linux-kernel - Re: [RFC] TIF_NOTIFY_RESUME, arch/*/*/*signal*.c and all such

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Message-ID: <20120427235023.GR6871@ZenIV.linux.org.uk>
Date:	Sat, 28 Apr 2012 00:50:23 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] TIF_NOTIFY_RESUME, arch/*/*/*signal*.c and all such

On Sat, Apr 28, 2012 at 12:15:26AM +0100, Al Viro wrote:

> I think all such architectures need that check lifted to do_notify_resume()
> (and the rest needs it killed, of course).  Including x86, by the look
> of it - we _probably_ can't get there with TIF_NOTIFY_RESUME and
> !user_mode(regs), but I'm not entirely sure of that.  arm is in about the
> same situation; alpha, ppc{32,64}, sparc{32,64} and m68k really can't get
> there like that (they all check it in the asm glue).  mips probably might,
> unless I'm misreading their ret_from_fork()...  Fun.

I'm completely unfamiliar with tile assembler, so I might be missing something
there, but AFAICS the following sequence can lead to do_signal() being
called for a kernel thread:
	kernel_thread()
		ret_from_fork() in child
			.Lresume_userspace
				do_work_pending()
					do_signal()
and unlike many other, tile does _not_ check for !user_mode() anywhere
relevant...  Broken?
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