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

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Message-ID: <20120427233235.GQ6871@ZenIV.linux.org.uk>
Date:	Sat, 28 Apr 2012 00:32:35 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Martin Schwidefsky <schwidefsky@...ibm.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.

	Speaking of user_mode() oddities - may I politely inquire what had
been smoked to inspire this (in arch/s390/kernel/signal.c):
        /* This is the legacy signal stack switching. */
        else if (!user_mode(regs) &&
                 !(ka->sa.sa_flags & SA_RESTORER) &&
                 ka->sa.sa_restorer) {
                sp = (unsigned long) ka->sa.sa_restorer;
        }
especially when all paths leading to that come through do_signal() that does
        if (!user_mode(regs))
                return;
on the same regs.  It had been like that since 2.3.99pre8 when s390 went
into the tree...  It looks vaguely similar to i386
                        /* This is the legacy signal stack switching. */
                        if ((regs->ss & 0xffff) != __USER_DS &&
                                !(ka->sa.sa_flags & SA_RESTORER) &&
                                        ka->sa.sa_restorer)
                                sp = (unsigned long) ka->sa.sa_restorer;
but there the code is at least not unreachable...
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