Re: RFC: ENOSYS vs SIGSYS for FreeBSD ABIs

On Mon, Apr 20, 2026 at 08:37:05AM +0000, Poul-Henning Kamp wrote:
> --------
> Brooks Davis writes:
> 
> > [???] but it would let you survive a kernel roll-back
> > with new syscalls in csu code.
> 
> I would expect precisely that to be the situation were we would
> be careful to handle that scenario :-)

You can if you use an out of band mechanism to detect the presence
of the syscall by checking __getosreldate, but that works base in
unmodified FreeBSD.  Downstream you may need an alternative mechanism
like checking a feature sysctl.

My frustration with SIGSYS is that it's an annoyance for developers and
requires special handling to avoid too many user complaints.  We then
forget to clean up the special handling once it's obsolete (which takes
years due to our de facto src upgrade support model).

-- Brooks