How does OS X generate a crash report?

Question

The material available from web, mail-list, books like Mac OS X Internals, and even source code is quite limited.

Now I know that xnu kernel raise an EXC_CRASH, which notify launched to start "Problem Reporter.app" (prior is Crash Reporter.app). Is this app using some debugging interface to generate the crash report, or is it kernel already generating the report and just notify the app to open the already-generated report?

Answer 1

Every Mach thread and/or task (the underlying kernel object on top of which the BSD layer process is implemented) has exception ports. three port levels are available: Thread, Task, and host. When an exception occurs, a Mach message is sent - first to the thread port, then - if none caught it - the task'S, and finally the host. If you obtain the port, you can catch the exception, debug it (as does gdb on OS X) or generate a crash dump (as does Crash Reporter). Specifically, launchd - the parent of all OS X system tasks - registers their exception ports, so it gets the messages, and then triggers CrashReporter (as you can see from the launchd's ReportCrash plist:

 <key>MachServices</key>
        <dict>
                <key>com.apple.ReportCrash.DirectoryService</key>
                <dict>
                        <key>DrainMessagesOnCrash</key>
                        <string>All</string>
                        <key>ExceptionServer</key>
                        <dict/>
                </dict>
        </dict>

The XNU kernel code is responsible for sending the message on EXC_CRASH. Specifically, proc_prepareexit does that:

  /* If a core should be generated, notify crash reporter */
        if (hassigprop(WTERMSIG(rv), SA_CORE) || ((p->p_csflags & CS_KILLED) != 0)) {
                /* 
                 * Workaround for processes checking up on PT_DENY_ATTACH:
                 * should be backed out post-Leopard (details in 5431025).
                 */
                if ((SIGSEGV == WTERMSIG(rv)) && 
                                (p->p_pptr->p_lflag & P_LNOATTACH)) {
                        goto skipcheck;
                }

                /*
                 * Crash Reporter looks for the signal value, original exception
                 * type, and low 20 bits of the original code in code[0] 
                 * (8, 4, and 20 bits respectively). code[1] is unmodified. 
                 */
                code = ((WTERMSIG(rv) & 0xff) << 24) |
                        ((ut->uu_exception & 0x0f) << 20) | 
                        ((int)ut->uu_code & 0xfffff);
                subcode = ut->uu_subcode;
                (void) task_exception_notify(EXC_CRASH, code, subcode); // <-- Sends the msg
        }