linux-kernel - Re: [PATCH v3 2/4] x86/syscalls: Specific usage of verify_pre_usermode

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Date:   Thu, 23 Mar 2017 12:14:56 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Garnier <thgarnie@...gle.com>
CC:     Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        David Howells <dhowells@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dave Hansen <dave.hansen@...el.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Petr Mladek <pmladek@...e.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Helge Deller <deller@....de>, Rik van Riel <riel@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Frederic@...or.com
Subject: Re: [PATCH v3 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

Weisbecker <fweisbec@...il.com>,Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>,Ingo Molnar <mingo@...hat.com>,Paolo Bonzini <pbonzini@...hat.com>,Dmitry Safonov <dsafonov@...tuozzo.com>,Borislav Petkov <bp@...en8.de>,Josh Poimboeuf <jpoimboe@...hat.com>,Brian Gerst <brgerst@...il.com>,Jan Beulich <JBeulich@...e.com>,Christian Borntraeger <borntraeger@...ibm.com>,Fenghua Yu <fenghua.yu@...el.com>,He Chen <he.chen@...ux.intel.com>,Russell King <linux@...linux.org.uk>,Vladimir Murzin <vladimir.murzin@....com>,Will Deacon <will.deacon@....com>,Catalin Marinas <catalin.marinas@....com>,Mark Rutland <mark.rutland@....com>,James Morse <james.morse@....com>,"David A . Long" <dave.long@...aro.org>,Pratyush Anand <panand@...hat.com>,Laura Abbott <labbott@...hat.com>,Andre Przywara <andre.przywara@....com>,Chris Metcalf <cmetcalf@...lanox.com>,linux-s390 <linux-s390@...r.kernel.org>,LKML <linux-kernel@...r.kernel.org>,Linux API <linux-api@...r.kernel.org>,the arch/x86 maintainers
<x86@...nel.org>,"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,Kernel Hardening <kernel-hardening@...ts.openwall.com>
From: hpa@...or.com
Message-ID: <E0C934E7-6789-4598-AF55-468C84B8568B@...or.com>

On March 22, 2017 2:11:12 PM PDT, Thomas Garnier <thgarnie@...gle.com> wrote:
>On Wed, Mar 22, 2017 at 1:49 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 03/22/17 13:41, Thomas Garnier wrote:
>>>>> with the change below for additional feedback.
>>>>
>>>> Can you specify what that means?
>>>
>>> If I set inline by default, the compiler chose not to inline it on
>>> x86. If I force inline the size impact was actually bigger (without
>>> the architecture specific code).
>>>
>>
>> That's utterly bizarre.  Something strange is going on there.  I
>suspect
>> the right thing to do is to out-of-line the error case only, but even
>> that seems strange.  It should be something like four instructions
>inline.
>>
>
>The compiler seemed to often inline other functions called by the
>syscall handlers. I assume the growth was due to changes in code
>optimization because the function is much larger at the end.
>
>>>>
>>>> On x86, where there is only one caller of this, it really seems
>like it
>>>> ought to reduce the overhead to almost zero (since it most likely
>is
>>>> hidden in the pipeline.)
>>>>
>>>> I would like to suggest defining it inline if
>>>> CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE is set; I really
>don't
>>>> care about an architecture which doesn't have it.
>>>
>>> But if there is only one caller, does the compiler is not suppose to
>>> inline the function based on options?
>>
>> If it is marked static in the same file, yes, but you have it in a
>> different file from what I can tell.
>
>If we do global optimization, it should. Having it as a static inline
>make it easier on all types of builds.
>
>>
>>> The assembly will call it too, so I would need an inline and a
>>> non-inline based on the caller.
>>
>> Where?  I don't see that anywhere, at least for x86.
>
>After the latest changes on x86, yes. On arm/arm64, we call it with
>the CHECK_DATA_CORRUPTION config.
>
>>
>>         -hpa
>>

If we do global optimization, yes, but global optimization (generally called link-time optimization, LTO, on Linux) is very much the exception and not the rule for the Linux kernel at this time.
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