Re: [PATCH] x86: kill handle_signal()->set_fs()

[Oleg Nesterov <oleg@redhat.com>]

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.