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11d7510 Nov 18, 2016
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Introduction

Breakpad is a library and tool suite that allows you to distribute an application to users with compiler-provided debugging information removed, record crashes in compact "minidump" files, send them back to your server, and produce C and C++ stack traces from these minidumps. Breakpad can also write minidumps on request for programs that have not crashed.

Breakpad is currently used by Google Chrome, Firefox, Google Picasa, Camino, Google Earth, and other projects.

Workflow

Breakpad has three main components:

  • The client is a library that you include in your application. It can write minidump files capturing the current threads' state and the identities of the currently loaded executable and shared libraries. You can configure the client to write a minidump when a crash occurs, or when explicitly requested.

  • The symbol dumper is a program that reads the debugging information produced by the compiler and produces a symbol file, in Breakpad's own format.

  • The processor is a program that reads a minidump file, finds the appropriate symbol files for the versions of the executables and shared libraries the minidump mentions, and produces a human-readable C/C++ stack trace.

The minidump file format

The minidump file format is similar to core files but was developed by Microsoft for its crash-uploading facility. A minidump file contains:

  • A list of the executable and shared libraries that were loaded in the process at the time the dump was created. This list includes both file names and identifiers for the particular versions of those files that were loaded.

  • A list of threads present in the process. For each thread, the minidump includes the state of the processor registers, and the contents of the threads' stack memory. These data are uninterpreted byte streams, as the Breakpad client generally has no debugging information available to produce function names or line numbers, or even identify stack frame boundaries.

  • Other information about the system on which the dump was collected: processor and operating system versions, the reason for the dump, and so on.

Breakpad uses Windows minidump files on all platforms, instead of the traditional core files, for several reasons:

  • Core files can be very large, making them impractical to send across a network to the collector for processing. Minidumps are smaller, as they were designed to be used this way.

  • The core file format is poorly documented. For example, the Linux Standards Base does not describe how registers are stored in PT_NOTE segments.

  • It is harder to persuade a Windows machine to produce a core dump file than it is to persuade other machines to write a minidump file.

  • It simplifies the Breakpad processor to support only one file format.

Overview/Life of a minidump

A minidump is generated via calls into the Breakpad library. By default, initializing Breakpad installs an exception/signal handler that writes a minidump to disk at exception time. On Windows, this is done via SetUnhandledExceptionFilter(); on OS X, this is done by creating a thread that waits on the Mach exception port; and on Linux, this is done by installing a signal handler for various exceptions like SIGILL, SIGSEGV etc.

Once the minidump is generated, each platform has a slightly different way of uploading the crash dump. On Windows & Linux, a separate library of functions is provided that can be called into to do the upload. On OS X, a separate process is spawned that prompts the user for permission, if configured to do so, and sends the file.

Terminology

In-process vs. out-of-process exception handling - it's generally considered that writing the minidump from within the crashed process is unsafe - key process data structures could be corrupted, or the stack on which the exception handler runs could �have been overwritten, etc. All 3 platforms support what's known as "out-of-process" exception handling.

Integration overview

Breakpad Code Overview

All the client-side code is found by visiting the Google Project at https://chromium.googlesource.com/breakpad/breakpad. The following directory structure is present in the src directory:

  • processor Contains minidump-processing code that is used on the server side and isn't of use on the client side
  • client Contains client minidump-generation libraries for all platforms
  • tools Contains source code & projects for building various tools on each platform.

(Among other directories)

Build process specifics(symbol generation)

This applies to all platforms. Inside src/tools/{platform}/dump_syms is a tool that can read debugging information for each platform (e.g. for OS X/Linux, DWARF and STABS, and for Windows, PDB files) and generate a Breakpad symbol file. This tool should be run on your binary before it's stripped(in the case of OS X/Linux) and the symbol files need to be stored somewhere that the minidump processor can find. There is another tool, symupload, that can be used to upload symbol files if you have written a server that can accept them.