90 lines
2.9 KiB
Plaintext
90 lines
2.9 KiB
Plaintext
Android Dynamic Linker Design Notes
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===================================
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Introduction:
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-------------
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This document provides several notes related to the design of the Android
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dynamic linker.
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Initialization and Termination functions:
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-----------------------------------------
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The Unix Sys V Binary Interface standard states that an
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executable can have the following entries in its .dynamic
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section:
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DT_INIT
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Points to the address of an initialization function
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that must be called when the file is loaded.
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DT_INIT_ARRAY
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Points to an array of function addresses that must be
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called, in-order, to perform initialization. Some of
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the entries in the array can be 0 or -1, and should
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be ignored.
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Note: this is generally stored in a .init_array section
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DT_INIT_ARRAYSZ
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The size of the DT_INITARRAY, if any
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DT_FINI
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Points to the address of a finalization function which
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must be called when the file is unloaded or the process
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terminated.
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DT_FINI_ARRAY
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Same as DT_INITARRAY but for finalizers. Note that the
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functions must be called in reverse-order though
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Note: this is generally stored in a .fini_array section
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DT_FINI_ARRAYSZ
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Size of FT_FINIARRAY
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DT_PREINIT_ARRAY
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An array similar to DT_INIT_ARRAY which must *only* be
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present in executables, not shared libraries, which contains
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a list of functions that need to be called before any other
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initialization function (i.e. DT_INIT and/or DT_INIT_ARRAY)
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in the executable or any of its libraries.
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Note: this is generally stored in a .preinit_array section
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DT_PREINIT_ARRAYSZ
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The size of DT_PREINIT_ARRAY
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If both a DT_INIT and DT_INITARRAY entry are present, the DT_INIT
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function must be called before the DT_INITARRAY functions.
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Consequently, the DT_FINIARRAY must be parsed in reverse order before
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the DT_FINI function, if both are available.
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Note that the implementation of static C++ constructors is very
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much processor dependent, and may use different ELF sections.
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On the ARM (see "C++ ABI for ARM" document), the static constructors
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must be called explicitly from the DT_INIT_ARRAY, and each one of them
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shall register a destructor by calling the special __eabi_atexit()
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function (provided by the C library). The DT_FINI_ARRAY is not used
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by static C++ destructors.
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On x86, the lists of constructors and destructors are placed in special
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sections named ".ctors" and ".dtors", and the DT_INIT / DT_FINI functions
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are in charge of calling them explicitly.
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Debugging:
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----------
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You can increase the verbosity of debug traces by defining the LD_DEBUG
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environment variable to a numeric value from 0 to 2 (corresponding to
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INFO, TRACE, and DEBUG calls in the source). This will only
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affect new processes being launched.
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By default, traces are sent to logcat, with the "linker" tag. You can
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change this to go to stdout instead by setting the definition of
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LINKER_DEBUG_TO_LOG to 0 in "linker_debug.h".
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