--- /dev/null
+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.
+
return 0;
}
-static void call_array(unsigned *ctor, int count)
+
+/* Please read the "Initialization and Termination functions" functions.
+ * of the linker design note in bionic/linker/README.TXT to understand
+ * what the following code is doing.
+ *
+ * The important things to remember are:
+ *
+ * DT_PREINIT_ARRAY must be called first for executables, and should
+ * not appear in shared libraries.
+ *
+ * DT_INIT should be called before DT_INIT_ARRAY if both are present
+ *
+ * DT_FINI should be called after DT_FINI_ARRAY if both are present
+ *
+ * DT_FINI_ARRAY must be parsed in reverse order.
+ */
+
+static void call_array(unsigned *ctor, int count, int reverse)
{
- int n;
- for(n = count; n > 0; n--){
- TRACE("[ %5d Looking at ctor *0x%08x == 0x%08x ]\n", pid,
+ int n, inc = 1;
+
+ if (reverse) {
+ ctor += (count-1);
+ inc = -1;
+ }
+
+ for(n = count; n > 0; n--) {
+ TRACE("[ %5d Looking at %s *0x%08x == 0x%08x ]\n", pid,
+ reverse ? "dtor" : "ctor",
(unsigned)ctor, (unsigned)*ctor);
- void (*func)() = (void (*)()) *ctor++;
+ void (*func)() = (void (*)()) *ctor;
+ ctor += inc;
if(((int) func == 0) || ((int) func == -1)) continue;
TRACE("[ %5d Calling func @ 0x%08x ]\n", pid, (unsigned)func);
func();
static void call_constructors(soinfo *si)
{
- /* TODO: THE ORIGINAL CODE SEEMED TO CALL THE INIT FUNCS IN THE WRONG ORDER.
- * Old order: init, init_array, preinit_array..
- * Correct order: preinit_array, init, init_array.
- * Verify WHY.
- */
-
if (si->flags & FLAG_EXE) {
TRACE("[ %5d Calling preinit_array @ 0x%08x [%d] for '%s' ]\n",
pid, (unsigned)si->preinit_array, si->preinit_array_count,
si->name);
- call_array(si->preinit_array, si->preinit_array_count);
+ call_array(si->preinit_array, si->preinit_array_count, 0);
TRACE("[ %5d Done calling preinit_array for '%s' ]\n", pid, si->name);
} else {
if (si->preinit_array) {
}
}
- // If we have an init section, then we should call it now, to make sure
- // that all the funcs in the .ctors section get run.
- // Note: For ARM, we shouldn't have a .ctor section (should be empty)
- // when we have an (pre)init_array section, but let's be compatible with
- // old (non-eabi) binaries and try the _init (DT_INIT) anyway.
if (si->init_func) {
TRACE("[ %5d Calling init_func @ 0x%08x for '%s' ]\n", pid,
(unsigned)si->init_func, si->name);
if (si->init_array) {
TRACE("[ %5d Calling init_array @ 0x%08x [%d] for '%s' ]\n", pid,
(unsigned)si->init_array, si->init_array_count, si->name);
- call_array(si->init_array, si->init_array_count);
+ call_array(si->init_array, si->init_array_count, 0);
TRACE("[ %5d Done calling init_array for '%s' ]\n", pid, si->name);
}
}
+
static void call_destructors(soinfo *si)
{
if (si->fini_array) {
TRACE("[ %5d Calling fini_array @ 0x%08x [%d] for '%s' ]\n", pid,
(unsigned)si->fini_array, si->fini_array_count, si->name);
- call_array(si->fini_array, si->fini_array_count);
+ call_array(si->fini_array, si->fini_array_count, 1);
TRACE("[ %5d Done calling fini_array for '%s' ]\n", pid, si->name);
}
- // If we have an fini section, then we should call it now, to make sure
- // that all the funcs in the .dtors section get run.
- // Note: For ARM, we shouldn't have a .dtor section (should be empty)
- // when we have an fini_array section, but let's be compatible with
- // old (non-eabi) binaries and try the _fini (DT_FINI) anyway.
if (si->fini_func) {
TRACE("[ %5d Calling fini_func @ 0x%08x for '%s' ]\n", pid,
(unsigned)si->fini_func, si->name);
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