1 /*
2 * Copyright (c) 2016, 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.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 */
23
24 /*
25 * A simple way to test JVMTI ClassFileLoadHook. See ../testlibrary_tests/SimpleClassFileLoadHookTest.java
26 * for an example.
27 */
28 #include <stdio.h>
29 #include <stdarg.h>
30 #include <stdlib.h>
31 #include <string.h>
32
33 #include <jvmti.h>
34 #include <jni.h>
35
36 static char* CLASS_NAME = NULL;
37 static char* FROM = NULL;
38 static char* TO = NULL;
39 static jvmtiEnv *jvmti = NULL;
40 static jvmtiEventCallbacks callbacks;
41
42 /**
43 * For all classes whose name equals to CLASS_NAME, we replace all occurrence of FROM to TO
44 * in the classfile data. CLASS_NAME must be a binary class name.
45 *
46 * FROM is usually chosen as part of a UTF8 string in the class file. For example, if the
47 * original class file has
48 * String getXXX() { return "theXXX";}
49 * You can set FROM=XXX, TO=YYY to rewrite the class to be
50 * String getYYY() { return "theYYY";}
51 *
52 * Please note that the replacement is NOT limited just the UTF8 strings, but rather applies
53 * to all the bytes in the classfile. So if you pick a very short FROM string like X,
54 * it may override any POP2 bytecodes, which have the value 88 (ascii 'X').
55 *
56 * A good FROM string to use is 'cellphone', where the first 4 bytes represent the bytecode
57 * sequence DADD/LSUB/IDIV/IDIV, which does not appear in valid bytecode streams.
58 */
59 void JNICALL
60 ClassFileLoadHook(jvmtiEnv *jvmti_env, JNIEnv *env, jclass class_beeing_redefined,
61 jobject loader, const char* name, jobject protection_domain,
62 jint class_data_len, const unsigned char* class_data,
63 jint *new_class_data_len, unsigned char** new_class_data) {
64
65 if (name != NULL && (strcmp(name, CLASS_NAME) == 0)) {
66 size_t n = strlen(FROM);
67 unsigned char* new_data;
68
69 if ((*jvmti)->Allocate(jvmti, class_data_len, &new_data) == JNI_OK) {
70 const unsigned char* s = class_data;
71 unsigned char* d = new_data;
72 unsigned char* end = d + class_data_len;
73 int count = 0;
74
75 fprintf(stderr, "found class to be hooked: %s - rewriting ...\n", name);
76
77 while (d + n < end) {
78 if (memcmp(s, FROM, n) == 0) {
79 memcpy(d, TO, n);
80 s += n;
81 d += n;
82 count++;
83 } else {
84 *d++ = *s++;
85 }
86 }
87 while (d < end) {
88 *d++ = *s++;
89 }
90
91 *new_class_data_len = class_data_len;
92 *new_class_data = new_data;
93
94 fprintf(stderr, "Rewriting done. Replaced %d occurrence(s)\n", count);
95 }
96 }
97 }
98
99 static jint init_options(char *options) {
100 char* class_name;
101 char* from;
102 char* to;
103
104 fprintf(stderr, "Agent library loaded with options = %s\n", options);
105 if ((class_name = options) != NULL &&
106 (from = strchr(class_name, ',')) != NULL && (from[1] != 0)) {
107 *from = 0;
108 from++;
109 if ((to = strchr(from, ',')) != NULL && (to[1] != 0)) {
110 *to = 0;
111 to++;
112 if (strchr(to, ',') == NULL &&
113 strlen(to) == strlen(from) &&
114 strlen(class_name) > 0 &&
115 strlen(to) > 0) {
116 CLASS_NAME = strdup(class_name);
117 FROM = strdup(from);
118 TO = strdup(to);
119 fprintf(stderr, "CLASS_NAME = %s, FROM = %s, TO = %s\n",
120 CLASS_NAME, FROM, TO);
121 return JNI_OK;
122 }
123 }
124 }
125 fprintf(stderr,
126 "Incorrect options. You need to start the JVM with -agentlib:ClassFileLoadHook=<classname>,<from>,<to>\n"
127 "where <classname> is the class you want to hook, <from> is the string in the classfile to be replaced\n"
128 "with <to>. <from> and <to> must have the same length. Example:\n"
129 " @run main/native -agentlib:ClassFileLoadHook=Foo,XXX,YYY ClassFileLoadHookTest\n");
130 return JNI_ERR;
131 }
132
133 static jint Agent_Initialize(JavaVM *jvm, char *options, void *reserved) {
134 int rc;
135
136 if ((rc = (*jvm)->GetEnv(jvm, (void **)&jvmti, JVMTI_VERSION_1_1)) != JNI_OK) {
137 fprintf(stderr, "Unable to create jvmtiEnv, GetEnv failed, error = %d\n", rc);
138 return JNI_ERR;
139 }
140 if ((rc = init_options(options)) != JNI_OK) {
141 return JNI_ERR;
142 }
143
144 (void) memset(&callbacks, 0, sizeof(callbacks));
145 callbacks.ClassFileLoadHook = &ClassFileLoadHook;
146 if ((rc = (*jvmti)->SetEventCallbacks(jvmti, &callbacks, sizeof(callbacks))) != JNI_OK) {
147 fprintf(stderr, "SetEventCallbacks failed, error = %d\n", rc);
148 return JNI_ERR;
149 }
150
151 if ((rc = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE,
152 JVMTI_EVENT_CLASS_FILE_LOAD_HOOK, NULL)) != JNI_OK) {
153 fprintf(stderr, "SetEventNotificationMode failed, error = %d\n", rc);
154 return JNI_ERR;
155 }
156
157 return JNI_OK;
158 }
159
160 JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *jvm, char *options, void *reserved) {
161 return Agent_Initialize(jvm, options, reserved);
162 }
163
164 JNIEXPORT jint JNICALL Agent_OnAttach(JavaVM *jvm, char *options, void *reserved) {
165 return Agent_Initialize(jvm, options, reserved);
166 }