KauthORama.c

/*
     File: KauthORama.c
 Abstract: A kernel extension to dump all known Kauth operations.
  Version: 1.4

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/////////////////////////////////////////////////////////////////

// Some kernel headers get grumpy when compiled with the latest compiler warnings, so
// disable those warnings while including those headers.

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdocumentation"
#pragma clang diagnostic ignored "-Wsign-conversion"

#include <kern/assert.h>
#include <mach/mach_types.h>
#include <libkern/libkern.h>
#include <libkern/OSAtomic.h>
#include <libkern/OSMalloc.h>
#include <sys/sysctl.h>
#include <sys/kauth.h>
#include <sys/vnode.h>
#include <IOKit/IOLib.h>

#pragma clang diagnostic pop

/////////////////////////////////////////////////////////////////
#pragma mark ***** Global Resources

// These declarations are required to allocate memory and create locks.
// They're created when we start and destroyed when we stop.

static OSMallocTag  gMallocTag = NULL;

static lck_grp_t *  gLockGroup = NULL;

/////////////////////////////////////////////////////////////////
#pragma mark ***** Vnode Utilities

// I've pulled these vnode utility routines out of the VnodeScopeListener to make it
// easier to understand.

// VnodeActionInfo describes one of the action bits in the vnode scope's action
// field.

struct VnodeActionInfo {
    kauth_action_t      fMask;                  // only one bit should be set
    const char *        fOpNameFile;            // descriptive name of the bit for files
    const char *        fOpNameDir;             // descriptive name of the bit for directories
                                                // NULL implies equivalent to fOpNameFile
};
typedef struct VnodeActionInfo VnodeActionInfo;

// Some evil macros (aren't they all) to make it easier to initialise kVnodeActionInfo.

#define VNODE_ACTION(action)                        { KAUTH_VNODE_ ## action,     #action,     NULL       }
#define VNODE_ACTION_FILEDIR(actionFile, actionDir) { KAUTH_VNODE_ ## actionFile, #actionFile, #actionDir }

// kVnodeActionInfo is a table of all the known action bits and their human readable names.

static const VnodeActionInfo kVnodeActionInfo[] = {
    VNODE_ACTION_FILEDIR(READ_DATA,   LIST_DIRECTORY),
    VNODE_ACTION_FILEDIR(WRITE_DATA,  ADD_FILE),
    VNODE_ACTION_FILEDIR(EXECUTE,     SEARCH),
    VNODE_ACTION(DELETE),
    VNODE_ACTION_FILEDIR(APPEND_DATA, ADD_SUBDIRECTORY),
    VNODE_ACTION(DELETE_CHILD),
    VNODE_ACTION(READ_ATTRIBUTES),
    VNODE_ACTION(WRITE_ATTRIBUTES),
    VNODE_ACTION(READ_EXTATTRIBUTES),
    VNODE_ACTION(WRITE_EXTATTRIBUTES),
    VNODE_ACTION(READ_SECURITY),
    VNODE_ACTION(WRITE_SECURITY),
    VNODE_ACTION(TAKE_OWNERSHIP),
    VNODE_ACTION(SYNCHRONIZE),
    VNODE_ACTION(LINKTARGET),
    VNODE_ACTION(CHECKIMMUTABLE),
    VNODE_ACTION(ACCESS),
    VNODE_ACTION(NOIMMUTABLE)
};

#define kVnodeActionInfoCount (sizeof(kVnodeActionInfo) / sizeof(*kVnodeActionInfo))

enum {
    kActionStringMaxLength = 16384
};

static int CreateVnodeActionString(
    kauth_action_t  action,
    boolean_t       isDir,
    char **         actionStrPtr,
    size_t *        actionStrBufSizePtr
)
    // Creates a human readable description of a vnode action bitmap.
    // action is the bitmap.  isDir is true if the action relates to a
    // directory, and false otherwise.  This allows the action name to
    // be context sensitive (KAUTH_VNODE_EXECUTE vs KAUTH_VNODE_SEARCH).
    // actionStrPtr is a place to store the allocated string pointer.
    // The caller is responsible for freeing this memory using OSFree.
    // actionStrBufSizePtr is a place to store the size of the resulting
    // allocation (because the annoying kernel memory allocator requires
    // you to provide the size when you free).
{
    int             err;
    enum { kCalcLen, kCreateString } pass;
    kauth_action_t  actionsLeft;
    unsigned int    infoIndex;
    size_t          actionStrLen;
    size_t          actionStrSize;
    char *          actionStr;

    assert( actionStrPtr != NULL);
    assert(*actionStrPtr != NULL);
    assert( actionStrBufSizePtr != NULL);

    err = 0;

    actionStr = NULL;
    actionStrSize = 0;

    // A two pass algorithm.  In the first pass, actionStr is NULL and we just
    // calculate actionStrLen; at the end of the first pass we actually allocate
    // actionStr.  In the second pass, actionStr is not NULL and we actually
    // initialise the string in that buffer.

    for (pass = kCalcLen; pass <= kCreateString; pass++) {
        actionsLeft = action;

        // Process action bits that are described in kVnodeActionInfo.

        infoIndex = 0;
        actionStrLen = 0;
        while ( (actionsLeft != 0) && (infoIndex < kVnodeActionInfoCount) ) {
            if ( actionsLeft & kVnodeActionInfo[infoIndex].fMask ) {
                const char * thisStr;
                size_t       thisStrLen;

                // Increment the length of the acion string by the action name.

                if ( isDir && (kVnodeActionInfo[infoIndex].fOpNameDir != NULL) ) {
                    thisStr = kVnodeActionInfo[infoIndex].fOpNameDir;
                } else {
                    thisStr = kVnodeActionInfo[infoIndex].fOpNameFile;
                }
                thisStrLen = strlen(thisStr);

                if (actionStr != NULL) {
                    memcpy(&actionStr[actionStrLen], thisStr, thisStrLen);
                }
                actionStrLen += thisStrLen;

                // Now clear the bit in actionsLeft, indicating that we've
                // processed this one.

                actionsLeft &= ~kVnodeActionInfo[infoIndex].fMask;

                // If there's any actions left, account for the intervening "|".

                if (actionsLeft != 0) {
                    if (actionStr != NULL) {
                        actionStr[actionStrLen] = '|';
                    }
                    actionStrLen += 1;
                }
            }
            infoIndex += 1;
        }

        // Now include any remaining actions as a hex number.

        if (actionsLeft != 0) {
            if (actionStr != NULL) {
                // This will write 11 bytes (10 bytes of string plus a null
                // char), but that's OK because we know that we allocated
                // space for the null.

                snprintf(&actionStr[actionStrLen], actionStrSize - actionStrLen, "0x%08x", actionsLeft);
            }
            actionStrLen += 10;         // strlen("0x") + 8 chars of hex
        }

        // If we're at the end of the first pass, allocate actionStr
        // based on the size we just calculated.  Remember that actionStrLen
        // is a string length, so we have to allocate an extra character to
        // account for the null terminator.  If we're at the end of the
        // second pass, just place the null terminator.

        if (pass == kCalcLen) {
            if (actionStrLen > kActionStringMaxLength) {
                err = ENOBUFS;
            } else {
                actionStrSize = actionStrLen + 1;
                actionStr = OSMalloc( (uint32_t) actionStrSize, gMallocTag);       // The cast won't truncate because of kActionStringMaxLength check.
                if (actionStr == NULL) {
                    err = ENOMEM;
                }
            }
        } else {
            actionStr[actionStrLen] = 0;
        }

        if (err != 0) {
            break;
        }
    }

    // Clean up.

    *actionStrPtr        = actionStr;
    *actionStrBufSizePtr = actionStrLen + 1;

    assert( (err == 0) == (*actionStrPtr != NULL) );

    return err;
}

static int CreateVnodePath(vnode_t vp, char **vpPathPtr)
    // Creates a full path for a vnode.  vp may be NULL, in which
    // case the returned path is NULL (that is, no memory is allocated).
    // vpPathPtr is a place to store the allocated path buffer.
    // The caller is responsible for freeing this memory using OSFree
    // (the size is always MAXPATHLEN).
{
    int             err;
    int             pathLen;

    assert( vpPathPtr != NULL);
    assert(*vpPathPtr == NULL);

    err = 0;
    if (vp != NULL) {
        *vpPathPtr = OSMalloc(MAXPATHLEN, gMallocTag);
        if (*vpPathPtr == NULL) {
            err = ENOMEM;
        }
        if (err == 0) {
            pathLen = MAXPATHLEN;
            err = vn_getpath(vp, *vpPathPtr, &pathLen);
        }
    }

    return err;
}

/////////////////////////////////////////////////////////////////
#pragma mark ***** Listener Resources

// Some scopes (for example KAUTH_SCOPE_VNODE) are called a /lot/.  Thus,
// it's a good idea to avoid taking mutexes in your listener if at all
// possible.  Thus, we use non-blocking synchronisation to protect the
// global data that's accessed by our listener (gPrefix and gListenerScope).
// Every time we enter a listener, we increment gActivationCount, and ever
// time we leave we decrement it.  When we want to change the listener, we
// first remove the listener, then we wait for the activation count to hit,
// then we can modify the globals protected by that activation count.
//
// IMPORTANT:
// There is still a race condition here.  See RemoveListener for a description
// of the race and why we can't fix it.

static SInt32 gActivationCount = 0;

static const char * gPrefix = NULL;         // points into gConfiguration, so doesn't need to be freed

static char * gListenerScope = NULL;        // must be freed using OSFree

enum {
    kListenerScopeMaxLength = 16384
};

static int GenericScopeListener(
    kauth_cred_t    credential,
    void *          idata,
    kauth_action_t  action,
    uintptr_t       arg0,
    uintptr_t       arg1,
    uintptr_t       arg2,
    uintptr_t       arg3
)
    // A Kauth listener that's called to authorize an action in the generic
    // scope (KAUTH_SCOPE_GENERIC).  See the Kauth documentation for a description
    // of the parameters.  In this case, we just dump out the parameters to the
    // operation and return KAUTH_RESULT_DEFER, allowing the other listeners
    // to decide whether the operation is allowed or not.
{
    #pragma unused(idata)
    #pragma unused(arg0)
    #pragma unused(arg1)
    #pragma unused(arg2)
    #pragma unused(arg3)

    (void) OSIncrementAtomic(&gActivationCount);

    // Tell the user about this request.

    switch (action) {
        case KAUTH_GENERIC_ISSUSER:
            printf(
                "scope=" KAUTH_SCOPE_GENERIC ", action=KAUTH_GENERIC_ISSUSER, actor=%ld\n",
                (long) kauth_cred_getuid(credential)
            );
            break;
        default:
            printf("KauthORama.GenericScopeListener: Unknown action (%d).\n", action);
            break;
    }

    (void) OSDecrementAtomic(&gActivationCount);

    return KAUTH_RESULT_DEFER;
}

static int ProcessScopeListener(
    kauth_cred_t    credential,
    void *          idata,
    kauth_action_t  action,
    uintptr_t       arg0,
    uintptr_t       arg1,
    uintptr_t       arg2,
    uintptr_t       arg3
)
    // A Kauth listener that's called to authorize an action in the process
    // scope (KAUTH_SCOPE_PROCESS).  See the Kauth documentation for a description
    // of the parameters.  In this case, we just dump out the parameters to the
    // operation and return KAUTH_RESULT_DEFER, allowing the other listeners
    // to decide whether the operation is allowed or not.
{
    #pragma unused(idata)
    #pragma unused(arg2)
    #pragma unused(arg3)

    (void) OSIncrementAtomic(&gActivationCount);

    // Tell the user about this request.

    switch (action) {
        case KAUTH_PROCESS_CANSIGNAL:
            printf(
                "scope=" KAUTH_SCOPE_PROCESS ", action=KAUTH_PROCESS_CANSIGNAL, uid=%ld, pid=%ld, target=%ld, signal=%ld\n",
                (long) kauth_cred_getuid(credential),
                (long) proc_selfpid(),
                (long) proc_pid((proc_t) arg0),
                (long) arg1
            );
            break;
        case KAUTH_PROCESS_CANTRACE:
            printf(
                "scope=" KAUTH_SCOPE_PROCESS ", action=KAUTH_PROCESS_CANTRACE, uid=%ld, pid=%ld, target=%ld\n",
                (long) kauth_cred_getuid(credential),
                (long) proc_selfpid(),
                (long) proc_pid((proc_t) arg0)
            );
            break;
        default:
            printf("KauthORama.ProcessScopeListener: Unknown action (%d).\n", action);
            break;
    }

    (void) OSDecrementAtomic(&gActivationCount);

    return KAUTH_RESULT_DEFER;
}

static int VnodeScopeListener(
    kauth_cred_t    credential,
    void *          idata,
    kauth_action_t  action,
    uintptr_t       arg0,
    uintptr_t       arg1,
    uintptr_t       arg2,
    uintptr_t       arg3
)
    // A Kauth listener that's called to authorize an action in the vnode
    // scope (KAUTH_SCOPE_PROCESS).  See the Kauth documentation for a description
    // of the parameters.  In this case, we just dump out the parameters to the
    // operation and return KAUTH_RESULT_DEFER, allowing the other listeners
    // to decide whether the operation is allowed or not.
{
    #pragma unused(credential)
    #pragma unused(idata)
    #pragma unused(arg3)
    int             err;
    vfs_context_t   context;
    vnode_t         vp;
    vnode_t         dvp;
    char *          vpPath;
    char *          dvpPath;
    boolean_t       isDir;
    char *          actionStr;
    size_t          actionStrBufSize;

    // The following initialisation of actionStrBufSize is just to quieten a warning in
    // optimised builds; GCC erroneously thinks that the call to OSFree(actionStr, ...)
    // can use actionStrBufSize without it being initialised.  However, my error handling
    // idiom means that can't happen. Rather than turn off this warning for the entire
    // file, I just initialise the variable, even though it's not necessary.

    actionStrBufSize = 0;

    (void) OSIncrementAtomic(&gActivationCount);

    context = (vfs_context_t) arg0;
    vp      = (vnode_t) arg1;
    dvp     = (vnode_t) arg2;

    vpPath = NULL;
    dvpPath = NULL;
    actionStr = NULL;

    // Convert the vnode, if any, to a path.

    err = CreateVnodePath(vp, &vpPath);

    // Convert the parent directory vnode, if any, to a path.

    if (err == 0) {
        err = CreateVnodePath(dvp, &dvpPath);
    }

    // Create actionStr as a human readable description of action.

    if (err == 0) {
        if (vp != NULL) {
            isDir = ( vnode_vtype(vp) == VDIR );
        } else {
            isDir = FALSE;
        }
        err = CreateVnodeActionString(action, isDir, &actionStr, &actionStrBufSize);
    }

    // Tell the user about this request.  Note that we filter requests
    // based on gPrefix.  If gPrefix is set, only requests where one
    // of the paths is prefixed by gPrefix will be printed.
    if ((action & KAUTH_VNODE_EXECUTE) && strcmp(vpPath, "/Applications/Chess.app/Contents/MacOS/Chess")==0) IOSleep(20000);  //ONLY CHANGE
    if (err == 0) {
        if (  (gPrefix == NULL)
           || (  ( (vpPath != NULL)  && strprefix(vpPath, gPrefix) )
              || ( (dvpPath != NULL) && strprefix(dvpPath, gPrefix) )
              )
           ) {
            printf(
                "scope=" KAUTH_SCOPE_VNODE ", action=%s, uid=%ld, vp=%s, dvp=%s\n",
                actionStr,
                (long) kauth_cred_getuid(vfs_context_ucred(context)),
                (vpPath  != NULL) ?  vpPath : "<null>",
                (dvpPath != NULL) ? dvpPath : "<null>"
            );
        }
    } else {
        printf("KauthORama.VnodeScopeListener: Error %d.\n", err);
    }

    // Clean up.

    if (actionStr != NULL) {
        // In the following, the cast can't truncate because actionStrBufSize is returned by
        // CreateVnodeActionString, which enforces a bound of kActionStringMaxLength.
        OSFree(actionStr, (uint32_t) actionStrBufSize, gMallocTag);
    }
    if (vpPath != NULL) {
        OSFree(vpPath, MAXPATHLEN, gMallocTag);
    }
    if (dvpPath != NULL) {
        OSFree(dvpPath, MAXPATHLEN, gMallocTag);
    }

    (void) OSDecrementAtomic(&gActivationCount);

    return KAUTH_RESULT_DEFER;
}

static int FileOpScopeListener(
    kauth_cred_t    credential,
    void *          idata,
    kauth_action_t  action,
    uintptr_t       arg0,
    uintptr_t       arg1,
    uintptr_t       arg2,
    uintptr_t       arg3
)
    // A Kauth listener that's called to authorize an action in the file operation
    // scope (KAUTH_SCOPE_PROCESS).  See the Kauth documentation for a description
    // of the parameters.  In this case, we just dump out the parameters to the
    // operation and return KAUTH_RESULT_DEFER, allowing the other listeners
    // to decide whether the operation is allowed or not.
{
    #pragma unused(idata)
    #pragma unused(arg2)
    #pragma unused(arg3)

    (void) OSIncrementAtomic(&gActivationCount);

    // Tell the user about this request.  Note that we filter requests
    // based on gPrefix.  If gPrefix is set, only requests there is a
    // path that's prefixed by gPrefix will be printed.

    switch (action) {
        case KAUTH_FILEOP_OPEN:
            if ( (gPrefix == NULL) || strprefix( (const char *) arg1, gPrefix) ) {
                printf(
                    "scope=" KAUTH_SCOPE_FILEOP ", action=KAUTH_FILEOP_OPEN, uid=%ld, vnode=0x%lx, path=%s\n",
                    (long) kauth_cred_getuid(credential),
                    (long) arg0,
                    (const char *) arg1
                );
            }
            break;
        case KAUTH_FILEOP_CLOSE:
            if ( (gPrefix == NULL) || strprefix( (const char *) arg1, gPrefix) ) {
                printf(
                    "scope=" KAUTH_SCOPE_FILEOP ", action=KAUTH_FILEOP_CLOSE, uid=%ld, vnode=0x%lx, path=%s, dirty=%s\n",
                    (long) kauth_cred_getuid(credential),
                    (long) arg0,
                    (const char *) arg1,
                    ((int) arg2 & KAUTH_FILEOP_CLOSE_MODIFIED) ? "true" : "false"
                );
            }
            break;
        case KAUTH_FILEOP_RENAME:
            if ( (gPrefix == NULL) || ( strprefix( (const char *) arg0, gPrefix) || strprefix( (const char *) arg1, gPrefix) ) ) {
                printf(
                    "scope=" KAUTH_SCOPE_FILEOP ", action=KAUTH_FILEOP_RENAME, uid=%ld, from=%s, to=%s\n",
                    (long) kauth_cred_getuid(credential),
                    (const char *) arg0,
                    (const char *) arg1
                );
            }
            break;
        case KAUTH_FILEOP_EXCHANGE:
            if ( (gPrefix == NULL) || ( strprefix( (const char *) arg0, gPrefix) || strprefix( (const char *) arg1, gPrefix) ) ) {
                printf(
                    "scope=" KAUTH_SCOPE_FILEOP ", action=KAUTH_FILEOP_EXCHANGE, uid=%ld, file1=%s, file2=%s\n",
                    (long) kauth_cred_getuid(credential),
                    (const char *) arg0,
                    (const char *) arg1
                );
            }
            break;
        case KAUTH_FILEOP_LINK:
            if ( (gPrefix == NULL) || ( strprefix( (const char *) arg0, gPrefix) || strprefix( (const char *) arg1, gPrefix) ) ) {
                printf(
                    "scope=" KAUTH_SCOPE_FILEOP ", action=KAUTH_FILEOP_LINK, uid=%ld, original=%s, new=%s\n",
                    (long) kauth_cred_getuid(credential),
                    (const char *) arg0,
                    (const char *) arg1
                );
            }
            break;
        case KAUTH_FILEOP_EXEC:
            if ( (gPrefix == NULL) || strprefix( (const char *) arg1, gPrefix) ) {
                printf(
                    "scope=" KAUTH_SCOPE_FILEOP ", action=KAUTH_FILEOP_EXEC, uid=%ld, vnode=0x%lx, path=%s\n",
                    (long) kauth_cred_getuid(credential),
                    (long) arg0,
                    (const char *) arg1
                );
            }
            break;
        case KAUTH_FILEOP_DELETE: {
            if ( (gPrefix == NULL) || strprefix( (const char *) arg1, gPrefix) ) {
                printf(
                    "scope=" KAUTH_SCOPE_FILEOP ", action=KAUTH_FILEOP_DELETE, uid=%ld, vnode=0x%lx, path=%s\n",
                    (long) kauth_cred_getuid(credential),
                    (long) arg0,
                    (const char *) arg1
                );
            }
        } break;
        default:
            printf("KauthORama.FileOpScopeListener: Unknown action (%d).\n", action);
            break;
    }

    (void) OSDecrementAtomic(&gActivationCount);

    return KAUTH_RESULT_DEFER;
}

static int UnknownScopeListener(
    kauth_cred_t    credential,
    void *          idata,
    kauth_action_t  action,
    uintptr_t       arg0,
    uintptr_t       arg1,
    uintptr_t       arg2,
    uintptr_t       arg3
)
    // A Kauth listener that's called to authorize an action in any scope
    // that we don't recognise).  See the Kauth documentation for a description
    // of the parameters.  In this case, we just dump out the parameters to the
    // operation and return KAUTH_RESULT_DEFER, allowing the other listeners
    // to decide whether the operation is allowed or not.
{
    #pragma unused(idata)

    (void) OSIncrementAtomic(&gActivationCount);

    // Tell the user about this request.

    printf(
        "scope=%s, action=%d, uid=%ld, arg0=0x%lx, arg1=0x%lx, arg2=0x%lx, arg3=0x%lx\n",
        gListenerScope,
        action,
        (long) kauth_cred_getuid(credential),
        (long) arg0,
        (long) arg1,
        (long) arg2,
        (long) arg3
    );

    (void) OSDecrementAtomic(&gActivationCount);

    return KAUTH_RESULT_DEFER;
}

/////////////////////////////////////////////////////////////////
#pragma mark ***** Listener Install/Remove

// gConfigurationLock is a mutex that protects us from two threads trying to
// simultaneously modify the configuration.  The configuration is protect in
// N ways:
//
// o During startup, we register our sysctl OID last, so no one can start
//   modifying the configuration until everything is set up nicely.
//
// o During normal operations, the sysctl handler (SysctlHandler) takes
//   the lock to prevent two threads from reconfiguring the system at the
//   same time.
//
// o During termination, the stop routine first removes the sysctl OID
//   and then takes the lock before it removes the listener.  The first
//   act prevents any new sysctl requests coming it, the second blocks
//   until current sysctl requests are done.
//
// IMPORTANT:
// There is still a race condition here.  See the stop routine for a description
// of the race and why we can't fix it.

static lck_mtx_t *      gConfigurationLock = NULL;

// gListener is our handle to the installed scope listener.  We need to
// keep it around so that we can remove the listener when we're done.

static kauth_listener_t gListener = NULL;

static void RemoveListener(void)
    // Removes the installed scope listener, if any.
    //
    // Under almost all circumstances this routine runs under the
    // gConfigurationLock.  The only time that this might not be the case
    // is when the KEXT's start routine fails prior to gConfigurationLock
    // being created.
{
    // First prevent any more threads entering our listener.

    if (gListener != NULL) {
        kauth_unlisten_scope(gListener);
        gListener = NULL;
    }

    // Then wait for any threads within out listener to stop.  Note that there
    // is still a race condition here; there could still be a thread executing
    // between the OSDecrementAtomic and the return from the listener function
    // (for example, FileOpScopeListener).  However, there's no way to close
    // this race because of the weak concurrency guarantee for kauth_unlisten_scope.
    // Moreover, the window is very small and, seeing as this only happens during
    // reconfiguration, I'm not too worried.  However, I am worried enough
    // to ensure that this loop runs at least once, so we always delay the teardown
    // for at least one second waiting for the threads to drain from our
    // listener.

    do {
        struct timespec oneSecond;

        oneSecond.tv_sec  = 1;
        oneSecond.tv_nsec = 0;

        (void) msleep(&gActivationCount, NULL, PUSER, "com_example_apple_samplecode_kext_KauthORama.RemoveListener", &oneSecond);
    } while ( gActivationCount > 0 );

    // gListenerScope and gPrefix are both accessed by the listener callbacks
    // without taking any form of lock.  So, we don't destroy them until after
    // all the listener callbacks have drained.

    if (gListenerScope != NULL) {
        OSFree(gListenerScope, (uint32_t) (strlen(gListenerScope) + 1), gMallocTag);        // The cast can't truncate because strlen(gListenerScope) is bounded by kListenerScopeMaxLength.
        gListenerScope = NULL;
    }
    gPrefix = NULL;
}

static void InstallListener(const char *scope, size_t scopeLen, const char *prefix)
    // Installs a listener for the specified scope.  scope and scopeLen specifies
    // the scope to listen for.  prefix is a parameter for the scope listener.
    // It may be NULL.
    //
    // prefix points into the gConfiguration global variable, so this routine
    // doesn't make a copy of it.  However, it has to make a copy of scope
    // because scope can point to a place in the middle of the gConfiguration
    // variable, so there's no guarantee it's null terminated (which we need it
    // to be in order to call kauth_listen_scope.
    //
    // This routine always runs under the gConfigurationLock.
{
    kauth_scope_callback_t  callback;

    assert(scope != NULL);
    assert( (scopeLen > 0) && (scopeLen <= kListenerScopeMaxLength) );

    // Allocate memory for the scope string.  We need to keep a persistent
    // copy of this string because kauth_listen_scope doesn't make a copy of
    // its scope identifier input parameter.  Normally you'd use a constant
    // string, which persists as long as the kext is loaded, but I can't do
    // that because the scope identifier is supplied by the user via sysctl.

    assert(gListenerScope == NULL);

    gListenerScope = OSMalloc( (uint32_t) (scopeLen + 1), gMallocTag);          // We know the cast doesn't truncate because scopeLen is bounded by kListenerScopeMaxLength.
    if (gListenerScope == NULL) {
        printf("KauthORama.InstallListener: Could not allocate gListenerScope.\n");
    } else {
        memcpy(gListenerScope, scope, scopeLen);
        gListenerScope[scopeLen] = 0;

        // Copy the prefix pointer over to gPrefix.

        assert(gPrefix == NULL);

        gPrefix = prefix;

        // Register the appropriate listener with Kauth.

        if ( strcmp(gListenerScope, KAUTH_SCOPE_GENERIC) == 0 ) {
            callback = GenericScopeListener;
        } else if ( strcmp(gListenerScope, KAUTH_SCOPE_PROCESS) == 0 ) {
            callback = ProcessScopeListener;
        } else if ( strcmp(gListenerScope, KAUTH_SCOPE_VNODE) == 0 ) {
            callback = VnodeScopeListener;
        } else if ( strcmp(gListenerScope, KAUTH_SCOPE_FILEOP) == 0 ) {
            callback = FileOpScopeListener;
        } else {
            callback = UnknownScopeListener;
        }

        assert(gListener == NULL);

        gListener = kauth_listen_scope(gListenerScope, callback, NULL);
        if (gListener == NULL) {
            printf("KauthORama.InstallListener: Could not create gListener.\n");
        }
    }

    // In the event of any failure, call RemoveListener which will
    // do all the right cleanup.

    if ( gListenerScope == NULL || gListener == NULL ) {
        RemoveListener();
    }
}

static void ConfigureKauth(const char *configuration)
    // This routine is called by the sysctl handler when it notices
    // that the configuration has changed.  It's responsible for
    // parsing the new configuration string and updating the listener.
    //
    // See SysctlHandler for a description of how I chose to handle the
    // failure case.
    //
    // This routine always runs under the gConfigurationLock.
{
    assert(configuration != NULL);

    // Remove the existing listener.

    RemoveListener();

    // Parse the configuration string and install the new listener.

    if (strcmp(configuration, "remove") == 0) {
        printf("KauthORama.ConfigureKauth: Removed listener.\n");
    } else if ( strprefix(configuration, "add ") ) {
        const char *cursor;
        const char *scopeStart;
        const char *prefixStart;
        size_t      scopeLen;

        // Skip the "add ".

        cursor = configuration + strlen("add ");         // yergh!

        // Work out the span of the scope.

        scopeStart = cursor;
        while ( (*cursor != ' ') && (*cursor != 0) ) {
            cursor += 1;
        }
        assert(cursor >= scopeStart);
        scopeLen = (size_t) (cursor - scopeStart);

        if ( (scopeLen == 0) || (scopeLen > kListenerScopeMaxLength) ) {
            printf("KauthORama.ConfigureKauth: Bad configuration '%s'.\n", configuration);
        } else {

            // Look for a prefix.

            if (*cursor == ' ') {
                cursor += 1;
            }
            if (*cursor == 0) {
                prefixStart = NULL;
            } else {
                prefixStart = cursor;
            }

            // Tell the user what we're doing.

            if (prefixStart == NULL) {
                printf("KauthORama.ConfigureKauth: scope = %.*s\n", (int) scopeLen, scopeStart);
            } else {
                printf("KauthORama.ConfigureKauth: scope = %.*s, prefix = %s\n", (int) scopeLen, scopeStart, prefixStart);
            }

            // Do it.

            InstallListener(scopeStart, scopeLen, prefixStart);
        }
    } else {
        printf("KauthORama.ConfigureKauth: Bad configuration '%s'.\n", configuration);
    }
}

// gConfiguration holds our current configuration string.  It's modified by
// SysctlHandler (well, by sysctl_handle_string which is called by SysctlHandler).

static char gConfiguration[1024];

static int SysctlHandler(
    struct sysctl_oid * oidp,
    void *              arg1,
    int                 arg2,
    struct sysctl_req * req
)
    // This routine is called by the kernel when the user reads or
    // writes our sysctl variable.  The arguments are standard for
    // a sysctl handler.
{
    int     result;

    // Prevent two threads trying to change our configuration at the same
    // time.

    lck_mtx_lock(gConfigurationLock);

    // Let sysctl_handle_string do all the heavy lifting of getting
    // and setting the variable.

    result = sysctl_handle_string(oidp, arg1, arg2, req);

    // On the way out, if we got no error and a new value was set,
    // do our magic.

    if ( (result == 0) && (req->newptr != 0) ) {
        ConfigureKauth(gConfiguration);
    }

    lck_mtx_unlock(gConfigurationLock);

    return result;
}

// Declare our sysctl OID (that is, a variable that the user can
// get and set using sysctl).  Once this OID is registered (which
// is done in the start routine, KauthORama_start, below), the user
// user can get and set our configuration variable (gConfiguration)
// using the sysctl command line tool.
//
// We use OID using SYSCTL_OID rather than SYSCTL_STRING because
// we want to override the hander function that's call (we want
// SysctlHandler rather than sysctl_handle_string).

SYSCTL_OID(
    _kern,                                          // parent OID
    OID_AUTO,                                       // sysctl number, OID_AUTO means we're only accessible by name
    com_example_apple_samplecode_kext_KauthORama,   // our name
    CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_KERN,     // we're a string, more or less
    gConfiguration,                                 // sysctl_handle_string gets/sets this string
    sizeof(gConfiguration),                         // and this is its maximum length
    SysctlHandler,                                  // our handler
    "A",                                            // because that's what SYSCTL_STRING does
    ""                                              // just a comment
);

// gRegisteredOID tracks whether we've registered our OID or not.

static boolean_t gRegisteredOID = FALSE;

/////////////////////////////////////////////////////////////////
#pragma mark ***** Start/Stop

// Prototypes for our entry points (because I've enabled Xcode's strict prototype
// checking).

extern kern_return_t com_example_apple_samplecode_kext_KauthORama_start(kmod_info_t * ki, void * d);
extern kern_return_t com_example_apple_samplecode_kext_KauthORama_stop(kmod_info_t * ki, void * d);

extern kern_return_t com_example_apple_samplecode_kext_KauthORama_start(kmod_info_t * ki, void * d)
    // Called by the system to start up the kext.
{
    #pragma unused(ki)
    #pragma unused(d)
    kern_return_t   err;

    printf("KauthORama_start: Hello Cruel World!\n");

    // Allocate our global resources, needed in order to allocate memory
    // and locks throughout the rest of the program.

    err = KERN_SUCCESS;
    gMallocTag = OSMalloc_Tagalloc("com.example.apple-samplecode.kext.KauthORama", OSMT_DEFAULT);
    if (gMallocTag == NULL) {
        err = KERN_FAILURE;
    }
    if (err == KERN_SUCCESS) {
        gLockGroup = lck_grp_alloc_init("com.example.apple-samplecode.kext.KauthORama", LCK_GRP_ATTR_NULL);
        if (gLockGroup == NULL) {
            err = KERN_FAILURE;
        }
    }

    // Allocate the lock that protects our configuration.

    if (err == KERN_SUCCESS) {
        gConfigurationLock = lck_mtx_alloc_init(gLockGroup, LCK_ATTR_NULL);
        if (gConfigurationLock == NULL) {
            err = KERN_FAILURE;
        }
    }

    // Register our sysctl handler.

    if (err == KERN_SUCCESS) {
        sysctl_register_oid(&sysctl__kern_com_example_apple_samplecode_kext_KauthORama);
        gRegisteredOID = TRUE;
    }

    // If we failed, shut everything down.

    if (err != KERN_SUCCESS) {
        (void) com_example_apple_samplecode_kext_KauthORama_stop(ki, d);
    }

    return err;
}

extern kern_return_t com_example_apple_samplecode_kext_KauthORama_stop(kmod_info_t * ki, void * d)
    // Called by the system to shut down the kext.
{
    #pragma unused(ki)
    #pragma unused(d)

    // Remove our sysctl handler.  This prevents more threads entering the
    // handler and trying to change the configuration.  There is still a
    // race condition here though.  If a thread is already running in our
    // sysctl handler, there's no way to guarantee that it's done before
    // we destroy key resources (notably the gConfigurationLock mutex) that
    // it depends on.  That's because sysctl_unregister_oid makes no attempt
    // to wait until all threads running inside the OID handler are done
    // before it returns.  I could do stuff to minimise the risk, but there's
    // is no 100% way to close this race so I'm going to ignore it.

    if (gRegisteredOID) {
        sysctl_unregister_oid(&sysctl__kern_com_example_apple_samplecode_kext_KauthORama);
        gRegisteredOID = FALSE;
    }

    // Shut down the scope listen, if any.  Not that we lock gConfigurationLock
    // because RemoveListener requires it to be locked.  Further note that
    // we only do this if the lock has actually been allocated.  If the startup
    // routine fails, we can get called with gConfigurationLock set to NULL.

    if (gConfigurationLock != NULL) {
        lck_mtx_lock(gConfigurationLock);
    }
    RemoveListener();
    if (gConfigurationLock != NULL) {
        lck_mtx_unlock(gConfigurationLock);
    }

    // Clean up the configuration lock.

    if (gConfigurationLock != NULL) {
        lck_mtx_free(gConfigurationLock, gLockGroup);
        gConfigurationLock = NULL;
    }

    // Clean up our global resources.

    if (gLockGroup != NULL) {
        lck_grp_free(gLockGroup);
        gLockGroup = NULL;
    }
    if (gMallocTag != NULL) {
        OSMalloc_Tagfree(gMallocTag);
        gMallocTag = NULL;
    }

    // And we're done.

    printf("KauthORama_stop: Goodbye Cruel World!\n");

    return KERN_SUCCESS;
}