linux-kernel - Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

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Message-ID: <55CD29C2.7060901@gmail.com>
Date:	Thu, 13 Aug 2015 16:35:30 -0700
From:	Raymond Jennings <shentino@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	Stas Sergeev <stsp@...t.ru>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu

On 08/13/15 16:18, Linus Torvalds wrote:
> On Thu, Aug 13, 2015 at 4:05 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> The _only_ thing that matters is that something broke.
> To clarify: things like test programs etc don't matter. Real
> applications, used by real users. That's what regressions cover. If
> you have a workflow that isn't just some random kernel test thing, and
> you depend on it, and we break it, the kernel is supposed to fix it.
>
> There are some (very few) exceptions.
>
> If it's a security issue, we may not be able to "fix" it, because
> other concerns can obviously take precedence.
>
> Also, sometimes the reports come in way too late - if you were running
> some stable distro kernel for several years, and updated, and notice a
> change that happened four years ago and modern applications now rely
> on the _new_ behavior, we may not be able to fix the regression any
> more.
>
> But no, "it was an unintentional kernel bug and clearly just stupid
> crap code, and we fixed it and now the kernel is much better and
> cleaner" is not a valid reason for regressions. We'll go back to the
> stupid and crap code if necessary, however much that may annoy us.
>
> For an example of the kind of things we may have to do, see commits
>
>      64f371bc3107 autofs: make the autofsv5 packet file descriptor use
> a packetized pipe
>      9883035ae7ed pipes: add a "packetized pipe" mode for writing
>
> and just wonder at the insanity. That's the kinds of things that
> happen when one application had actively worked around a bug in
> compatibility handling, and then trying to "fix" that bug just caused
> another application to break instead.
>
>                         Linus
Is there a way to temporally confine the bad crap code just to the 
applications that depend on it, or does a userspace app latching onto 
bad behavior effectively lock down the abi for the future?

I know that some features in the kernel get deprecated over a process 
(devfs for example) once userspace is given an alternative...would there 
be a process like that to deal with userspace code that is pinning a 
piece of crap in the kernel?
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