Re: Moving forward with sh2/nommu

[Rich Felker <dalias@libc.org>]

On Thu, Jun 11, 2015 at 11:12:52AM -0400, Rich Felker wrote:
> > >> 3. We need sh/vfork.s since the default vfork.c just uses fork, which
> > >>    won't work. I have a version locally but it doesn't make sense to
> > >>    commit without runtime trap number selection.
> > > 
> > > Done and updated to use runtime selection in the (ugly) patch.
> > 
> > If they ask for vfork() they should get vfork()...?
> 
> Yes. The "runtime selection" is about the syscall trap number, not
> whether or not to use vfork. I committed vfork to upstream musl now,
> but with a SH3/4 trap number to be consistent with the code that's
> upstream now. Later I'll either convert them all to trap 31 (0x1f) if
> that ends up being acceptable, or merge the runtime-selection code,
> but I think it makes sense to make the change across all files at
> once, whichever way it's done.

Ah, maybe I misunderstood. If you were asking abaout the original
remark that the default vfork.c uses fork, the reason is simply that
you can't write vfork() in C. The return from vfork() in the child
will clobber vfork's stack frame, which may contain the return address
or saved registers, and then when the parent resumes, very bad things
will happen. vfork() has to be implemented in asm to ensure that any
state it needs to be able to return in the parent is kept in registers
rather than memory. Thus, each arch needs an arch-specific version,
and we just hadn't gotten around to adding the sh version yet.

Rich