Fix the parsing of the .fini_array in the dynamic linker (it should happen in reverse order).

Also add a small design overview document for the linker. The latter contains details on
how the initialization/finalization is supposed to work.

1 files changed, 114 insertions, 0 deletions

diff --git a/linker/README.TXT b/linker/README.TXT
new file mode 100644
index 0000000..4fff14e
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+++ b/linker/README.TXT
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+Android Dynamic Linker Design Notes
+===================================
+
+Introduction:
+-------------
+
+This document provides several notes related to the design of the Android
+dynamic linker.
+
+
+Prelinking:
+-----------
+
+System libraries in Android are internally prelinked, which means that
+any internal relocations within them are stripped from the corresponding
+shared object, in order to reduce size and speed up loading.
+
+Such libraries can only be loaded at the very specific virtual memory address
+they have been prelinked to (during the build process). The list of prelinked
+system libraries and their corresponding virtual memory address is found in
+the file:
+
+   build/core/prelink-linux-<arch>.map
+
+It should be updated each time a new system library is added to the
+system.
+
+The prelink step happens at build time, and uses the 'soslim' and 'apriori'
+tools:
+
+  - 'apriori' is the real prelink tool which removes relocations from the
+    shared object, however, it must be given a list of symbols to remove
+    from the file.
+
+  - 'soslim' is used to find symbols in an executable ELF file
+    and generate a list that can be passed to 'apriori'.
+
+By default, these tools are only used to remove internal symbols from
+libraries, though they have been designed to allow more aggressive
+optimizations (e.g. 'global' prelinking and symbol stripping, which
+prevent replacing individual system libraries though).
+
+You can disable prelinking at build time by modifying your Android.mk with
+a line like:
+
+    LOCAL_PRELINK_MODULE := false
+
+
+Initialization and Termination functions:
+-----------------------------------------
+
+The Unix Sys V Binary Interface standard states that an
+executable can have the following entries in its .dynamic
+section:
+
+  DT_INIT
+      Points to the address of an initialization function
+      that must be called when the file is loaded.
+
+  DT_INIT_ARRAY
+      Points to an array of function addresses that must be
+      called, in-order, to perform initialization. Some of
+      the entries in the array can be 0 or -1, and should
+      be ignored.
+
+      Note: this is generally stored in a .init_array section
+
+  DT_INIT_ARRAYSZ
+      The size of the DT_INITARRAY, if any
+
+  DT_FINI
+      Points to the address of a finalization function which
+      must be called when the file is unloaded or the process
+      terminated.
+
+  DT_FINI_ARRAY
+      Same as DT_INITARRAY but for finalizers. Note that the
+      functions must be called in reverse-order though
+
+      Note: this is generally stroed in a .fini_array section
+
+  DT_FINI_ARRAYSZ
+      Size of FT_FINIARRAY
+
+  DT_PREINIT_ARRAY
+      An array similar to DT_INIT_ARRAY which must *only* be
+      present in executables, not shared libraries, which contains
+      a list of functions that need to be called before any other
+      initialization function (i.e. DT_INIT and/or DT_INIT_ARRAY)
+
+      Note: this is generally stroed in a .preinit_array section
+
+  DT_PREINIT_ARRAYSZ
+      The size of DT_PREINIT_ARRAY
+
+If both a DT_INIT and DT_INITARRAY entry are present, the DT_INIT
+function must be called before the DT_INITARRAY functions.
+
+Consequently, the DT_FINIARRAY must be parsed in reverse order before
+the DT_FINI function, if both are available.
+
+Note that the implementation of static C++ constructors is very
+much processor dependent, and may use different ELF sections.
+
+On the ARM (see "C++ ABI for ARM" document), the static constructors
+must be called explicitely from the DT_INIT_ARRAY, and each one of them
+shall register a destructor by calling the special __eabi_atexit()
+function (provided by the C library). The DT_FINI_ARRAY is not used
+by static C++ destructors.
+
+On x86, the lists of constructors and destructors are placed in special
+sections named ".ctors" and ".dtors", and the DT_INIT / DT_FINI functions
+are in charge of calling them explicitely.
+