linux-kernel - Re: [PATCH v3 2/2] arch: wire-up clone3() syscall

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Message-ID: <20190621111839.v5yqlws6iw7mx4aa@brauner.io>
Date:   Fri, 21 Jun 2019 13:18:41 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Florian Weimer <fweimer@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Adrian Reber <adrian@...as.de>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v3 2/2] arch: wire-up clone3() syscall

On Fri, Jun 21, 2019 at 11:37:50AM +0200, Arnd Bergmann wrote:
> On Fri, Jun 21, 2019 at 12:10 AM Christian Brauner <christian@...uner.io> wrote:
> > On Thu, Jun 20, 2019 at 11:44:51AM -0700, Guenter Roeck wrote:
> > > On Tue, Jun 04, 2019 at 06:09:44PM +0200, Christian Brauner wrote:
> >
> > clone3() was placed under __ARCH_WANT_SYS_CLONE. Most architectures
> > simply define __ARCH_WANT_SYS_CLONE and are done with it.
> > Some however, such as nios2 and h8300 don't define it but instead
> > provide a sys_clone stub of their own because of architectural
> > requirements (or tweaks) and they are mostly written in assembly. (That
> > should be left to arch maintainers for sys_clone3.)
> >
> > The build failures were on my radar already. I hadn't yet replied
> > since I haven't pushed the fixup below.
> > The solution is to define __ARCH_WANT_SYS_CLONE3 and add a
> > cond_syscall(clone3) so we catch all architectures that do not yet
> > provide clone3 with a ENOSYS until maintainers have added it.
> >
> > diff --git a/arch/arm/include/asm/unistd.h b/arch/arm/include/asm/unistd.h
> > index 7a39e77984ef..aa35aa5d68dc 100644
> > --- a/arch/arm/include/asm/unistd.h
> > +++ b/arch/arm/include/asm/unistd.h
> > @@ -40,6 +40,7 @@
> >  #define __ARCH_WANT_SYS_FORK
> >  #define __ARCH_WANT_SYS_VFORK
> >  #define __ARCH_WANT_SYS_CLONE
> > +#define __ARCH_WANT_SYS_CLONE3
> 
> I never really liked having __ARCH_WANT_SYS_CLONE here
> because it was the only one that a new architecture needed to
> set: all the other __ARCH_WANT_* are for system calls that
> are already superseded by newer ones, so a new architecture
> would start out with an empty list.
> 
> Since __ARCH_WANT_SYS_CLONE3 replaces
> __ARCH_WANT_SYS_CLONE for new architectures, how about
> leaving __ARCH_WANT_SYS_CLONE untouched but instead

__ARCH_WANT_SYS_CLONE is left untouched. :)

> coming up with the reverse for clone3 and mark the architectures
> that specifically don't want it (if any)?

Afaict, your suggestion is more or less the same thing what is done
here. So I'm not sure it buys us anything apart from future
architectures not needing to set __ARCH_WANT_SYS_CLONE3.

I expect the macro above to be only here temporarily until all arches
have caught up and we're sure that they don't require assembly stubs
(cf. [1]). A decision I'd leave to the maintainers (since even for
nios2 we were kind of on the fence what exactly the sys_clone stub was
supposed to do).

But I'm happy to take a patch from you if it's equally or more simple
than this one right here.

In any case, linux-next should be fine on all arches with this fixup
now.

Christian


[1]: Architectures such as nios2 or h8300 simply take the asm-generic
     syscall definitions and generate their syscall table from it. But
     since they don't define __ARCH_WANT_SYS_CLONE the build would fail
     complaining about sys_clone3 missing. The reason this doesn't
     happen for legacy clone is that nios2 and h8300 provide assembly
     stubs for sys_clone but they don't for sys_clone3.