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README.TXT

1Introduction:
2-------------
3
4A custom dynamic linker for Android programs that adds a few interesting
5features compared to /system/bin/linker:
6
7  - Support changing the library search path. The system linker, when used
8    inside Android applications, is limited to the value of LD_LIBRARY_PATH
9    at boot time, that only looks into system directories, not application
10    ones.
11
12    This linker allows the client to add application paths before the
13    default system ones, this has two benefits:
14
15      - Library dependencies are loaded automatically in the right order.
16
17      - Libraries from the application paths are favored over system ones.
18        This avoids conflicts when one of your application's libraries
19        has the same name than a system one (which happens randomly
20        on certain devices due to system application bundling).
21
22    (Note: The system linker issue above has been fixed in Android 4.3).
23
24  - Supports any number of shared libraries. On older Android platforms,
25    the system linker will refuse to load more than 64 or 128 libraries
26    in a single process (Note: Fixed in Android 4.3).
27
28  - Supports loading a library at an explicit (page-aligned) memory
29    address. The system linker always randomizes the address. Note that
30    this is generally a _bad_ idea for security reasons. Consider using
31    this only when using shared RELROs (see below).
32
33  - Supports loading a library from an explicit (page-aligned) file
34    offset. This can be useful to load a library directly from an .apk,
35    provided that it is uncompressed and at a page-aligned offset.
36
37  - Support sharing of RELRO sections. When two processes load the same
38    library at exactly the same address, the content of its RELRO section
39    is identical. By default, each instance uses private RAM pages to host
40    it, but it is possible to use a single ashmem region to share the same
41    data instead.
42
43    WARNING: This feature will not work on certain older kernels. See
44             the documentation for crazy_system_can_share_relro() for
45             more details.
46
47See include/crazy_linker.h for the API and its documentation.
48
49See LICENSE file for full licensing details (hint: BSD)
50
51A few notes:
52
53  - Do not use this if you don't know what you're doing. Read the API
54    documentation first, and look at the test programs for usage examples.
55
56  - The crazy linker will always use the system linker to load NDK-exposed
57    system libraries (e.g. liblog.so and others). This avoids having two
58    instances of the same library in the same process, and correctly
59    resolving any symbols from system libraries.
60
61  - Any library loaded by the crazy linker, and which uses functions of
62    libdl.so will continue to work. However, calls to dlopen(), dlsym(),
63    et al will be redirected to the crazy linker's own wrappers.
64
65    This ensures that if a library is loaded by the crazy linker, any of
66    its dependencies will be loaded by it too.
67
68  - Libraries loaded with the crazy linker are visible to GDB, or Breakpad,
69    and stack unwinding / C++ exception propagation should just work.
70
71
72Caveats:
73--------
74
75  You can't call the crazy_linker code directly from Java in your Android
76  application (it's a static library). You need to put it into your own
77  shared library, loaded with System.loadLibrary() instead (or alternatively,
78  inside your NativeActivity's shared library).
79
80  Also, libraries loaded with the crazy linker are not visible to the system
81  one. In practice, it means that lazy native method lookup will not work. I.e.:
82
83  The first time you call a native method like:
84
85    'mypackage.MyClass.myNativeMethod()'
86
87  The VM will look into existing native libraries with dlsym() for a
88  function symbol named like:
89
90    Java_mypackage_MyClass_myNativeMethod
91
92  This will not work if the symbol is inside a library loaded with the
93  crazy_linker.
94
95  To work-around this, register the native methods explicitely
96  in your JNI_OnLoad() by calling env->RegisterNatives() with the
97  appropriate parameters.
98
99
100Usage instructions:
101-------------------
102
103  1/ Add the following to your module definition in your project's Android.mk:
104
105        LOCAL_STATIC_LIBRARIES := crazy_linker
106
107  2/ Also Include the top-level crazy_linker Android.mk, as in:
108
109        include /path/to/crazy_linker/Android.mk
110
111  3/ In your C or C++ source:
112
113        #include <crazy_linker.h>
114
115    Read the header for API documentation.
116
117  If your library implements native methods, it must explicitely register
118  them with env->RegisterNatives() before they become usable.
119
120BUGS & TODOs:
121-------------
122
123  - Libraries loaded by the crazy linker are not automatically closed when
124    the process exits.
125
126  - dlopen() when called inside a library loaded by the crazy linker doesn't
127    support RTLD_MAIN or RTLD_NEXT.
128
129Testing:
130--------
131
132  If you modify this code, check your changes by running the test suite using:
133
134    cd $NDK
135    tests/run-tests.sh crazy_linker
136
137  See DESIGN.TXT for an overview of the library's design.
138