Re: [RFC 16/22] x86/percpu: Adapt percpu for PIE support

From: "H. Peter Anvin" <hpa@zytor.com>
To: Thomas Garnier <thgarnie@google.com>, Brian Gerst <brgerst@gmail.com>
Cc: "Michal Hocko" <mhocko@suse.com>,
	"kvm list" <kvm@vger.kernel.org>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Christopher Li" <sparse@chrisli.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"Paul Gortmaker" <paul.gortmaker@windriver.com>,
	"Pavel Machek" <pavel@ucw.cz>,
	"Kernel Hardening" <kernel-hardening@lists.openwall.com>,
	"Christoph Lameter" <cl@linux.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Chris Metcalf" <cmetcalf@mellanox.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	"Herbert Xu" <herbert@gondor.apana.org.au>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Matthew Wilcox" <mawilcox@microsoft.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	"Peter Foley" <pefoley2@pefoley.com>,
	"Christian Borntraeger" <borntraeger@de.ibm.com>,
	linux-sparse@vger.kernel.org,
	"Matthias Kaehlcke" <mka@chromium.org>
Subject: Re: [RFC 16/22] x86/percpu: Adapt percpu for PIE support
Date: Wed, 19 Jul 2017 20:03:16 -0700	[thread overview]
Message-ID: <b9bb87ee-7c85-3f94-3ef5-a3ca432f302d__5145.28452554277$1500521275$gmane$org@zytor.com> (raw)
In-Reply-To: <9b152d55-8a1a-75b5-58df-0fe33458107a@zytor.com>

On 07/19/17 19:21, H. Peter Anvin wrote:
> On 07/19/17 16:33, H. Peter Anvin wrote:
>>>
>>> I agree that it is odd but that's how the compiler generates code. I
>>> will re-explore PIC options with mcmodel=small or medium, as mentioned
>>> on other threads.
>>
>> Why should the way compiler generates code affect the way we do things
>> in assembly?
>>
>> That being said, the compiler now has support for generating this kind
>> of code explicitly via the __seg_gs pointer modifier.  That should let
>> us drop the __percpu_prefix and just use variables directly.  I suspect
>> we want to declare percpu variables as "volatile __seg_gs" to account
>> for the possibility of CPU switches.
>>
>> Older compilers won't be able to work with this, of course, but I think
>> that it is acceptable for those older compilers to not be able to
>> support PIE.
>>
> 
> Grump.  It turns out that the compiler doesn't do the right thing for
> symbols marked with the __seg_[fg]s markers.  __thread does the right
> thing, but __thread a) has %fs: hard-coded, still, and b) I believe can
> still cache %seg:0 arbitrarily long.

I filed this bug report for gcc:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81490

It might still be possible to work around this by playing really ugly
games with __thread, but I haven't yet figured out how best to do that.

	-hpa

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