Re: For review: documentation of clone3() system call

[Christian Brauner <christian.brauner@ubuntu.com>]

On Mon, Oct 28, 2019 at 08:09:13PM +0100, Jann Horn wrote:
> On Mon, Oct 28, 2019 at 6:21 PM Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
> > On Mon, Oct 28, 2019 at 04:12:09PM +0100, Jann Horn wrote:
> > > On Fri, Oct 25, 2019 at 6:59 PM Michael Kerrisk (man-pages)
> > > <mtk.manpages@gmail.com> wrote:
> > > > I've made a first shot at adding documentation for clone3(). You can
> > > > see the diff here:
> > > > https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=faa0e55ae9e490d71c826546bbdef954a1800969
> [...]
> > > You might want to note somewhere that its flags can't be
> > > seccomp-filtered because they're stored in memory, making it
> > > inappropriate to use in heavily sandboxed processes.
> >
> > Hm, I don't think that belongs on the clone manpage. Granted that
> > process creation is an important syscall but so are a bunch of others
> > that aren't filterable because of pointer arguments.
> > We can probably mention on the seccomp manpage that seccomp can't filter
> > on pointer arguments and then provide a list of examples. If you setup a
> > seccomp filter and don't know that you can't filter syscalls with
> > pointer args that seems pretty bad to begin with.
> 
> Fair enough.
> 
> [...]
> > One thing I never liked about clone() was that userspace had to know
> > about stack direction. And there is a lot of ugly code in userspace that
> > has nasty clone() wrappers like:
> [...]
> > where stack + stack_size is addition on a void pointer which usually
> > clang and gcc are not very happy about.
> > I wanted to bring this up on the mailing list soon: If possible, I don't
> > want userspace to need to know about stack direction and just have stack
> > point to the beginning and then have the kernel do the + stack_size
> > after the copy_clone_args_from_user() if the arch needs it. For example,
> > by having a dumb helder similar to copy_thread_tls()/coyp_thread() that
> > either does the + stack_size or not. Right now, clone3() is supported on
> > parisc and afaict, the stack grows upwards for it. I'm not sure if there
> > are obvious reasons why that won't work or it would be a bad idea...
> 
> That would mean adding a new clone flag that redefines how those
> parameters work and describing the current behavior in the manpage as
> the behavior without the flag (which doesn't exist on 5.3), right?

I would break API and if someone reports breakage we'll revert and go
the more complicated route you outlined (see [1]).
But I don't think it will a big deal. First, we haven't documented how
stack needs to be passed so who knows what people currently do. Second,
clone3() has not been out for a long time and currently does _not_
provide features that legacy clone() does not provide apart from a
cleaner interface. So userspace has no incentive to use clone3() over
clone() right now. That'll change latest with v5.5 where we have new
features on top of clone3() (CLONE_CLEAR_SIGHAND). So let's just try and
fix it.

[1]: This is basically what Linus has repeatedly said: it's not about
     never breaking api in principle but rather about whether this
     breaks someones usecase. And if it does break, we need to revert.

Christian