” as form data. The
+postmortem minidump is then attached as an “application/octet- stream”
+attachment with the name “upload_file_minidump”. The entirety of the request
+body, including the minidump, can be gzip-compressed to reduce transmission time
+and increase transmission reliability. Note that by convention there is a set of
+“crash keys” that are used to communicate the product, version, client ID and
+other relevant data about the client, to the server. Crashpad normally stores
+these values in the minidump file itself, but retrieves them from the minidump
+and supplies them as form data for compatibility with the Breakpad-style server.
+
+This is a temporary compatibility measure to allow the current Breakpad-based
+upstream server to handle Crashpad reports. In the fullness of time, the wire
+protocol is expected to change to remove this redundant transmission and
+processing of the Annotations.
+
+#### Transport
+
+The embedding client controls the URL of the collection server by the command
+line passed to the handler. The handler can upload crashes with HTTP or HTTPS,
+depending on client’s preference. It’s strongly suggested use HTTPS transport
+for crash uploads to protect the user’s privacy against man-in-the-middle
+snoopers.
+
+TODO(mmentovai): Certificate pinning.
+
+#### Throttling & Retry Strategy
+
+To protect both the collection server from DDoS as well as to protect the
+clients from unreasonable data transfer demands, the handler implements a
+client-side throttling strategy. At the moment, the strategy is very simplistic,
+it simply limits uploads to one upload per hour, and failed uploads are aborted.
+
+An experiment has been conducted to lift all throttling. Analysis on the
+aggregate data this produced shows that multiple crashes within a short timespan
+on the same client are nearly always due to the same cause. Therefore there is
+very little loss of signal due to the throttling, though the ability to
+reconstruct at least the full crash count is highly desirable.
+
+The lack of retry is expected to [change
+soon](https://crashpad.chromium.org/bug/23), as this creates blind spots for
+client crashes that exclusively occur on e.g. network down events, during
+suspend and resume and such.
+
+### Extensibility
+
+#### Client Extensibility
+
+Clients are able to extend the generated crash reports in two ways, by
+manipulating their CrashpadInfo structure.
+The two extensibility points are:
+1. Nominating a set of address ranges for inclusion in the crash report.
+2. Adding user-defined minidump streams for inclusion in the crash report.
+
+In both cases the CrashpadInfo structure has to be updated before a crash
+occurs.
+
+##### Embedder Extensibility
+
+Additionally, embedders of the handler can provide "user stream data source"
+instances to the handler's main function. Any time a minidump is written, these
+instances get called.
+
+Each data source may contribute a custom stream to the minidump, which can be
+computed from e.g. system or application state relevant to the crash.
+
+As a case in point, it can be handy to know whether the system was under memory
+or other resource duress at the time of crash.
+
+### Dependencies
+
+Aside from system headers and APIs, when used outside of Chromium, Crashpad has
+a dependency on “mini_chromium”, which is a subset of the Chromium base library.
+This is to allow non-Chromium clients to use Crashpad, without taking a direct
+dependency on the Chromium base, while allowing Chromium projects to use
+Crashpad with minimum code duplication or hassle. When using Crashpad as part of
+Chromium, Chromium’s own copy of the base library is used instead of
+mini_chromium.
+
+The downside to this is that mini_chromium must be kept up to date with
+interface and implementation changes in Chromium base, for the subset of
+functionality used by Crashpad.
+
+## Caveats
+
+TODO(anyone): You may need to describe what you did not do or why simpler
+approaches don't work. Mention other things to watch out for (if any).
+
+## Security Considerations
+
+Crashpad may be used to capture the state of sandboxed processes and it writes
+minidumps to disk. It may therefore straddle security boundaries, so it’s
+important that Crashpad handle all data it reads out of the crashed process with
+extreme care. The Crashpad handler takes care to access client address spaces
+through specially-designed accessors that check pointer validity and enforce
+accesses within prescribed bounds. The flow of information into the Crashpad
+handler is exclusively one-way: Crashpad never communicates anything back to
+its clients, aside from providing single-bit indications of completion.
+
+## Privacy Considerations
+
+Crashpad may capture arbitrary contents from crashed process’ memory, including
+user IDs and passwords, credit card information, URLs and whatever other content
+users have trusted the crashing program with. The client program must acquire
+and honor the user’s consent to upload crash reports, and appropriately manage
+the upload state in Crashpad’s database.
+
+Crashpad must also be careful not to upload crashes for arbitrary processes on
+the user’s system. To this end, Crashpad will never upload a process that hasn’t
+registered with the handler, but note that registrations are inherited by child
+processes on some operating systems.