Re: [PATCH v2] x86: Preserve iopl on fork and execve

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Alex Henrie <alexhenrie24@gmail.com>,
	One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
	Kees Cook <keescook@chromium.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Doug Johnson <dougvj@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Tyler Hicks <tyhicks@canonical.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Borislav Petkov <bp@alien8.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Arjan van de Ven <arjan@infradead.org>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Brian Gerst <brgerst@gmail.com>
Subject: Re: [PATCH v2] x86: Preserve iopl on fork and execve
Date: Tue, 12 May 2015 08:13:35 -0700	[thread overview]
Message-ID: <CA+55aFx3RswnjmCErk8QhCo0KrCvxZnuES3WALBR1NkPbUZ8qw@mail.gmail.com> (raw)
In-Reply-To: <20150512064032.GA25097@gmail.com>

On Mon, May 11, 2015 at 11:40 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
>  - Nothing actually broke that people cared about in the last 2.5
>    years, thus this might be one of the (very very rare) cases where
>    preserving a breakage is the right thing to do.

Indeed. The Linux "no regressions" rule is not about some theoretical
"the ABI changed". It's about actual observed regressions.

So if we can improve the ABI without any user program or workflow
breaking, that's fine.

How was this detected? Was it just from code inspection? Because if
so, I think the "don't preserve iopl" is indeed the better ABI and we
should keep it, accidental or not, since it restricts the impact.

But if it turns out somebody was actually depending on it, it's a
regression. Of course, 2.5 years later, that is unlikely, but hey,
some usages clearly end up updating kernels much too seldom.

                             Linus