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1 /*
2  * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4  *
5  * This code is free software; you can redistribute it and/or modify it
6  * under the terms of the GNU General Public License version 2 only, as
7  * published by the Free Software Foundation.  Oracle designates this
8  * particular file as subject to the "Classpath" exception as provided
9  * by Oracle in the LICENSE file that accompanied this code.
10  *
11  * This code is distributed in the hope that it will be useful, but WITHOUT
12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14  * version 2 for more details (a copy is included in the LICENSE file that
15  * accompanied this code).
16  *
17  * You should have received a copy of the GNU General Public License version
18  * 2 along with this work; if not, write to the Free Software Foundation,
19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20  *
21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22  * or visit www.oracle.com if you need additional information or have any
23  * questions.
24  */
25 
26 #if defined(_ALLBSD_SOURCE)
27 #include <stdint.h>                     /* for uintptr_t */
28 #endif
29 
30 #include "util.h"
31 #include "commonRef.h"
32 
33 /*
34  * ANDROID-CHANGED: This was modified for android to avoid any use of weak
35  * global (jweak) references. On Android hosts the number of jweak
36  * references active at any one time is limited. By using jweaks to keep
37  * track of objects here we could hit the jweak limit on some very large
38  * apps. The implementation is compatible with any JVMTI implementation
39  * that provides the 'can_tag_objects' and
40  * 'can_generate_object_free_events' capabilities. This works by watching
41  * for the ObjectFree events on tagged objects and storing them in a list
42  * of things that have been deleted.
43  *
44  * Each object sent to the front end is tracked with the RefNode struct
45  * (see util.h).
46  * External to this module, objects are identified by a jlong id which is
47  * simply the sequence number. A JVMTI tag is usually used so that
48  * the presence of a debugger-tracked object will not prevent
49  * its collection. Once an object is collected, its RefNode may be
50  * deleted (these may happen in * either order). Using the sequence number
51  * as the object id prevents ambiguity in the object id when the weak ref
52  * is reused. The RefNode* is stored with the object as it's JVMTI Tag.
53  * This tag also provides the weak-reference behavior.
54  *
55  * The ref member is changed from weak to strong when gc of the object is
56  * to be prevented. Whether or not it is strong, it is never exported
57  * from this module.
58  *
59  * A reference count of each jobject is also maintained here. It tracks
60  * the number times an object has been referenced through
61  * commonRef_refToID. A RefNode is freed once the reference
62  * count is decremented to 0 (with commonRef_release*), even if the
63  * corresponding object has not been collected.
64  *
65  * One hash table is maintained. The mapping of ID to RefNode* is handled
66  * with one hash table that will re-size itself as the number of RefNode's
67  * grow.
68  */
69 
70 /* Initial hash table size (must be power of 2) */
71 #define HASH_INIT_SIZE 512
72 /* If element count exceeds HASH_EXPAND_SCALE*hash_size we expand & re-hash */
73 #define HASH_EXPAND_SCALE 8
74 /* Maximum hash table size (must be power of 2) */
75 #define HASH_MAX_SIZE  (1024*HASH_INIT_SIZE)
76 
77 /* Map a key (ID) to a hash bucket */
78 static jint
hashBucket(jlong key)79 hashBucket(jlong key)
80 {
81     /* Size should always be a power of 2, use mask instead of mod operator */
82     /*LINTED*/
83     return ((jint)key) & (gdata->objectsByIDsize-1);
84 }
85 
86 /* Generate a new ID */
87 static jlong
newSeqNum(void)88 newSeqNum(void)
89 {
90     return gdata->nextSeqNum++;
91 }
92 
93 /* ANDROID-CHANGED: This helper function is unique to android.
94  * This function gets a local-ref to object the node is pointing to. If the node's object has been
95  * collected it will return NULL. The caller is responsible for calling env->DeleteLocalRef or
96  * env->PopLocalFrame to clean up the reference. This function makes no changes to the passed in
97  * node.
98  */
99 static jobject
getLocalRef(JNIEnv * env,const RefNode * node)100 getLocalRef(JNIEnv *env, const RefNode* node) {
101     if (node->isStrong) {
102         return JNI_FUNC_PTR(env,NewLocalRef)(env, node->ref);
103     }
104     jint count = -1;
105     jobject *objects = NULL;
106     jlong tag = ptr_to_jlong(node);
107     jvmtiError error = JVMTI_FUNC_PTR(gdata->jvmti,GetObjectsWithTags)
108             (gdata->jvmti, 1, &tag, &count, &objects, NULL);
109     if (error != JVMTI_ERROR_NONE) {
110         EXIT_ERROR(error,"GetObjectsWithTags");
111     }
112     if (count != 1 && count != 0) {
113         EXIT_ERROR(AGENT_ERROR_INTERNAL,
114                    "GetObjectsWithTags returned multiple objects unexpectedly");
115     }
116     jobject res = (count == 0) ? NULL : objects[0];
117     JVMTI_FUNC_PTR(gdata->jvmti,Deallocate)(gdata->jvmti,(unsigned char*)objects);
118     return res;
119 }
120 
121 /* ANDROID-CHANGED: Handler function for objects being freed. */
commonRef_handleFreedObject(jlong tag)122 void commonRef_handleFreedObject(jlong tag) {
123     RefNode* node = (RefNode*)jlong_to_ptr(tag);
124     debugMonitorEnterNoSuspend(gdata->refLock); {
125         // Delete the node and remove it from the hashmap.
126         // If we raced with a deleteNode call and lost the next and prev will be null but we will
127         // not be at the start of the bucket. This is fine.
128         jint slot = hashBucket(node->seqNum);
129         if (node->next != NULL ||
130                 node->prev != NULL ||
131                 gdata->objectsByID[slot] == node) {
132             /* Detach from id hash table */
133             if (node->prev == NULL) {
134                 gdata->objectsByID[slot] = node->next;
135             } else {
136                 node->prev->next = node->next;
137             }
138             /* Also fixup back links. */
139             if (node->next != NULL) {
140                 node->next->prev = node->prev;
141             }
142             gdata->objectsByIDcount--;
143         }
144         jvmtiDeallocate(node);
145     } debugMonitorExit(gdata->refLock);
146 }
147 
148 /* Create a fresh RefNode structure, and tag the object (creating a weak-ref to it).
149  * ANDROID-CHANGED: The definition of RefNode was changed slightly so that node->ref is only for
150  * a strong reference. For weak references we use the node as a tag on the object to keep track if
151  * it.
152  * ANDROID-CHANGED: ref must be a local-reference held live for the duration of this method until it
153  * is fully in the objectByID map.
154  */
155 static RefNode *
createNode(JNIEnv * env,jobject ref)156 createNode(JNIEnv *env, jobject ref)
157 {
158     RefNode   *node;
159     jvmtiError error;
160 
161     if (ref == NULL) {
162         return NULL;
163     }
164 
165     /* Could allocate RefNode's in blocks, not sure it would help much */
166     node = (RefNode*)jvmtiAllocate((int)sizeof(RefNode));
167     if (node == NULL) {
168         return NULL;
169     }
170 
171     /* ANDROID-CHANGED: Use local reference to make sure we have a reference. We will use this
172      * reference to set a tag to the node to use as a weak-reference and keep track of the ref.
173      * ANDROID-CHANGED: Set node tag on the ref. This tag now functions as the weak-reference to the
174      * object.
175      */
176     error = JVMTI_FUNC_PTR(gdata->jvmti, SetTag)(gdata->jvmti, ref, ptr_to_jlong(node));
177     if ( error != JVMTI_ERROR_NONE ) {
178         jvmtiDeallocate(node);
179         return NULL;
180     }
181 
182     /* Fill in RefNode */
183     node->ref      = NULL;
184     node->isStrong = JNI_FALSE;
185     node->count    = 1;
186     node->seqNum   = newSeqNum();
187 
188     /* Count RefNode's created */
189     gdata->objectsByIDcount++;
190     return node;
191 }
192 
193 /* Delete a RefNode allocation, delete weak/global ref and clear tag */
194 static void
deleteNode(JNIEnv * env,RefNode * node)195 deleteNode(JNIEnv *env, RefNode *node)
196 {
197     /* ANDROID-CHANGED: use getLocalRef to get a local reference to the node. */
198     WITH_LOCAL_REFS(env, 1) {
199         jobject localRef = getLocalRef(env, node);
200         LOG_MISC(("Freeing %d\n", (int)node->seqNum));
201 
202         /* Detach from id hash table */
203         if (node->prev == NULL) {
204             gdata->objectsByID[hashBucket(node->seqNum)] = node->next;
205         } else {
206             node->prev->next = node->next;
207         }
208         /* Also fixup back links. */
209         if (node->next != NULL) {
210             node->next->prev = node->prev;
211         }
212 
213         // If we don't get the localref that means the ObjectFree event is being called and the
214         // node will be deleted there.
215         if ( localRef != NULL ) {
216             /* Clear tag */
217             (void)JVMTI_FUNC_PTR(gdata->jvmti,SetTag)
218                                 (gdata->jvmti, localRef, NULL_OBJECT_ID);
219             if (node->isStrong) {
220                 JNI_FUNC_PTR(env,DeleteGlobalRef)(env, node->ref);
221             }
222 
223             jvmtiDeallocate(node);
224         } else {
225             // We are going to let the object-free do the final work. Mark this node as not in the
226             // list with both null links but not in the bucket.
227             node->prev = NULL;
228             node->next = NULL;
229         }
230         gdata->objectsByIDcount--;
231     } END_WITH_LOCAL_REFS(env);
232 }
233 
234 /* Change a RefNode to have a strong reference */
235 static jobject
strengthenNode(JNIEnv * env,RefNode * node)236 strengthenNode(JNIEnv *env, RefNode *node)
237 {
238     if (!node->isStrong) {
239         /* ANDROID-CHANGED: We need to find and fill in the node->ref when we strengthen a node. */
240         WITH_LOCAL_REFS(env, 1) {
241             /* getLocalRef will return NULL if the referent has been collected. */
242             jobject localRef = getLocalRef(env, node);
243             if (localRef != NULL) {
244                 node->ref = JNI_FUNC_PTR(env,NewGlobalRef)(env, localRef);
245                 if (node->ref == NULL) {
246                     EXIT_ERROR(AGENT_ERROR_NULL_POINTER,"NewGlobalRef");
247                 }
248                 node->isStrong = JNI_TRUE;
249             }
250         } END_WITH_LOCAL_REFS(env);
251     }
252     return node->ref;
253 }
254 
255 /* Change a RefNode to have a weak reference
256  * ANDROID-CHANGED: This is done by deleting the strong reference. We already have a tag in
257  * to the node from when we created the node. Since this is never removed we can simply delete the
258  * global ref, reset node->isStrong & node->ref, and return. Since no part of this can fail we can
259  * change this function to be void too.
260  */
261 static void
weakenNode(JNIEnv * env,RefNode * node)262 weakenNode(JNIEnv *env, RefNode *node)
263 {
264     if (node->isStrong) {
265         JNI_FUNC_PTR(env,DeleteGlobalRef)(env, node->ref);
266         node->ref      = NULL;
267         node->isStrong = JNI_FALSE;
268     }
269 }
270 
271 /*
272  * Returns the node which contains the common reference for the
273  * given object. The passed reference should not be a weak reference
274  * managed in the object hash table (i.e. returned by commonRef_idToRef)
275  * because no sequence number checking is done.
276  */
277 static RefNode *
findNodeByRef(JNIEnv * env,jobject ref)278 findNodeByRef(JNIEnv *env, jobject ref)
279 {
280     jvmtiError error;
281     jlong      tag;
282 
283     tag   = NULL_OBJECT_ID;
284     error = JVMTI_FUNC_PTR(gdata->jvmti,GetTag)(gdata->jvmti, ref, &tag);
285     if ( error == JVMTI_ERROR_NONE ) {
286         RefNode   *node;
287 
288         node = (RefNode*)jlong_to_ptr(tag);
289         return node;
290     }
291     return NULL;
292 }
293 
294 /* Locate and delete a node based on ID */
295 static void
deleteNodeByID(JNIEnv * env,jlong id,jint refCount)296 deleteNodeByID(JNIEnv *env, jlong id, jint refCount)
297 {
298     /* ANDROID-CHANGED: Rewrite for double-linked list. Also remove ALL_REFS since it's not needed
299      * since the free-callback will do the work of cleaning up when an object gets collected. */
300     jint     slot;
301     RefNode *node;
302 
303     slot = hashBucket(id);
304     node = gdata->objectsByID[slot];
305 
306     while (node != NULL) {
307         if (id == node->seqNum) {
308             node->count -= refCount;
309             if (node->count <= 0) {
310                 if ( node->count < 0 ) {
311                     EXIT_ERROR(AGENT_ERROR_INTERNAL,"RefNode count < 0");
312                 }
313                 deleteNode(env, node);
314             }
315             break;
316         }
317         node = node->next;
318     }
319 }
320 
321 /*
322  * Returns the node stored in the object hash table for the given object
323  * id. The id should be a value previously returned by
324  * commonRef_refToID.
325  *
326  *  NOTE: It is possible that a match is found here, but that the object
327  *        is garbage collected by the time the caller inspects node->ref.
328  *        Callers should take care using the node->ref object returned here.
329  *
330  */
331 static RefNode *
findNodeByID(JNIEnv * env,jlong id)332 findNodeByID(JNIEnv *env, jlong id)
333 {
334     /* ANDROID-CHANGED: Rewrite for double-linked list */
335     jint     slot;
336     RefNode *node;
337 
338     slot = hashBucket(id);
339     node = gdata->objectsByID[slot];
340 
341     while (node != NULL) {
342         if ( id == node->seqNum ) {
343             if ( node->prev != NULL ) {
344                 /* Re-order hash list so this one is up front */
345                 node->prev->next = node->next;
346                 node->prev->prev = node->prev;
347                 node->next = gdata->objectsByID[slot];
348                 node->next->prev = node;
349                 node->prev = NULL;
350                 gdata->objectsByID[slot] = node;
351             }
352             break;
353         }
354         node = node->next;
355     }
356     return node;
357 }
358 
359 /* Initialize the hash table stored in gdata area */
360 static void
initializeObjectsByID(int size)361 initializeObjectsByID(int size)
362 {
363     /* Size should always be a power of 2 */
364     if ( size > HASH_MAX_SIZE ) size = HASH_MAX_SIZE;
365     gdata->objectsByIDsize  = size;
366     gdata->objectsByIDcount = 0;
367     gdata->objectsByID      = (RefNode**)jvmtiAllocate((int)sizeof(RefNode*)*size);
368     (void)memset(gdata->objectsByID, 0, (int)sizeof(RefNode*)*size);
369 }
370 
371 /* hash in a RefNode */
372 static void
hashIn(RefNode * node)373 hashIn(RefNode *node)
374 {
375     /* ANDROID-CHANGED: Modify for double-linked list */
376     jint     slot;
377 
378     /* Add to id hashtable */
379     slot                     = hashBucket(node->seqNum);
380     node->next               = gdata->objectsByID[slot];
381     node->prev               = NULL;
382     if (node->next != NULL) {
383         node->next->prev     = node;
384     }
385     gdata->objectsByID[slot] = node;
386 }
387 
388 /* Allocate and add RefNode to hash table
389  * ANDROID-CHANGED: Requires that ref be a held-live local ref.*/
390 static RefNode *
newCommonRef(JNIEnv * env,jobject ref)391 newCommonRef(JNIEnv *env, jobject ref)
392 {
393     RefNode *node;
394 
395     /* Allocate the node and set it up */
396     node = createNode(env, ref);
397     if ( node == NULL ) {
398         return NULL;
399     }
400 
401     /* See if hash table needs expansion */
402     if ( gdata->objectsByIDcount > gdata->objectsByIDsize*HASH_EXPAND_SCALE &&
403          gdata->objectsByIDsize < HASH_MAX_SIZE ) {
404         RefNode **old;
405         int       oldsize;
406         int       newsize;
407         int       i;
408 
409         /* Save old information */
410         old     = gdata->objectsByID;
411         oldsize = gdata->objectsByIDsize;
412         /* Allocate new hash table */
413         gdata->objectsByID = NULL;
414         newsize = oldsize*HASH_EXPAND_SCALE;
415         if ( newsize > HASH_MAX_SIZE ) newsize = HASH_MAX_SIZE;
416         initializeObjectsByID(newsize);
417         /* Walk over old one and hash in all the RefNodes */
418         for ( i = 0 ; i < oldsize ; i++ ) {
419             RefNode *onode;
420 
421             onode = old[i];
422             while (onode != NULL) {
423                 RefNode *next;
424 
425                 next = onode->next;
426                 hashIn(onode);
427                 onode = next;
428             }
429         }
430         jvmtiDeallocate(old);
431     }
432 
433     /* Add to id hashtable */
434     hashIn(node);
435     return node;
436 }
437 
438 /* Initialize the commonRefs usage */
439 void
commonRef_initialize(void)440 commonRef_initialize(void)
441 {
442     gdata->refLock = debugMonitorCreate("JDWP Reference Table Monitor");
443     gdata->nextSeqNum       = 1; /* 0 used for error indication */
444     initializeObjectsByID(HASH_INIT_SIZE);
445 }
446 
447 /* Reset the commonRefs usage */
448 void
commonRef_reset(JNIEnv * env)449 commonRef_reset(JNIEnv *env)
450 {
451     debugMonitorEnter(gdata->refLock); {
452         int i;
453 
454         for (i = 0; i < gdata->objectsByIDsize; i++) {
455             RefNode *node;
456 
457             for (node = gdata->objectsByID[i]; node != NULL; node = gdata->objectsByID[i]) {
458                 deleteNode(env, node);
459             }
460             gdata->objectsByID[i] = NULL;
461         }
462 
463         /* Toss entire hash table and re-create a new one */
464         jvmtiDeallocate(gdata->objectsByID);
465         gdata->objectsByID      = NULL;
466         gdata->nextSeqNum       = 1; /* 0 used for error indication */
467         initializeObjectsByID(HASH_INIT_SIZE);
468 
469     } debugMonitorExit(gdata->refLock);
470 }
471 
472 /*
473  * Given a reference obtained from JNI or JVMTI, return an object
474  * id suitable for sending to the debugger front end.
475  */
476 jlong
commonRef_refToID(JNIEnv * env,jobject ref)477 commonRef_refToID(JNIEnv *env, jobject ref)
478 {
479     jlong id;
480 
481     if (ref == NULL) {
482         return NULL_OBJECT_ID;
483     }
484 
485     id = NULL_OBJECT_ID;
486     debugMonitorEnter(gdata->refLock); {
487         RefNode *node;
488 
489         node = findNodeByRef(env, ref);
490         if (node == NULL) {
491             WITH_LOCAL_REFS(env, 1) {
492                 node = newCommonRef(env, JNI_FUNC_PTR(env,NewLocalRef)(env, ref));
493                 if ( node != NULL ) {
494                     id = node->seqNum;
495                 }
496             } END_WITH_LOCAL_REFS(env);
497         } else {
498             id = node->seqNum;
499             node->count++;
500         }
501     } debugMonitorExit(gdata->refLock);
502     return id;
503 }
504 
505 /*
506  * Given an object ID obtained from the debugger front end, return a
507  * strong, global reference to that object (or NULL if the object
508  * has been collected). The reference can then be used for JNI and
509  * JVMTI calls. Caller is resposible for deleting the returned reference.
510  */
511 jobject
commonRef_idToRef(JNIEnv * env,jlong id)512 commonRef_idToRef(JNIEnv *env, jlong id)
513 {
514     jobject ref;
515 
516     ref = NULL;
517     debugMonitorEnter(gdata->refLock); {
518         RefNode *node;
519 
520         node = findNodeByID(env, id);
521         if (node != NULL) {
522             if (node->isStrong) {
523                 saveGlobalRef(env, node->ref, &ref);
524             } else {
525                 jobject lref;
526 
527                 /* ANDROID-CHANGED: Use getLocalRef helper to get a local-reference to the object
528                  * this node weakly points to. It will return NULL if the object has been GCd
529                  */
530                 lref = getLocalRef(env, node);
531                 if ( lref != NULL ) {
532                     /* ANDROID-CHANGED: Use lref to save the global ref since that is the only real
533                      * jobject we have.
534                      */
535                     saveGlobalRef(env, lref, &ref);
536                     JNI_FUNC_PTR(env,DeleteLocalRef)(env, lref);
537                 }
538                 /* ANDROID-CHANGED: Otherwise the object was GC'd shortly after we found the node.
539                  * The free callback will deal with cleanup once we return.
540                  */
541             }
542         }
543     } debugMonitorExit(gdata->refLock);
544     return ref;
545 }
546 
547 /* Deletes the global reference that commonRef_idToRef() created */
548 void
commonRef_idToRef_delete(JNIEnv * env,jobject ref)549 commonRef_idToRef_delete(JNIEnv *env, jobject ref)
550 {
551     if ( ref==NULL ) {
552         return;
553     }
554     tossGlobalRef(env, &ref);
555 }
556 
557 
558 /* Prevent garbage collection of an object */
559 jvmtiError
commonRef_pin(jlong id)560 commonRef_pin(jlong id)
561 {
562     jvmtiError error;
563 
564     error = JVMTI_ERROR_NONE;
565     if (id == NULL_OBJECT_ID) {
566         return error;
567     }
568     debugMonitorEnter(gdata->refLock); {
569         JNIEnv  *env;
570         RefNode *node;
571 
572         env  = getEnv();
573         node = findNodeByID(env, id);
574         if (node == NULL) {
575             error = AGENT_ERROR_INVALID_OBJECT;
576         } else {
577             jobject strongRef;
578 
579             strongRef = strengthenNode(env, node);
580             if (strongRef == NULL) {
581                 /*
582                  * Referent has been collected, clean up now.
583                  * ANDROID-CHANGED: The node will be cleaned up by the object-free callback.
584                  */
585                 error = AGENT_ERROR_INVALID_OBJECT;
586             }
587         }
588     } debugMonitorExit(gdata->refLock);
589     return error;
590 }
591 
592 /* Permit garbage collection of an object */
593 jvmtiError
commonRef_unpin(jlong id)594 commonRef_unpin(jlong id)
595 {
596     jvmtiError error;
597 
598     error = JVMTI_ERROR_NONE;
599     debugMonitorEnter(gdata->refLock); {
600         JNIEnv  *env;
601         RefNode *node;
602 
603         env  = getEnv();
604         node = findNodeByID(env, id);
605         if (node != NULL) {
606             // ANDROID-CHANGED: weakenNode was changed to never fail.
607             weakenNode(env, node);
608         }
609     } debugMonitorExit(gdata->refLock);
610     return error;
611 }
612 
613 /* Release tracking of an object by ID */
614 void
commonRef_release(JNIEnv * env,jlong id)615 commonRef_release(JNIEnv *env, jlong id)
616 {
617     debugMonitorEnter(gdata->refLock); {
618         deleteNodeByID(env, id, 1);
619     } debugMonitorExit(gdata->refLock);
620 }
621 
622 void
commonRef_releaseMultiple(JNIEnv * env,jlong id,jint refCount)623 commonRef_releaseMultiple(JNIEnv *env, jlong id, jint refCount)
624 {
625     debugMonitorEnter(gdata->refLock); {
626         deleteNodeByID(env, id, refCount);
627     } debugMonitorExit(gdata->refLock);
628 }
629 
630 /* Get rid of RefNodes for objects that no longer exist */
631 void
commonRef_compact(void)632 commonRef_compact(void)
633 {
634     // NO-OP.
635 }
636 
637 /* Lock the commonRef tables */
638 void
commonRef_lock(void)639 commonRef_lock(void)
640 {
641     debugMonitorEnter(gdata->refLock);
642 }
643 
644 /* Unlock the commonRef tables */
645 void
commonRef_unlock(void)646 commonRef_unlock(void)
647 {
648     debugMonitorExit(gdata->refLock);
649 }
650