uclibc-ng/docs/porting.txt

Some notes to help future porters.  Replace 'ARCH' with whatever arch
you are hacking on.

====================
=== Config Files ===
====================
- create extra/Configs/Config.ARCH
   See the other arch files for some good examples.  powerpc/sparc/alpha
   should be pretty simple templates.
- add ARCH to the 'Target Architecture' list in extra/Configs/Config.in
- Initially you will want to disable shared libraries, since making
   the shared library loader work requires you first have basic architecture
   support working.  Thus you should add ARCH_HAS_NO_SHARED and
   ARCH_HAS_NO_LDSO to Config.ARCH's TARGET_ARCH
- When static pie support is added this TARGET_arch can be appended to the 
   list in extra/Configs/Config.in

====================
=== libc sysdeps ===
====================
(note: if glibc has already been ported to your arch, you can usually just
       copy a lot of files from them rather than coding from scratch)
- create libc/sysdeps/linux/ARCH
- copy Makefile and Makefile.arch from libc/sysdeps/linux/i386/
- set CSRC and SSRC to nothing in Makefile.arch for now

- create crt1.S which defines the _start function ... you will probably want
  to clear the frame pointer to make gdb happy, and then you will want to call
  the funcion __uClibc_main() which takes these parameters:
   __uClibc_main(main(), argc, argv, _init(), _fini())
  Initially if you wish to make things easier on yourself, you can disable the
  UCLIBC_CTOR_DTOR option and just set the init/fini arguments to NULL.
  glibc generally stores this function in libc/sysdeps/ARCH/elf/start.S

- create these additional files in ARCH/bits/

  (template versions can be found in common/bits/ for you to tweak)
  endian.h  fcntl.h  setjmp.h  stackinfo.h  uClibc_arch_features.h  wordsize.h

  kernel_types.h should be created based upon linux asm-ARCH/posix_types.h

  copy linux asm-ARCH/stat.h to bits/kernel_stat.h

  create syscalls.h based upon linux's unistd.h / glibc's sysdeps.h ... really
  you just want to define the _syscall[0-6] macros.  It is important that
  these syscalls should be PIC safe (or you should provide a PIC and non-PIC
  version) if you wish to properly support shared libraries.

- at this point, you should have enough to generate a working HELLO WORLD
  static binary

- if you want UCLIBC_CTOR_DTOR support, you will need to create crti.S and
  crtn.S files which define function prologues/epilogues.

- for a more stable static port, you will need to create these files (and
  update the Makefile.arch values accordingly)
    __longjmp  bsd-_setjmp  bsd-setjmp  brk  clone  setjmp  syscall  vfork
  usually these are written in assembler, but you may be able to cheat and
  write them in C ... see other ports for more information

- Once static and pie executables are stable, static-pie support can be 
  added by modifying crt1.S to calculate the address that the kernel loaded 
  the main elf. Once the elf load address is found, call reloc_static_pie to
  perform all the dynamic relocations normally handled by ldso. This new
  code should be placed at the begining of _start and surrounded by defines
  so that it is only compiled into rcrt1.o and not the static or shared 
  versions. This is usually done by using the special L_rcrt1 preprocessor 
  define. i386 and x86_64 have good reference implementations.

====================
=== ldso sysdeps ===
====================
- elf.h - presumably you've already taught binutils all about the random ELF
  relocations your arch needs, so now you need to make sure the defines exist
  for uClibc.  make sure the EM_### define exists and all of the R_###_###
  reloc defines.

- enable ldso/shared options in your extra/Configs/Config.ARCH file
- you will need to create the following files in ldso/ldso/ARCH/
  dl-startup.h  dl-syscalls.h  dl-sysdep.h  elfinterp.c  resolve.S

- dl-startup.h:
  - define the _start function which should call _dl_start which takes just one
    parameter ... a pointer to argc (usually on the stack)
    glibc stores this function in sysdeps/ARCH/dl-machine.h as RTLD_START
  - define the GET_ARGV() macro which calculates the value of argv based upon
    the parameter passed to _dl_start (usually it's simply just ARGS+1)
  - define PERFORM_BOOTSTRAP_RELOC() macro which will handle just the relocs
    that the ldso itself will generate

- dl-syscalls.h:
  if you wrote your bits/syscalls.h file correctly in the libc step above, you
  can simply copy this file from another arch and be done ... otherwise you
  will have to define the syscall[0-6] macros again, but this time setting
  _dl_errno instead of just errno

- dl-sysdep.h:
  misc cruft goes in here ... you want to:
  - either define or undefine ELF_USES_RELOCA
  - define the INIT_GOT macro
  - define MAGIC1 to the EM_### value your ELF arch uses
  - define ELF_TARGET to a string name for your arch
  - define the do_rem() macro
  - define misc ALIGN macro's
  - define elf_machine_type_class() macro
  - define the inline functions elf_machine_dynamic, elf_machine_load_address,
    and elf_machine_relative
  glibc stores a bunch of these values in sysdeps/ARCH/dl-machine.h

- elfinterp.c:
  define all the relocation functions ... it's best if you just copy from
  another arch which uses the same type of relocations (REL or RELA) and
  start from there.

- resolve.S:
  front end of lazy relocation ... define the _dl_linux_resolve symbol which
  is called by a PLT entry which has yet to be setup ... you will want to:
  - set up arguments for _dl_linux_resolver()
  - call _dl_linux_resolver()
  - clean up after call
  - jump to function address now stored in PLT
  glibc stores this function in sysdeps/ARCH/dl-trampoline.S

- utils/ldd.c - if you want support for ldso cache files (spoiler: you do),
  then you'll need to teach ldd a little.  generally, the fallback code
  should be smart and "just work", but you should be explicit.  just pop
  it open and add an appropriate ifdef for your arch and set MATCH_MACHINE()
  and ELFCLASSM.  there are plenty examples and you're (hopefully) smart.

====================
===  Misc Cruft  ===
====================
- MAINTAINERS - presumably you're going to submit this code back to mainline
  and since you're the only one who cares about this arch (right now), you
  should add yourself to the toplevel MAINTAINERS file.  do it.