linux-kernel - Re: what's papered over by set_fs(USER

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Message-ID: <AANLkTikKtK1s8mGF4chhLtXRJW1G43BKAAY3wC-EVbQH@mail.gmail.com>
Date:	Fri, 24 Sep 2010 23:51:11 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, tglx@...utronix.de,
	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: what's papered over by set_fs(USER_DS) in amd64 signal delivery?

On Fri, Sep 24, 2010 at 10:48 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Sep 24, 2010 at 10:25:15PM -0400, Brian Gerst wrote:
>> > + ?? ?? ?? ?? ?? ?? ?? __asm__("mov %w0,%%fs ; mov %w0,%%gs":"=r" (seg) :"0" (seg));
>> > + ?? ?? ?? ?? ?? ?? ?? set_fs(seg);
>> > + ?? ?? ?? ?? ?? ?? ?? regs->xds = seg;
>> > + ?? ?? ?? ?? ?? ?? ?? regs->xes = seg;
>> > + ?? ?? ?? ?? ?? ?? ?? regs->xss = seg;
>> > + ?? ?? ?? ?? ?? ?? ?? regs->xcs = USER_CS;
>> > in 2.1.2. ??And that's when we had
>> > ?? ?? ?? ??* fs and gs evicted from pt_regs
>> > ?? ?? ?? ??* fs and gs not saved restored on kernel entry/exit
>> > ?? ?? ?? ??* just introduced set_fs() to start with (that went in 2.1.0)
>> >
>> > A bit before my time, so I'm not sure what's been going on there...
>>
>> I believe it can be safely removed.  Looking through the history, the
>> corresponding set_fs() calls were removed from 32-bit by commit
>> b93b6ca3.  This is just an artifact from ancient i386 code where
>> set_fs (which is grossly misnamed now) really did set the %fs
>> register.
>
> Not quite.  If you look at the tree where it has shown up (2.1.2), you'll see
> that
>        a) by that time it _wasn't_ an assignment to %fs
>        b) the same patch that has introduced that call there does direct
> assignment to %fs right next to that set_fs().  See that __asm__ above?
>
> Again, I agree that it almost certainly can be dropped.  I really wonder
> about the history, though.  It predates git and bk by far (late 1996).
> Linus, do you have any recollection regarding that stuff?
>

In the beginning, the i386 kernel used a non-flat segmented memory
layout.  USER_[CD]S were 3GB segments at base 0, and KERNEL_[CD]S were
1GB segments at base 3GB.  This meant that the kernel could not access
userspace addresses without using a fs segment override (%fs was saved
in pt_regs, reloaded with USER_DS on kernel entry, and restored on
kernel exit).  You had to reload %fs with KERNEL_DS for the *_user
functions to address the kernel segment.

v2.1.2 introduced the modern flat memory layout with 4GB segments at
base 0.  %fs no longer was used for userspace access, so it wasn't
saved in pt_regs or touched in any way until a task switch.  Instead
of the hardware enforcing the limit, the check was moved to software.

Originally the signal handler had to set regs->xfs = USER_DS so that
the signal handler had a known state when it ran.  That had nothing to
do with the kernel's userspace access mechanism.  It was converted to
do both the immediate reloading of the %fs register (since it was no
longer saved in pt_regs and wouldn't get restored on kernel exit), and
to a new set_fs(USER_DS) call which meant something completely
different.  That is the origin of the code we are trying to remove
now.

--
Brian Gerst
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