linux-kernel - Re: [PATCH v3] kernel/signal.c: fix BUG

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Date:	Tue, 25 Jun 2013 23:13:30 +0100
From:	James Hogan <james.hogan@...tec.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	David Daney <ddaney@...iumnetworks.com>,
	David Daney <ddaney.cavm@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	David Daney <david.daney@...ium.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>, linux-mips@...ux-mips.org
Subject: Re: [PATCH v3] kernel/signal.c: fix BUG_ON with SIG128 (MIPS)

On 25 June 2013 22:40, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Mon, 24 Jun 2013 10:10:08 +0100 James Hogan <james.hogan@...tec.com> wrote:
>
>> On 22/06/13 20:09, Oleg Nesterov wrote:
>> > On 06/21, David Daney wrote:
>> >> I am proposing that we just reduce the number of usable signals such
>> >> that existing libc status checking macros/functions don't change in any
>> >> way.
>> >
>> > And I fully agree! Absolutely, sorry for confusion.
>> >
>> >
>> > What I tried to say, _if_ we change the ABI instead, lets make this
>> > change sane.
>>
>> I agree that this approach isn't very nice (I was really just trying to
>> explore the options) and reducing the number of signals is nicer. But is
>> anybody here confident enough that the number of signals changing under
>> the feet of existing binaries/libc won't actually break anything real?
>> I.e. anything trying to use SIGRTMAX() to get a lower priority signal.
>
> Meanwhile, unprivileged users can make a MIPS kernel go BUG.
>
> How much of a problem is this?  Obviously less of a problem with MIPS
> than it would be with some other CPU types, but I'd imagine it's still
> awkward in some environments.
>
> If this _is_ considered a problem, can we think of some nasty little
> hack which at least makes the effects less damaging, which we can also
> put into -stable kernels?

The first rfc patch I sent sort of satisfies that by passing 127 if
sig==128, or slightly better would be passing 126 if sig>=127 (so that
SIFSIGNALED returns true). Effectively #ifdef'ing it on _NSIG>127 as
this patch does may be preferable too.

That's probably the minimum change necessary to evade the BUG_ON
without removing it. The wait status code will still be wrong, but it
wasn't exactly right before so it's no worse.

IMO changing the ABI by reducing _NSIG to 127 or 126 isn't appropriate
for stable.

Cheers
James
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