am 8215679b: 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.
Merge commit '8215679b355efe3829bab571bd566dc818ea4ccc' * commit '8215679b355efe3829bab571bd566dc818ea4ccc': Fix the parsing of the .fini_array in the dynamic linker (it should happen in reverse order).
This commit is contained in:
David 'Digit' Turner
The Android Open Source Project
commit
d7f5035959
2 changed files with 148 additions and 24 deletions
114
linker/README.TXT
Normal file
114
linker/README.TXT
Normal file
|
|
@ -0,0 +1,114 @@
|
|||
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.
|
||||
|
||||
|
|
@ -1248,13 +1248,38 @@ static int reloc_library(soinfo *si, Elf32_Rel *rel, unsigned count)
|
|||
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();
|
||||
|
|
@ -1263,17 +1288,11 @@ static void call_array(unsigned *ctor, int count)
|
|||
|
||||
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) {
|
||||
|
|
@ -1283,11 +1302,6 @@ static void call_constructors(soinfo *si)
|
|||
}
|
||||
}
|
||||
|
||||
// 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);
|
||||
|
|
@ -1298,25 +1312,21 @@ static void call_constructors(soinfo *si)
|
|||
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);
|
||||
|
|
|
|||
Loading…
Reference in a new issue