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[原创]android linker 解读2 ---- find_library
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发表于: 2020-12-31 13:34
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我们跟踪创建Loadtask过程。
该函数调用LoadTask的重载函数。
该函数完成各个成员的赋值，简单看下哪些成员被赋值，name，needed_by ，elf_readermap , start_from.
start_with
追溯needed_by 来历， needed_by传入时实参为start_with , 继续往上跟，发现caller_addr 通过__builtin_return_address来赋值。
查看__builtin_return_address() 的说明 , 返回调用函数的地址，g++的内建函数 ，可以获取到调用函数时的ret地址。就可以获取到调用android_dlopen_exit的函数地址。
在该函数将其转换为soinfo 。
find_containing_library 根据地址的偏移计算来返回调用函数所在模块的soinfo 。
我们跟踪创建Loadtask过程。
该函数调用LoadTask的重载函数 ， 同时分配空间。
该函数完成各个成员的赋值，简单看下哪些成员被赋值，名称，needed_by ，elf_readermap , start_from 。
readers_map 就有意思了 ， 在调用load过程中可以用来解析elf文件。
该过程在find_library中被生成 。
该步骤给列表中增加so。
筛选要载入的load_list 并解析elf文件的so
这个elf_reader 就是对应so用来解析elf文件的。
Dynamic entry
预连接所有DT_NEEDED 库, 逻辑简单，没有被连接过，则调用prelink_image()。
[x] prelink_image。 依次看全太肝了 ， 后期在这里查字段。
略
涉及到重定位的镜像连接。
在Step3中找这rel_ 和pltrel 的赋值，分别为DT_JMPREL 和 DT_REL 对应.rel.plt 和.rel.dyn段 。
参数中有 plain_reloc_iterator ，传入rel的迭代器 。
Elf32_Rel的结构体。
对于DT_REL,该函数获取到rel.dyn 段, 到这里重定位的内容就很简单了。
start: =
addr : = 0
重定位的类型：
重定位运算方法:
验证重定位过程。
以类型为R_ARM_RELATIVE 为例。
偏移为1a610 类型为R_ARM_RELATIVE ，该偏移的值为19758 , 则修正 base+1a610 的地址的值为 19758 + base。
重定位结果。
但是重定位算法解决了，但是对于不同的类型含义是什么？？ 可以确定R_ARM_RELATIVE 对应内部符号重定位 ， R_ARM_GLOB_DAT，R_ARM_JMP_SLOT对应外部符号 ，其中R_ARM_GLOB_DAT和R_ARM_JMP_SLOT的差别是？？
https://bbs.pediy.com/upload/attach/202012/790193_GT99SKKNDDFX66S.png
连接过程结束后，在依次返回到上层函数，就可以明白一个so的加载过程为 分配空间-> 解析elf -> 重定位空间 -> 调用init -> 存在则调用Jni_Onload .
https://docs.oracle.com/cd/E19120-01/open.solaris/819-0690/chapter6-26/index.html 重定位类型
http://nicephil.blinkenshell.org/my_book/ch07s04.html ELF 相关知识
// Step 0: prepare.
LoadTaskList load_tasks;for (size_t i = 0; i < library_names_count; ++i) {
const char* name = library_names[i];
LD_LOG(kLogDlopen,"[linker.cpp] step 1 ,so_name",name);
load_tasks.push_back(LoadTask::create(name, start_with, ns, &readers_map));
}// If soinfos array is null allocate one on stack.
// The array is needed in case of failure; for example
// when library_names[] = {libone.so, libtwo.so} and libone.so
// is loaded correctly but libtwo.so failed for some reason.
// In this case libone.so should be unloaded on return.
// See also implementation of failure_guard below.
if (soinfos == nullptr) {
size_t soinfos_size = sizeof(soinfo*)*library_names_count;
soinfos = reinterpret_cast<soinfo**>(alloca(soinfos_size));
memset(soinfos, 0, soinfos_size);
}// list of libraries to link - see step 2.
size_t soinfos_count = 0;
auto scope_guard = android::base::make_scope_guard([&]() {
for (LoadTask* t : load_tasks) {
LD_LOG(kLogDlopen,"[linker.cpp] before call deleter %s",t->get_name());
LoadTask::deleter(t);
}
});auto failure_guard = android::base::make_scope_guard([&]() {
// Housekeeping
soinfo_unload(soinfos, soinfos_count);
});ZipArchiveCache zip_archive_cache;// Step 0: prepare.
LoadTaskList load_tasks;for (size_t i = 0; i < library_names_count; ++i) {
const char* name = library_names[i];
LD_LOG(kLogDlopen,"[linker.cpp] step 1 ,so_name",name);
load_tasks.push_back(LoadTask::create(name, start_with, ns, &readers_map));
}// If soinfos array is null allocate one on stack.
// The array is needed in case of failure; for example
// when library_names[] = {libone.so, libtwo.so} and libone.so
// is loaded correctly but libtwo.so failed for some reason.
// In this case libone.so should be unloaded on return.
// See also implementation of failure_guard below.
if (soinfos == nullptr) {
size_t soinfos_size = sizeof(soinfo*)*library_names_count;
soinfos = reinterpret_cast<soinfo**>(alloca(soinfos_size));
memset(soinfos, 0, soinfos_size);
}// list of libraries to link - see step 2.
size_t soinfos_count = 0;
auto scope_guard = android::base::make_scope_guard([&]() {
for (LoadTask* t : load_tasks) {
LD_LOG(kLogDlopen,"[linker.cpp] before call deleter %s",t->get_name());
LoadTask::deleter(t);
}
});auto failure_guard = android::base::make_scope_guard([&]() {
// Housekeeping
soinfo_unload(soinfos, soinfos_count);
});ZipArchiveCache zip_archive_cache;static LoadTask* create(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map) {
LoadTask* ptr = TypeBasedAllocator<LoadTask>::alloc();
return new (ptr) LoadTask(name, needed_by, start_from, readers_map);
}static LoadTask* create(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map) {
LoadTask* ptr = TypeBasedAllocator<LoadTask>::alloc();
return new (ptr) LoadTask(name, needed_by, start_from, readers_map);
}LoadTask(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map)
: name_(name), needed_by_(needed_by), si_(nullptr),
fd_(-1), close_fd_(false), file_offset_(0), elf_readers_map_(readers_map),
is_dt_needed_(false), start_from_(start_from) {}
LoadTask(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map)
: name_(name), needed_by_(needed_by), si_(nullptr),
fd_(-1), close_fd_(false), file_offset_(0), elf_readers_map_(readers_map),
is_dt_needed_(false), start_from_(start_from) {}
void* android_dlopen_ext(const char* filename, int flag, const android_dlextinfo* extinfo) {
const void* caller_addr = __builtin_return_address(0);
return __loader_android_dlopen_ext(filename, flag, extinfo, caller_addr);
}void* android_dlopen_ext(const char* filename, int flag, const android_dlextinfo* extinfo) {
const void* caller_addr = __builtin_return_address(0);
return __loader_android_dlopen_ext(filename, flag, extinfo, caller_addr);
}soinfo* find_containing_library(const void* p) {
ElfW(Addr) address = reinterpret_cast<ElfW(Addr)>(p);
for (soinfo* si = solist_get_head(); si != nullptr; si = si->next) {
if (address >= si->base && address - si->base < si->size) {
return si;
}
}
return nullptr;
}soinfo* find_containing_library(const void* p) {
ElfW(Addr) address = reinterpret_cast<ElfW(Addr)>(p);
for (soinfo* si = solist_get_head(); si != nullptr; si = si->next) {
if (address >= si->base && address - si->base < si->size) {
return si;
}
}
return nullptr;
}static LoadTask* create(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map) {
LoadTask* ptr = TypeBasedAllocator<LoadTask>::alloc();
return new (ptr) LoadTask(name, needed_by, start_from, readers_map);
}static LoadTask* create(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map) {
LoadTask* ptr = TypeBasedAllocator<LoadTask>::alloc();
return new (ptr) LoadTask(name, needed_by, start_from, readers_map);
}LoadTask(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map)
: name_(name), needed_by_(needed_by), si_(nullptr),
fd_(-1), close_fd_(false), file_offset_(0), elf_readers_map_(readers_map),
is_dt_needed_(false), start_from_(start_from) {}
LoadTask(const char* name,
soinfo* needed_by,
android_namespace_t* start_from,
std::unordered_map<const soinfo*, ElfReader>* readers_map)
: name_(name), needed_by_(needed_by), si_(nullptr),
fd_(-1), close_fd_(false), file_offset_(0), elf_readers_map_(readers_map),
is_dt_needed_(false), start_from_(start_from) {}
// readers_map is shared across recursive calls to find_libraries.
std::unordered_map<const soinfo*, ElfReader> readers_map;
// readers_map is shared across recursive calls to find_libraries.
std::unordered_map<const soinfo*, ElfReader> readers_map;
// Step 1: expand the list of load_tasks to include
// all DT_NEEDED libraries (do not load them just yet)
for (size_t i = 0; i<load_tasks.size(); ++i) {
LoadTask* task = load_tasks[i];
soinfo* needed_by = task->get_needed_by();
bool is_dt_needed = needed_by != nullptr && (needed_by != start_with || add_as_children);
task->set_extinfo(is_dt_needed ? nullptr : extinfo);
task->set_dt_needed(is_dt_needed);
/*
try to find the load.Note: start from the namespace that is stored in the LoadTask. This namespace is different from the current namespace when the LoadTask is for a transitive dependency and the lib that created the LoadTask is not found in the current namespace but in one of the linked namespace.
*/
if (!find_library_internal(const_cast<android_namespace_t*>(task->get_start_from()),
task,
&zip_archive_cache,
&load_tasks,
rtld_flags,
search_linked_namespaces || is_dt_needed)) {
return false;
}
soinfo* si = task->get_soinfo();
if (is_dt_needed) {
needed_by->add_child(si);
if (si->is_linked()) {
si->increment_ref_count();
}
}
// When ld_preloads is not null, the first
// ld_preloads_count libs are in fact ld_preloads.
if (ld_preloads != nullptr && soinfos_count < ld_preloads_count) {
ld_preloads->push_back(si);
}
if (soinfos_count < library_names_count) {
soinfos[soinfos_count++] = si;
}
}
// Step 1: expand the list of load_tasks to include
// all DT_NEEDED libraries (do not load them just yet)
for (size_t i = 0; i<load_tasks.size(); ++i) {
LoadTask* task = load_tasks[i];
soinfo* needed_by = task->get_needed_by();
bool is_dt_needed = needed_by != nullptr && (needed_by != start_with || add_as_children);
task->set_extinfo(is_dt_needed ? nullptr : extinfo);
task->set_dt_needed(is_dt_needed);
/*
try to find the load.Note: start from the namespace that is stored in the LoadTask. This namespace is different from the current namespace when the LoadTask is for a transitive dependency and the lib that created the LoadTask is not found in the current namespace but in one of the linked namespace.
*/
if (!find_library_internal(const_cast<android_namespace_t*>(task->get_start_from()),
task,
&zip_archive_cache,
&load_tasks,
rtld_flags,
search_linked_namespaces || is_dt_needed)) {
return false;
}
soinfo* si = task->get_soinfo();
if (is_dt_needed) {
needed_by->add_child(si);
if (si->is_linked()) {
si->increment_ref_count();
}
}
// When ld_preloads is not null, the first
// ld_preloads_count libs are in fact ld_preloads.
if (ld_preloads != nullptr && soinfos_count < ld_preloads_count) {
ld_preloads->push_back(si);
}
if (soinfos_count < library_names_count) {
soinfos[soinfos_count++] = si;
}
}
// Step 2: Load libraries in random order (see b/24047022)
LoadTaskList load_list;for (auto&& task : load_tasks) {
soinfo* si = task->get_soinfo();
auto pred = [&](const LoadTask* t) {
return t->get_soinfo() == si;
};
if (!si->is_linked() &&
std::find_if(load_list.begin(), load_list.end(), pred) == load_list.end() ) {
load_list.push_back(task);
}
}shuffle(&load_list);for (auto&& task : load_list) {
//*** 看看这个loda函数
if (!task->load()) {
return false;
}
}// Step 2: Load libraries in random order (see b/24047022)
LoadTaskList load_list;for (auto&& task : load_tasks) {
soinfo* si = task->get_soinfo();
auto pred = [&](const LoadTask* t) {
return t->get_soinfo() == si;
};
if (!si->is_linked() &&
std::find_if(load_list.begin(), load_list.end(), pred) == load_list.end() ) {
load_list.push_back(task);
}
}shuffle(&load_list);for (auto&& task : load_list) {
//*** 看看这个loda函数
if (!task->load()) {
return false;
}
}bool load() {
ElfReader& elf_reader = get_elf_reader();
//在elf_read.Load 下完成elf文件空间的分配和解析 ，还不能算载入了so。
if (!elf_reader.Load(extinfo_)) {
return false;
}
si_->base = elf_reader.load_start();
si_->size = elf_reader.load_size();
si_->set_mapped_by_caller(elf_reader.is_mapped_by_caller());
si_->load_bias = elf_reader.load_bias();
LD_LOG(kLogDlopen,"[linker.cpp] Step2 load_bias %p ",si_->load_bias);
si_->phnum = elf_reader.phdr_count();
si_->phdr = elf_reader.loaded_phdr();
return true;
}bool load() {
ElfReader& elf_reader = get_elf_reader();
//在elf_read.Load 下完成elf文件空间的分配和解析 ，还不能算载入了so。
if (!elf_reader.Load(extinfo_)) {
return false;
}
si_->base = elf_reader.load_start();
si_->size = elf_reader.load_size();
si_->set_mapped_by_caller(elf_reader.is_mapped_by_caller());
si_->load_bias = elf_reader.load_bias();
LD_LOG(kLogDlopen,"[linker.cpp] Step2 load_bias %p ",si_->load_bias);
si_->phnum = elf_reader.phdr_count();
si_->phdr = elf_reader.loaded_phdr();
return true;
}typedef struct{ Elf64_Sxword d_tag; /* Dynamic entry type */
union
{
Elf64_Xword d_val; /* Integer value */
Elf64_Addr d_ptr; /* Address value */
} d_un;
} Elf64_Dyn;typedef struct{ Elf64_Sxword d_tag; /* Dynamic entry type */
union
{
Elf64_Xword d_val; /* Integer value */
Elf64_Addr d_ptr; /* Address value */
} d_un;
} Elf64_Dyn;// Step 3: pre-link all DT_NEEDED libraries in breadth first order.
for (auto&& task : load_tasks) {
soinfo* si = task->get_soinfo();
if (!si->is_linked() && !si->prelink_image()) {
return false;
}
}// Step 3: pre-link all DT_NEEDED libraries in breadth first order.
for (auto&& task : load_tasks) {
soinfo* si = task->get_soinfo();
if (!si->is_linked() && !si->prelink_image()) {
return false;
}
}// Step 5: link libraries that are not destined to this namespace.
// Do this by recursively calling find_libraries on the namespace where the lib
// was found during Step 1.
for (auto&& task : load_tasks) {
soinfo* si = task->get_soinfo();
if (si->get_primary_namespace() != ns) {
const char* name = task->get_name();
if (find_libraries(si->get_primary_namespace(), task->get_needed_by(), &name, 1,
nullptr /* soinfos */, nullptr /* ld_preloads */, 0 /* ld_preload_count */,
rtld_flags, nullptr /* extinfo */, false /* add_as_children */,
false /* search_linked_namespaces */, readers_map, namespaces)) {
// If this lib is directly needed by one of the libs in this namespace,
// then increment the count
soinfo* needed_by = task->get_needed_by();
if (needed_by != nullptr && needed_by->get_primary_namespace() == ns && si->is_linked()) {
si->increment_ref_count();
}
} else {
return false;
}
}
} //连接这个命名空间的库文件
// Step 6: link libraries in this namespace
soinfo_list_t local_group;walk_dependencies_tree( (start_with != nullptr && add_as_children) ? &start_with : soinfos,
(start_with != nullptr && add_as_children) ? 1 : soinfos_count,
[&] (soinfo* si) {
if (ns->is_accessible(si)) {
local_group.push_back(si);
return kWalkContinue;
} else {
return kWalkSkip;
}
});soinfo_list_t global_group = ns->get_global_group();
bool linked = local_group.visit([&](soinfo* si) {
if (!si->is_linked()) {
LD_LOG(kLogDlopen,"so %s is not linked , now try to link ",si->get_soname());
if (!si->link_image(global_group, local_group, extinfo) ||
!get_cfi_shadow()->AfterLoad(si, solist_get_head())) {
return false;
}
}
return true;
});if (linked) {
local_group.for_each([](soinfo* si) {
LD_LOG(kLogDlopen,"travser local_group list %s" , si->get_soname());
if (!si->is_linked()) {
si->set_linked();
}
});
failure_guard.Disable();
}return linked;
// Step 5: link libraries that are not destined to this namespace.
// Do this by recursively calling find_libraries on the namespace where the lib
// was found during Step 1.
for (auto&& task : load_tasks) {
soinfo* si = task->get_soinfo();
if (si->get_primary_namespace() != ns) {
const char* name = task->get_name();
if (find_libraries(si->get_primary_namespace(), task->get_needed_by(), &name, 1,
nullptr /* soinfos */, nullptr /* ld_preloads */, 0 /* ld_preload_count */,
rtld_flags, nullptr /* extinfo */, false /* add_as_children */,
false /* search_linked_namespaces */, readers_map, namespaces)) {
// If this lib is directly needed by one of the libs in this namespace,
// then increment the count
soinfo* needed_by = task->get_needed_by();
if (needed_by != nullptr && needed_by->get_primary_namespace() == ns && si->is_linked()) {
si->increment_ref_count();
}
} else {
return false;
}
}
} //连接这个命名空间的库文件
// Step 6: link libraries in this namespace
soinfo_list_t local_group;walk_dependencies_tree( (start_with != nullptr && add_as_children) ? &start_with : soinfos,
(start_with != nullptr && add_as_children) ? 1 : soinfos_count,
[&] (soinfo* si) {
if (ns->is_accessible(si)) {
local_group.push_back(si);
return kWalkContinue;
} else {
return kWalkSkip;
}
});soinfo_list_t global_group = ns->get_global_group();
bool linked = local_group.visit([&](soinfo* si) {
if (!si->is_linked()) {
LD_LOG(kLogDlopen,"so %s is not linked , now try to link ",si->get_soname());
if (!si->link_image(global_group, local_group, extinfo) ||
!get_cfi_shadow()->AfterLoad(si, solist_get_head())) {
return false;
}
}
return true;
});if (linked) {
local_group.for_each([](soinfo* si) {
LD_LOG(kLogDlopen,"travser local_group list %s" , si->get_soname());
if (!si->is_linked()) {
si->set_linked();
}
});
failure_guard.Disable();
}return linked;
bool soinfo::link_image(const soinfo_list_t& global_group, const soinfo_list_t& local_group,
const android_dlextinfo* extinfo) {
// 这里仅保留了大家比较熟悉的类型。常规都是rel。
#if defined(USE_RELA) if (rela_ != nullptr) {
DEBUG("[ relocating %s ]", get_realpath());
if (!relocate(version_tracker,
plain_reloc_iterator(rela_, rela_count_), global_group, local_group)) {
return false;
}
}
if (plt_rela_ != nullptr) {
DEBUG("[ relocating %s plt ]", get_realpath());
if (!relocate(version_tracker,
plain_reloc_iterator(plt_rela_, plt_rela_count_), global_group, local_group)) {
return false;
}
}
#else //把重心放在这里
if ( != nullptr) {
DEBUG("[ relocating %s ]", get_realpath());
if (!relocate(version_tracker,
plain_reloc_iterator(rel_, rel_count_), global_group, local_group)) {
return false;
}
}
if (plt_rel_ != nullptr) {
LD_LOG(kLogDlopen,"[ relocating %s plt ]", get_realpath());
if (!relocate(version_tracker,
plain_reloc_iterator(plt_rel_, plt_rel_count_), global_group, local_group)) {
return false;
}
}
#endifbool soinfo::link_image(const soinfo_list_t& global_group, const soinfo_list_t& local_group,
const android_dlextinfo* extinfo) {
// 这里仅保留了大家比较熟悉的类型。常规都是rel。
#if defined(USE_RELA) if (rela_ != nullptr) {
DEBUG("[ relocating %s ]", get_realpath());
if (!relocate(version_tracker,
plain_reloc_iterator(rela_, rela_count_), global_group, local_group)) {
return false;
}
}
if (plt_rela_ != nullptr) {
DEBUG("[ relocating %s plt ]", get_realpath());
if (!relocate(version_tracker,
plain_reloc_iterator(plt_rela_, plt_rela_count_), global_group, local_group)) {
return false;
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最后于 2020-12-31 16:51
被pareto编辑
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