adding a syscall to libc?

[Konstantin Belousov]

On Sat, Jun 08, 2019 at 02:57:27AM +0000, Rick Macklem wrote:
> Hi,
> 
> I've started working of a copy_file_range() syscall for FreeBSD. I think I have the
> kernel patched and ready for some testing.
> However, I'm confused about what I need to do in src/lib/libc/sys?
> - Some syscalls have little .c files, but other ones do not.
>   When is one of these little .c files needed and, when not needed, what else
>   needs to be done? (I notice that syscall.mk in src/sys/sys automagically, but
>   I can't see what else, if anything, needs to be done?)
Most important is to add the new syscall public symbol to sys/Symbol.map
into the correct version, FBSD_1.6 for CURRENT-13.  Do no bother with
__sys_XXX and __XXX aliases.

'Tiny .c files' are typically used for one of two purposes:
- Convert raw kernel interface into something expected by userspace,
  often this coversion uses more generic and non-standard interface to
  implement more usual function.  Examples are open(2) or waitid(2)
  which are really tiny wrappers around openat(2) and wait6(2) in
  today libc.
- Allow libthr to hook into libc to provide additional services.  Libthr
  often has to modify semantic of raw syscall, and libc contains the
  tables redirecting to implementation, the tables are patched on libthr
  load.  Since tables must fill entries with some address in case libthr
  is not loaded, tiny functions which wrap syscalls are created for
  use in that tables.

I think you do not need anything that complications for start, in which
case adding new syscall consists of the following steps:
- Add the syscall to sys/kern/syscalls.master, and if reasonable,
  to sys/compat/freebsd32/syscalls.master.
- Consider if the syscall makes sense in capsicumized environment,
  and if yes, list the syscall in sys/kern/capabilities.conf.  Typically,
  if syscall provides access to the global files namespace, it must be not
  allowed.  On the other hand, if syscall only operates on already opened
  file descriptors, then it is suitable (but of course there are lot of
  nuances).
- Add syscall prototype to the user-visible portion of header,
  hiding it under the proper visibility check.
- Add syscall symbol to lib/libc/sys/Symbol.ver.
- Implement the syscall.  There are some additional details that might
  require attention:
	- If compat32 syscall going to be implemented, or you know
	  that Linuxolator needs to implement same syscall and would
	  like to take advantage of the code, provide
		int kern_YOURSYSCALL();
	  wrapper and declare it in sys/syscallsubr.h.  Real implementations
	  of host-native and compat32 sys_YOURSYSCALL() should be just
	  decoding of uap members and call into kern_YOURSYSCALL.
	- Consider the need to add auditing for new syscall.
- Add man page for the syscall, at lib/libc/sys/YOURSYSCALL.2, and connect
  it to the build in lib/libc/sys/Makefile.inc.
- When creating review for the change, do not include diff for generated
  files after make sysent.  Similarly, when doing the commit, first commit
  everything non-generated, then do make -C sys/kern sysent (and
  make sysent -C sys/compat/freebsd32 sysent if appropriate) and commit
  the generated files in follow-up.

> 
> Thanks in advance for your help, rick
> ps: I am using the Linux man pages for the syscall ABI. At some point, I'll put this
>       in phabricator and post here for comments/review.
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