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Dynamic Linker

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WALDEMAR KOZACZUK edited this page Oct 21, 2020 · 31 revisions

Introduction

OSv is made of many components but the dynamic linker is probably the most essential one as it interacts with and ties all other components together and is responsible for bootstrapping an application. In essence, it involves locating an ELF file on the filesystem, loading it into memory using mmap(), processing its headers and segments to relocate symbols, configuring TLS, executing its init functions, loading any dependant ELF objects, and finally starting the app. Please note that unlike Linux, the dynamic linker is an integral part of the OSv kernel. Most of the dynamic linker code is located in core/elf.cc, arch/<arch>/arch-elf.cc and core/app.cc.

Concepts

  • elf::program - represent the dynamic linker's view of the running program. Typically there is only one instance of it created by elf::create_main_program() called from loader.cc. The program constructor (??) sets the program base in memory, initializes _core - an instance of elf::memory_image to represent the ELF of OSv kernel, and finally sets up a default set of "supplied" modules like libc.so.6, libpthread.so.0, etc. The default main program is stored in the s_program global variable, so effectively the elf::program is a singleton, but it is possible to create multiple instances for new ELF namespaces (see below). The key methods of the program class are:

    • std::shared_ptr<object> program::get_library(std::string name, std::vector<std::string> extra_path, bool delay_init) - the main method called from osv::application constructor and libc/dlfcn.cc:dlopen(); it instantiates an elf::object to represent the loaded ELF by delegating to program::load_object(..) (see below), builds static TLS template by calling init_static_tls() on the instantiated object and finally invokes the 'INIT' functions of ELF objects.
    • std::shared_ptr<elf::object> program::load_object(std::string name, ..) - locates an ELF file on the filesystem, creates an instance of elf::object to represent it and finally invokes number of key object methods on it (load_segments(), process_headers(), load_needed(), relocate() and fix_permissions()) to mmap segment of the ELF into memory and process its relocations among other things.
    • symbol_module program::lookup(const char* name) - iterates over all objects in elf::program::modules_list and calls object::lookup_symbol(name) for each to finally return symbol_module for the first found occurrence

  • elf::object - represents an ELF object; implements logic to load an ELF file into memory and process its headers and relocations. The main method is `

    • void object::relocate() - the top method that calls relocate_rela() and relocate_pltgot() to process the ELF object relocations
    • void object::relocate_rela() - iterates over the table of relocation entries (dynamic_ptr<Elf64_Rela>(DT_RELA)) and calls object::arch_relocate_rela() for each Elf64_Rela* and passes its relocation type (p->r_info & 0xffffffff), index in the symbol table of the object being relocated (p->r_info >> 32), address of the relocation (_base + p->r_offset: where to write the relocation value to) and addend (p->r_addend)
    • bool object::arch_relocate_rela(u32 type, u32 sym, void *addr, Elf64_Sxword addend) - based on the relocation type (type argument) determines the relocation value (symbol relocated address or object module index or st_value (?) for TLS) and writes it to the relocation address (addr argument):
      • R_X86_64_COPY - calls object::symbol_other(sym) to find symbol in other objects
      • R_X86_64_64 - calls object::symbol(sym, true) to find symbol in all objects (see below) and calculates the value as symbol.relocated_addr() + addend
      • R_X86_64_RELATIVE - calculates the value as _base + addend
      • R_X86_64_JUMP_SLOT, R_X86_64_GLOB_DAT - calls object::symbol(sym, true) to find symbol in all objects (see below) and calculates the value as symbol.relocated_addr()
      • R_X86_64_DTPMOD64 - calls object::symbol(sym, true) to find symbol in all objects (see below) and calculates value as the module index of the object where symbol was found in; for STN_UNDEF uses index of this object
      • R_X86_64_DTPOFF64 - (TLS) ???
      • R_X86_64_TPOFF64 - (TLS)???
    • void object::relocate_pltgot() - called from object:relocate() to process the PLT (Procedure Linkage Table) relocations mostly for functions; it iterates over entries in DT_JMPREL and either calls object::arch_relocate_jump_slot() if bind_now or sets the jump slots to resolve lazily later (PLT_GOT)
    • bool object::arch_relocate_jump_slot(u32 sym, void *addr, Elf64_Sxword addend, bool ignore_missing) - calls object::symbol(sym, true) to find symbol in all objects (see below) and writes symbol.relocated_addr() to the relocation jump slot address (addr argument)
    • void* object::resolve_pltgot(unsigned index) - finds relocation info under dynamic_ptr<Elf64_Rela>(DT_JMPREL) and symbol index and finds symbol by calling object::symbol() and calls object::arch_relocate_jump_slot() to write the symbol`s relocated address
    • symbol_module object::symbol(unsigned idx, bool ignore_missing) - entry point to symbol lookup; accepts symbol index, finds its name in the object symbols table (dynamic_ptr<Elf64_Sym>(DT_SYMTAB)) and searches for a symbol by name in all objects programs knows about by calling program::lookup(name); if symbol not found it aborts if ignore_missing is false otherwise just warns; returns symbol_module that is a tuple of the object the symbol resides and the symbol definition (Elf64_Sym *); called by following methods during relocation phase:
      • object::arch_relocate_rela()
      • object::arch_relocate_jump_slot()
      • object::resolve_pltgot()
    • Elf64_Sym* object::lookup_symbol(const char* name) - looks up symbol by name by delegating to either lookup_symbol_old or lookup_symbol_gnu; bails out if object not visible (during construction)
    • Elf64_Sym* object::lookup_symbol_old(const char* name) - ???
    • Elf64_Sym* object::lookup_symbol_gnu(const char* name) - uses GNU hashmap

  • elf::file

  • elf::memory_image

  • elf::symbol_module

  • osv::app_registry

  • osv::application - represents a running program and its _program member points to the program the application was created for.

Flow

The program::get_library() is the critical point where the dynamic linker gets involved in instantiating new application.

The main program (kernel?) gets instantiated by elf::create_main_program() called from loader.cc

application::new_program() instantiates new program for new ELF namespace with new base address.

TLS (Thread Local Storage)

Thread local storage (TLS) is a mechanism that allows applications and shared libraries to use variables stored in memory area specific to a given thread. These include variables marked with __thread and C++ thread_local modifiers. For TLS variables to work correctly, OSv dynamic linker needs to recognize TLS segments in an ELF file, construct static TLS blocks in memory, process relevant relocations and provide certain functions like __tls_get_addr among other things.

Before we delve into what OSv dynamic linker does to support TLS, it is important to understand two different formats of static TLS block layout - so-called Variant I and Variant II - and 4 different models of accessing TLS variables: local-exec, initial-exec, general-dynamic and local-dynamic.

Static TLS block is an area of memory allocated for each thread independently, intended to store thread-local variables and built from a template derived when loading the main application and its dependant libraries. The template, in essence, specifies the total size of the TLS block, the offsets in it for each ELF object including OSv kernel, and any initial values for the TLS variables in those objects. The static TLS does not change once the thread is created and running; that is why it is called "static" after all. In Variant I (used in AArch64 port) the data is laid out from left-to-right (local-exec for PIEs, kernel followed with other objects). In Variant II (used in X86_64 port) the data is laid out from right-to-left which is exactly opposite to Variant I.

  • Variant I
    // (1) - TLS memory area layout with app as shared library
    // |------|--------------|-----|-----|-----|
    // |<NONE>|KERNEL        |SO_1 |SO_2 |SO_3 |
    // |------|--------------|-----|-----|-----|

    // (2) - TLS memory area layout with PIE or position dependant executable
    // |------|--------------|-----|-----|
    // | EXE  |KERNEL        |SO_2 |SO_3 |
    // |------|--------------|-----|-----|
  • Variant II
    // (1) - TLS memory area layout with app shared library
    // |-----|-----|-----|--------------|------|
    // |SO_3 |SO_2 |SO_1 |KERNEL        |<NONE>|
    // |-----|-----|-----|--------------|------|

    // (2) - TLS memory area layout with PIE or position dependant executable
    //       |-----|-----|---------------------|
    //       |SO_3 |SO_2 |KERNEL        | EXE  |
    //       |-----|-----|--------------|------|

Flow

The role of the dynamic linker with respect to TLS handling is to connect the "dots" which at a high-level can be divided into 4 phases:

  • processing TLS program header to detect the size and other specifics of TLS data,
  • processing TLS-related relocations,
  • building memory blueprint for TLS - so-called template,
  • and finally allocating and initializing TLS blocks for each thread before it is started.

In the 1st phase, as OSv dynamic linker loads an ELF object in core/elf.cc:std::shared_ptr<elf::object> program::load_object(..), it first mmaps all PT_LOAD segments and then processes all headers (see core/elf.cc:void object::process_headers()) to detect any TLS segment and capture its size, alignment, and its address in memory.

In the 2nd phase, the dynamic linker processes all relocations including those in both GOT (Global Offset Table, see core/elf.cc:void object::relocate_rela()) and PLT (Procedure Linkage Table, see core/elf.cc:void object::relocate_pltgot()). Some of those relocations are TLS specific and are processed in a specific way:

  • R_X86_64_DTPMOD64: to identify the module ID (unique id assigned to each ELF object) where the given TLS variable referenced by this relocation is located; please note that the R_X86_64_DTPMOD64 relocation may be in one ELF object and its definition and location may be in another one or the same; in addition, if index == STN_UNDEF, the relocation applies to hidden (static) TLS variables in given module and is used to determine its module index passed later to __tls_get_addr() - offset in TLS block is known in advance,
  • R_X86_64_DTPOFF64: to calculate the offset in the TLS block of the object where the variable is going to "live"; the values determined for both R_X86_64_DTPMOD64 and R_X86_64_DTPOFF64 are stored in the relocation placeholders in the GOT and then referenced as input to the __tls_get_addr() function provided by OSv dynamic linker; please note that even though both relocations are intended for the dynamic model based on __tls_get_addr(), the variables can live in either static TLS or lazily allocated block if the object was dlopen()-ed,
  • R_X86_64_TPOFF64/R_AARCH64_TLS_TPREL64: to calculate offset of the TLS variable in static TLS - this means that variable will "live" in the statically allocated memory area; in addition, each of these relocations triggers a call to void object::alloc_static_tls() to determine the offset of the TLS block of the object owning the variable and eventually fed in order to properly build application TLS template,
  • R_AARCH64_TLSDESC: to place address of static TLS "resolver" function and its only argument - offset of the variable in static TLS.

In the 3rd phase, after std::shared_ptr<elf::object> program::load_object(..) completes loading ELF in memory, OSv dynamic linker calls void object::init_static_tls() (see std::shared_ptr<object> program::get_library(..)) to build the TLS template for the main app object based on its own TLS segment and any of its dependencies. More specifically it calculates total TLS template size (stored in object::_initial_tls_size member variable) and iterates over all its dependant objects to copy their TLS data to object::_initial_tls buffer by delegating to the void object::prepare_initial_tls(..) or void object::prepare_local_tls(..) methods that are arch (AArch64 or X86_64) specific. At the same time, it also stores the offsets of TLS data for each object in object::_initial_tls_offsets vector. So at the end the init_static_tls() sets 3 key variables - object::_initial_tls, object::_initial_tls_size and object::_initial_tls_offsets.

Finally, for each thread created, its constructor (see core/sched.cc::thread::thread()) allocates so called TCB (Thread Control Block) which includes static TLS block by delegating to arch-specific setup_tcb() (see arch/*/arch-switch.hh for details). The static TLS block (thread::_tcb->tls_base) is in essence an instance of the TLS template built in previous phases. Finally, the index (_tls) vector is populated with offsets for each module based on object::_initial_tls_offsets.

TLS descriptors

__tls_get_addr()

ELF Namespaces

References

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