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diff --git a/04_dynld_nostd/README.md b/04_dynld_nostd/README.md new file mode 100644 index 0000000..de021dd --- /dev/null +++ b/04_dynld_nostd/README.md @@ -0,0 +1,171 @@ +# `dynld` no-std + +### Goals +- Create a `no-std` shared library `libgreet.so` which exposes some functions + and variables. +- Create a `no-std` user executable which dynamically links against + `libgreet.so` and uses exposed functions and variables. +- Create a dynamic linker `dynld.so` which can prepare the execution + environment, by mapping the shared library dependency and resolving all + relocations. + +--- + +## Creating the shared library `libgreet.so` + +To challenge the dynamic linker at least a little bit, the shared library will +contain different functionality to generate different kinds of relocations. + +The first part consists of a global variable `gCalled` and a global function +`get_greet`. Since the global variable is referenced in the function and the +variable does not have `internal` linkage, this will generate a relocation in +the shared library object. +```cpp +int gCalled = 0; + +const char* get_greet() { + // Reference global variable -> generates RELA relocation (R_X86_64_GLOB_DAT). + ++gCalled; + return "Hello from libgreet.so!"; +} +``` + +Additionally the shared library contains a `constructor` and `destructor` +function which will be added to the `.init_array` and `.fini_array` sections +accordingly. The dynamic linkers task is to run these function during +initialization and shutdown of the shared library. +```cpp +// Definition of `static` function which is referenced from the `DT_INIT_ARRAY` +// dynamic section entry -> generates R_X86_64_RELATIVE relocation. +__attribute__((constructor)) static void libinit() { + pfmt("libgreet.so: libinit\n"); +} + +// Definition of `non static` function which is referenced from the +// `DT_FINI_ARRAY` dynamic section entry -> generates R_X86_64_64 relocation. +__attribute__((destructor)) void libfini() { + pfmt("libgreet.so: libfini\n"); +} +``` +> `constructor` / `destructor` are function attributes and their definition is +> described in [gcc common function attributes][gcc-fn-attributes]. + +The generated relocations can be seen in the `readelf` output of the shared +library ELF file. +```bash +> readelf -r libgreet.so + +Relocation section '.rela.dyn' at offset 0x3f0 contains 3 entries: + Offset Info Type Sym. Value Sym. Name + Addend +000000003e88 000000000008 R_X86_64_RELATIVE 1064 +000000003e90 000300000001 R_X86_64_64 000000000000107c libfini + 0 +000000003ff8 000400000006 R_X86_64_GLOB_DAT 0000000000004020 gCalled + 0 + +Relocation section '.rela.plt' at offset 0x438 contains 1 entry: + Offset Info Type Sym. Value Sym. Name + Addend +000000004018 000100000007 R_X86_64_JUMP_SLO 0000000000000000 pfmt + 0 +``` + +Dumping the `.dynamic` section of the shared library, it can be see that there +are `INIT_*` / `FINI_*` entries. These are generated as result of the +`constructor` / `destructor` functions. +The dynamic linker can make use of those entries at runtime to locate the +`.init_array` / `.fini_array` sections and run the functions accordingly. +```bash +> readelf -d libgreet.so + +Dynamic section at offset 0x2e98 contains 18 entries: + Tag Type Name/Value + 0x0000000000000019 (INIT_ARRAY) 0x3e88 + 0x000000000000001b (INIT_ARRAYSZ) 8 (bytes) + 0x000000000000001a (FINI_ARRAY) 0x3e90 + 0x000000000000001c (FINI_ARRAYSZ) 8 (bytes) + -- snip -- + 0x0000000000000002 (PLTRELSZ) 24 (bytes) + 0x0000000000000014 (PLTREL) RELA + 0x0000000000000017 (JMPREL) 0x438 + 0x0000000000000007 (RELA) 0x3f0 + 0x0000000000000008 (RELASZ) 72 (bytes) + 0x0000000000000009 (RELAENT) 24 (bytes) + 0x0000000000000000 (NULL) 0x0 +``` + +The full source code of the shared library is available in +[libgreet.c](./libgreet.c). + +## Creating the user executable + +The user program looks as follows, it will just make use of the `libgreet.so` +global variable and functions. +```cpp +// API of `libgreet.so`. +extern const char* get_greet(); +extern const char* get_greet2(); +extern int gCalled; + +void _start() { + pfmt("Running _start() @ %s\n", __FILE__); + + // Call function from libgreet.so -> generates PLT relocations (R_X86_64_JUMP_SLOT). + pfmt("get_greet() -> %s\n", get_greet()); + pfmt("get_greet2() -> %s\n", get_greet2()); + + // Reference global variable from libgreet.so -> generates RELA relocation (R_X86_64_COPY). + pfmt("libgreet.so called %d times\n", gCalled); +} +``` + +Inspecting the relocations again with `readelf` it can be seen that they +contain entries for the referenced variable and functions of the shared +library. +```bash +> readelf -r main + +Relocation section '.rela.dyn' at offset 0x478 contains 1 entry: + Offset Info Type Sym. Value Sym. Name + Addend +000000404028 000300000005 R_X86_64_COPY 0000000000404028 gCalled + 0 + +Relocation section '.rela.plt' at offset 0x490 contains 2 entries: + Offset Info Type Sym. Value Sym. Name + Addend +000000404018 000200000007 R_X86_64_JUMP_SLO 0000000000000000 get_greet + 0 +000000404020 000400000007 R_X86_64_JUMP_SLO 0000000000000000 get_greet2 + 0 +``` + +The last important piece is to dynamically link the user program against +`libgreet.so` which will generate a `DT_NEEDED` entry in the `.dynamic` +section. +```bash +> readelf -r -d main + +Dynamic section at offset 0x2ec0 contains 15 entries: + Tag Type Name/Value + 0x0000000000000001 (NEEDED) Shared library: [libgreet.so] + -- snip --- + 0x0000000000000000 (NULL) 0x0 +``` + +The full source code of the user program is available in +[main.c](./main.c). + +## Creating the dynamic linker `dynld.so` + +The dynamic linker developed here is kept simple and mainly used to explore the +mechanics of dynamic linking. That said, it means that it is tailored +specifically for the previously developed executable and won't support things as +- Multiple shared library dependencies. +- Dynamic resolve during runtime (lazy). +- Threads locals storage (TLS). + +However with a little effort this dynamic linker could easily be extend. + +Before diving into details, let's first define the high-level tasks of +`dynld.so`: +1. Parse out necessary information from the initial process context ([`SystemV` + context](../02_process_init/README.md#stack-state-on-process-entry)). +1. Map `libgreet.so` shared library dependency. +1. Resolve all relocations of `libgreet.so` and `main`. +1. Run `INIT` functions of `libgreet.so` and `main`. +1. Transfer control to `main` user program. +1. Run `FINI` functions of `libgreet.so` and `main`. + +[gcc-fn-attributes]: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes |