linux-kernel - Re: For review: documentation of clone3() system call

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Message-ID: <20191029160525.2zn6vnnyai5ahdj3@wittgenstein>
Date:   Tue, 29 Oct 2019 17:05:26 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...il.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: For review: documentation of clone3() system call

On Tue, Oct 29, 2019 at 04:20:37PM +0100, Jann Horn wrote:
> On Tue, Oct 29, 2019 at 3:26 PM Christian Brauner
> <christian.brauner@...ntu.com> wrote:
> > On Tue, Oct 29, 2019 at 12:27:07PM +0100, Christian Brauner wrote:
> > > 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@...ntu.com> wrote:
> > > > > 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]).
> >
> > @Jann, I think the following patch might even be enough?...
> [...]
> > +static inline void clone3_prepare_stack(struct kernel_clone_args *kargs)
> > +{
> > +#if !defined(CONFIG_STACK_GROWSUP) && !defined(CONFIG_IA64)
> > +       kargs->stack += kargs->stack_size;
> > +#endif
> > +}
> 
> I guess it might work as long as nobody is actually using clone3 yet
> and you can get this patch into the 5.3 stable tree and any distro
> kernels on 5.3 before people do start using clone3?

Yes, that would be my preferred approach. As I said doing it this way is
pretty common. A recent example where we did this is the file_max
sysctl.

Christian