Re: [PATCHv2 1/2] clone: Support passing tls argument via C rather than pt_regs magic

From: josh@joshtriplett.org
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
	x86@kernel.org
Subject: Re: [PATCHv2 1/2] clone: Support passing tls argument via C rather than pt_regs magic
Date: Tue, 12 May 2015 17:53:37 -0700	[thread overview]
Message-ID: <20150513005337.GB14292@cloud> (raw)
In-Reply-To: <20150512142250.dcb053da81855ae1b5861173@linux-foundation.org>

On Tue, May 12, 2015 at 02:22:50PM -0700, Andrew Morton wrote:
> On Mon, 11 May 2015 12:29:19 -0700 Josh Triplett <josh@joshtriplett.org> wrote:
> 
> > Introduce a new CONFIG_HAVE_COPY_THREAD_TLS for architectures to opt
> > into, and a new copy_thread_tls that accepts the tls parameter as an
> > additional unsigned long (syscall-argument-sized) argument.
> > Change sys_clone's tls argument to an unsigned long (which does
> > not change the ABI), and pass that down to copy_thread_tls.
> > 
> > Architectures that don't opt into copy_thread_tls will continue to
> > ignore the C argument to sys_clone in favor of the pt_regs captured at
> > kernel entry, and thus will be unable to introduce new versions of the
> > clone syscall.
> 
> What happens quite frequently is that we do something for x86 with the
> expectation that other architectures will follow along, but this
> doesn't happen.  The arch maintainers simply didn't know about it or
> nobody nags them.  Nothing happens and inconsistencies hang around for
> years.  eg, http://lkml.iu.edu/hypermail/linux/kernel/1504.2/04993.html

In this case, there will be a very clear incentive to switch to
CONFIG_HAVE_COPY_THREAD_TLS: if you don't, you can't enable new
syscalls.

- Josh Triplett