linux-kernel - Re: [PATCH] x86: kill handle_signal()->set

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Message-ID: <20110714190229.GA20030@redhat.com>
Date:	Thu, 14 Jul 2011 21:02:29 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: kill handle_signal()->set_fs()

So, Peter, do you agree we can remove this set_fs() ?

Of course, this doesn't matter from the perfomance pov.
But it is very confusing, especially with CONFIG_X86_64.


On 07/10, Oleg Nesterov wrote:
>
> On 07/10, H. Peter Anvin wrote:
> >
> > On 07/10/2011 09:44 AM, Oleg Nesterov wrote:
> > > handle_signal()->set_fs() has a nice comment which explains what
> > > set_fs() is, but it doesn't explain why it is needed and why it
> > > depends on CONFIG_X86_64.
> > >
> > > Afaics, the history of this confusion is:
> > >
> > > 	1. I guess today nobody can explain why it was needed
> > > 	   in arch/i386/kernel/signal.c, perhaps it was always
> > > 	   wrong. This predates 2.4.0 kernel.
> > >
> > > 	2. then it was copy-and-past'ed to the new x86_64 arch.
> > >
> > > 	3. then it was removed from i386 (but not from x86_64)
> > > 	   by b93b6ca3 "i386: remove unnecessary code".
> > >
> > > 	4. then it was reintroduced under CONFIG_X86_64 when x86
> > > 	   unified i386 and x86_64, because the patch above didn't
> > > 	   touch x86_64.
> > >
> > > Remove it. ->addr_limit should be correct. Even if it was possible
> > > that it is wrong, it is too late to fix it after setup_rt_frame().
> > >
> >
> > The main reason I could think of why this would be necessary is if we
> > take an event while we have fs == KERNEL_DS inside the kernel
>
> this is possible if we are the kernel thread, or set_fs(KERNEL_DS) was
> called.
>
> > which is
> > then promoted to a signal.
>
> How? We are going to return to the user-space. Obviously this is not
> possible with the kernel thread. So I think this can only happen if
> we already have a bug with unbalanced set_fs().
>
> Are you absolutely sure that can't happen?
>
> > In particular, there should be a setting upstream of this, as you're
> > correctly pointing out that it's too late.  If not, we might actually
> > have a problem.
>
> Hmm... Now I recall, this was already discussed 5 years ago. Thanks to
> google, see http://lkml.org/lkml/2007/7/17/321
>
> In particular, Linus sayd:
>
> 	Heh. I think it's entirely historical.
>
> 	Please realize that the whole reason that function is called "set_fs()" is
> 	that it literally used to set the %fs segment register, not
> 	"->addr_limit".
>
> 	So I think the "set_fs(USER_DS)" is there _only_ to match the other
>
> 		regs->xds = __USER_DS;
> 		regs->xes = __USER_DS;
> 		regs->xss = __USER_DS;
> 		regs->xcs = __USER_CS;
>
> 	things, and never mattered. And now it matters even less, and has been
> 	copied to all other architectures where it is just totally insane.
>
> Oleg.

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