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diff --git a/01_dynamic_linking/README.md b/01_dynamic_linking/README.md new file mode 100644 index 0000000..6c0dd5d --- /dev/null +++ b/01_dynamic_linking/README.md @@ -0,0 +1,103 @@ +# Hello dynamic linking + +In `dynamic linking` a program can use code that is not contained in the +program file itself but rather in separate library files, so called shared +objects. + +In comparison a statically linked program contains all the `code` & `data` that +it needs to run from start until completion. The program will be loaded by the +Linux Kernel from the disk into the virtual address space and control is handed +over to the mapped program which then executes. +```text + @vm + | | + @disk |--------| ++--------+ execve(2) | | <- $rip +| prog A | ------------> | prog A | ++--------+ | | + |--------| + | | +``` + +A dynamically linked program on the other hand needs to specify a `dynamic +linker` which is basically a runtime interpreter. The Linux Kernel will additionally load +that interpreter into the virtual address space and give control to the +interpreter rather than the user program. +The interpreter will prepare the execution environment for the user program by +for example loading dependencies and running initialization routines. After the +environment is set up the dynamic linker passes control to the user program. +```text + @vm @vm + | | | | + @disk |--------| |----------| ++--------------+ execve(2) | | | | <- $rip +| prog A | ------------> | prog A | | prog A | ++--------------+ | | load deps | | +| interp ldso | |--------| ------------> |----------| ++--------------+ | | | | +| dep libgreet | |--------| |----------| ++--------------+ | ldso | <- $rip | ldso | + |--------| |----------| + | | + |----------| + | libgreet | + |----------| +``` +> NOTE: Technically the Linux Kernel does not need to load the dynamically +> linked user program itself, but that detail is not important here. + +In the `ELF` binary format the name of the dynamic linker is specified as a +string in the special section `.interp`. +```bash +readelf -W --string-dump .interp main + +String dump of section '.interp': + [ 0] /lib64/ld-linux-x86-64.so.2 +``` + +The `.interp` section is referenced by the `PT_INTERP` segment in the program +headers. This segment is used by the Linux Kernel during the `execve(2)` +syscall in the [`load_elf_binary`][load_elf_binary] function to check if the +program needs a dynamic linker and if so to retrieve its name. +```bash +readelf -W --sections --program-headers main + +Section Headers: + [Nr] Name Type Address Off Size ES Flg Lk Inf Al + [ 0] NULL 0000000000000000 000000 000000 00 0 0 0 + [ 1] .interp PROGBITS 00000000000002a8 0002a8 00001c 00 A 0 0 1 + ... + +Program Headers: + Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align + PHDR 0x000040 0x0000000000000040 0x0000000000000040 0x000268 0x000268 R 0x8 + INTERP 0x0002a8 0x00000000000002a8 0x00000000000002a8 0x00001c 0x00001c R 0x1 + [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2] + ... +``` + +With the use of `gdb` it can be easily verified that the control is first +passed to the dynamic linker and not the user program. This is shown by +stopping at the first instruction of the new process (`starti`) and examining +the backtrace (`bt`). Where `ld-linux-x86-64.so` is the dynamic linker as shown +in the `.interp` section above. +```bash +gdb -q --batch -ex 'starti' -ex 'bt' ./main + +Program stopped. +0x00007ffff7fd2090 in _start () from /lib64/ld-linux-x86-64.so.2 +#0 0x00007ffff7fd2090 in _start () from /lib64/ld-linux-x86-64.so.2 +#1 0x0000000000000001 in ?? () +#2 0x00007fffffffe43e in ?? () +#3 0x0000000000000000 in ?? () +``` +> NOTE: Frames `#1 - #3` don't actually exist, gdb's unwinder just tried to further unwind the stack. + +## Things to remember +- Dynamically linked programs use code contained in separate library files. +- The `dynamic linker` is an interpreter loaded by the Linux Kernel and gets + control before the user program. +- A dynamically linked program specifies the dynamic linker needed in the + `.interp` section. + +[load_elf_binary]: https://elixir.bootlin.com/linux/v5.9.8/source/fs/binfmt_elf.c#L850 |