Re: [musl] Re: [PATCH V9 13/24] LoongArch: Add system call support

[Rich Felker <dalias@libc.org>]

On Thu, May 12, 2022 at 09:21:13AM +0200, Arnd Bergmann wrote:
> On Wed, May 11, 2022 at 11:12 PM Rich Felker <dalias@libc.org> wrote:
> > On Wed, May 11, 2022 at 09:11:56AM +0200, Arnd Bergmann wrote:
> > > On Mon, May 9, 2022 at 12:00 PM Christian Brauner <brauner@kernel.org> wrote:
> > > .....
> > > > I can try and move a poc for this up the todo list.
> > > >
> > > > Without an approach like this certain sandboxes will fallback to
> > > > ENOSYSing system calls they can't filter. This is a generic problem
> > > > though with clone3() being one promiment example.
> > >
> > > Thank you for the detailed reply. It sounds to me like this will eventually have
> > > to get solved anyway, so we could move ahead without clone() on loongarch,
> > > and just not have support for Chrome until this is fully solved.
> > >
> > > As both the glibc and musl ports are being proposed for inclusion right
> > > now, we should try to come to a decision so the libc ports can adjust if
> > > necessary. Adding both mailing lists to Cc here, the discussion is archived
> > > at [1].
> > >
> > >          Arnd
> > >
> > > [1] https://lore.kernel.org/linux-arch/20220509100058.vmrgn5fkk3ayt63v@wittgenstein/
> >
> > Having read about the seccomp issue, I think it's a very strong
> > argument that __NR_clone should be kept permanently for all future
> > archs.
> 
> Ok, let's keep clone() around for all architectures then. We should probably
> just remove the __ARCH_WANT_SYS_CLONE macro and build the
> code into the kernel unconditionally, but at the moment there
> are still private versions for ia64 and sparc with the same name as
> the generic version. Both are also still lacking support for clone3() and
> don't have anyone actively working on them.
> 
> In this case, we probably don't need to change clone3() to allow the
> zero-length stack after all, and the wrapper that was added to the
> musl port should get removed again.

I still think disallowing a zero length (unknown length with caller
providing the start address only) stack is a gratuitous limitation on
the clone3 interface, and would welcome leaving the change to allow
zero-length in place. There does not seem to be any good justification
for forbidding it, and it does pose other real-world obstruction to
use. For example if your main thread had exited (or if you're forking
from a non-main thread) and you wanted to create a new process using
the old main thread stack as your stack, you would not know a
size/lowest-address, only a starting address from which it extends
some long (and possibly expanding) amount.

Rich