<center><font color=#006666 size=3 face="黑体">**读ELF文件头**</font></center>
``` C++
// Loads the program header table from an ELF file into a read-only private
// anonymous mmap-ed block.
bool ElfReader::ReadProgramHeader() {
phdr_num_ = header_.e_phnum;
// Like the kernel, we only accept program header tables that
// are smaller than 64KiB.
if (phdr_num_ < 1 || phdr_num_ > 65536/sizeof(Elf32_Phdr)) {
DL_ERR("\"%s\" has invalid e_phnum: %d", name_, phdr_num_);
return false;
}
Elf32_Addr page_min = PAGE_START(header_.e_phoff); //页的起始地址
Elf32_Addr page_max = PAGE_END(header_.e_phoff + (phdr_num_ * sizeof(Elf32_Phdr))); //页的结束地址
Elf32_Addr page_offset = PAGE_OFFSET(header_.e_phoff); //程序头部在页中的偏移
phdr_size_ = page_max - page_min;
void* mmap_result = mmap(NULL, phdr_size_, PROT_READ, MAP_PRIVATE, fd_, page_min); //将程序头映射到内存
if (mmap_result == MAP_FAILED) {
DL_ERR("\"%s\" phdr mmap failed: %s", name_, strerror(errno));
return false;
}
phdr_mmap_ = mmap_result;
phdr_table_ = reinterpret_cast<Elf32_Phdr*>(reinterpret_cast<char*>(mmap_result) + page_offset); //程序头表在内存中的地址
return true;
}
// Reserve a virtual address range big enough to hold all loadable
// segments of a program header table. This is done by creating a
// private anonymous mmap() with PROT_NONE.
bool ElfReader::ReserveAddressSpace() {
Elf32_Addr min_vaddr;
load_size_ = phdr_table_get_load_size(phdr_table_, phdr_num_, &min_vaddr); //根据页对齐来计算Load段所占用的大小
if (load_size_ == 0) {
DL_ERR("\"%s\" has no loadable segments", name_);
return false;
}
uint8_t* addr = reinterpret_cast<uint8_t*>(min_vaddr);
int mmap_flags = MAP_PRIVATE | MAP_ANONYMOUS; //匿名私有
void* start = mmap(addr, load_size_, PROT_NONE, mmap_flags, -1, 0); //调用mmap为动态库分配一块内存空间
if (start == MAP_FAILED) {
DL_ERR("couldn't reserve %d bytes of address space for \"%s\"", load_size_, name_);
return false;
}
load_start_ = start;
load_bias_ = reinterpret_cast<uint8_t*>(start) - addr; //真实的加载地址与计算出来的(读ELF程序头中的p_vaddr)加载地址之差
return true;
}
// Map all loadable segments in process' address space.
// This assumes you already called phdr_table_reserve_memory to
// reserve the address space range for the library.
// TODO: assert assumption.
bool ElfReader::LoadSegments() {
for (size_t i = 0; i < phdr_num_; ++i) {
const Elf32_Phdr* phdr = &phdr_table_[i];
if (phdr->p_type != PT_LOAD) {
continue;
}
// Segment addresses in memory.
Elf32_Addr seg_start = phdr->p_vaddr + load_bias_;
Elf32_Addr seg_end = seg_start + phdr->p_memsz;
Elf32_Addr seg_page_start = PAGE_START(seg_start);
Elf32_Addr seg_page_end = PAGE_END(seg_end);
Elf32_Addr seg_file_end = seg_start + phdr->p_filesz;
// File offsets.
Elf32_Addr file_start = phdr->p_offset;
Elf32_Addr file_end = file_start + phdr->p_filesz;
Elf32_Addr file_page_start = PAGE_START(file_start);
Elf32_Addr file_length = file_end - file_page_start;
if (file_length != 0) {
void* seg_addr = mmap((void*)seg_page_start, //将Load Segment映射到内存,大小为在ELF文件中所占用的长度
file_length,
PFLAGS_TO_PROT(phdr->p_flags),
MAP_FIXED|MAP_PRIVATE,
fd_,
file_page_start);
if (seg_addr == MAP_FAILED) {
DL_ERR("couldn't map \"%s\" segment %d: %s", name_, i, strerror(errno));
return false;
}
}
// if the segment is writable, and does not end on a page boundary,
// zero-fill it until the page limit.
if ((phdr->p_flags & PF_W) != 0 && PAGE_OFFSET(seg_file_end) > 0) {
memset((void*)seg_file_end, 0, PAGE_SIZE - PAGE_OFFSET(seg_file_end)); //如果这块Segment是可写的,且在内存中的结束地址不在页的边界上,则将后面的数据都填充0
}
seg_file_end = PAGE_END(seg_file_end);
// seg_file_end is now the first page address after the file
// content. If seg_end is larger, we need to zero anything
// between them. This is done by using a private anonymous
// map for all extra pages.
if (seg_page_end > seg_file_end) {
void* zeromap = mmap((void*)seg_file_end, //如果seg_end大于它在文件中的长度,则继续为多出的那部分申请内存空间,并填充0。这里应该是主要针对bss段
seg_page_end - seg_file_end,
PFLAGS_TO_PROT(phdr->p_flags),
MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE,
-1,
0);
if (zeromap == MAP_FAILED) {
DL_ERR("couldn't zero fill \"%s\" gap: %s", name_, strerror(errno));
return false;
}
}
}
return true;
}
static soinfo* load_library(const char* name) {
// Open the file.
int fd = open_library(name);
if (fd == -1) {
DL_ERR("library \"%s\" not found", name);
return NULL;
}
// Read the ELF header and load the segments.
ElfReader elf_reader(name, fd);
if (!elf_reader.Load()) {
return NULL;
}
const char* bname = strrchr(name, '/');
soinfo* si = soinfo_alloc(bname ? bname + 1 : name);
if (si == NULL) {
return NULL;
}
si->base = elf_reader.load_start();
si->size = elf_reader.load_size();
si->load_bias = elf_reader.load_bias();
si->flags = 0;
si->entry = 0;
si->dynamic = NULL;
si->phnum = elf_reader.phdr_count();
si->phdr = elf_reader.loaded_phdr();
return si;
}
static soinfo* soinfo_alloc(const char* name) {
if (strlen(name) >= SOINFO_NAME_LEN) {
DL_ERR("library name \"%s\" too long", name);
return NULL;
}
if (!ensure_free_list_non_empty()) {
DL_ERR("out of memory when loading \"%s\"", name);
return NULL;
}
// Take the head element off the free list.
soinfo* si = gSoInfoFreeList;
gSoInfoFreeList = gSoInfoFreeList->next;
// Initialize the new element.
memset(si, 0, sizeof(soinfo));
strlcpy(si->name, name, sizeof(si->name));
sonext->next = si;
sonext = si;
TRACE("name %s: allocated soinfo @ %p", name, si);
return si;
}
void soinfo::CallConstructors() {
if (constructors_called) {
return;
}
// We set constructors_called before actually calling the constructors, otherwise it doesn't
// protect against recursive constructor calls. One simple example of constructor recursion
// is the libc debug malloc, which is implemented in libc_malloc_debug_leak.so:
// 1. The program depends on libc, so libc's constructor is called here.
// 2. The libc constructor calls dlopen() to load libc_malloc_debug_leak.so.
// 3. dlopen() calls the constructors on the newly created
// soinfo for libc_malloc_debug_leak.so.
// 4. The debug .so depends on libc, so CallConstructors is
// called again with the libc soinfo. If it doesn't trigger the early-
// out above, the libc constructor will be called again (recursively!).
constructors_called = true;
if ((flags & FLAG_EXE) == 0 && preinit_array != NULL) {
// The GNU dynamic linker silently ignores these, but we warn the developer.
PRINT("\"%s\": ignoring %d-entry DT_PREINIT_ARRAY in shared library!",
name, preinit_array_count);
}
if (dynamic != NULL) {
for (Elf32_Dyn* d = dynamic; d->d_tag != DT_NULL; ++d) {
if (d->d_tag == DT_NEEDED) {
const char* library_name = strtab + d->d_un.d_val;
TRACE("\"%s\": calling constructors in DT_NEEDED \"%s\"", name, library_name);
find_loaded_library(library_name)->CallConstructors();
}
}
}
TRACE("\"%s\": calling constructors", name);
// DT_INIT should be called before DT_INIT_ARRAY if both are present.
CallFunction("DT_INIT", init_func);
CallArray("DT_INIT_ARRAY", init_array, init_array_count, false);
}
/**
* Loads and links the library with the specified name. The mapping of the
* specified library name to the full path for loading the library is
* implementation-dependent.
*
* @param libName
* the name of the library to load.
* @throws UnsatisfiedLinkError
* if the library can not be loaded.
*/
public void loadLibrary(String libName) {
loadLibrary(libName, VMStack.getCallingClassLoader());
}
/*
* Searches for a library, then loads and links it without security checks.
*/
void loadLibrary(String libraryName, ClassLoader loader) {
if (loader != null) {
String filename = loader.findLibrary(libraryName);
if (filename == null) {
throw new UnsatisfiedLinkError("Couldn't load " + libraryName +
" from loader " + loader +
": findLibrary returned null");
}
String error = doLoad(filename, loader);
if (error != null) {
throw new UnsatisfiedLinkError(error);
}
return;
}
String filename = System.mapLibraryName(libraryName);
List<String> candidates = new ArrayList<String>();
String lastError = null;
for (String directory : mLibPaths) {
String candidate = directory + filename;
candidates.add(candidate);
if (IoUtils.canOpenReadOnly(candidate)) {
String error = doLoad(candidate, loader);
if (error == null) {
return; // We successfully loaded the library. Job done.
}
lastError = error;
}
}
if (lastError != null) {
throw new UnsatisfiedLinkError(lastError);
}
throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
}
private static final Runtime mRuntime = new Runtime();
/**
* Holds the library paths, used for native library lookup.
*/
private final String[] mLibPaths = initLibPaths();
private static String[] initLibPaths() {
String javaLibraryPath = System.getProperty("java.library.path");
if (javaLibraryPath == null) {
return EmptyArray.STRING;
}
String[] paths = javaLibraryPath.split(":");
// Add a '/' to the end of each directory so we don't have to do it every time.
for (int i = 0; i < paths.length; ++i) {
if (!paths[i].endsWith("/")) {
paths[i] += "/";
}
}
return paths;
}
/**
* Returns the absolute path of the native library with the specified name,
* or {@code null}. If this method returns {@code null} then the virtual
* machine searches the directories specified by the system property
* "java.library.path".
* <p>
* This implementation always returns {@code null}.
* </p>
*
* @param libName
* the name of the library to find.
* @return the absolute path of the library.
*/
protected String findLibrary(String libName) {
return null;
}