Re: [PATCH v3 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

["H. Peter Anvin" <hpa@zytor.com>]

Weisbecker <fweisbec@gmail.com>,Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>,Ingo Molnar <mingo@redhat.com>,Paolo Bonzini <pbonzini@redhat.com>,Dmitry Safonov <dsafonov@virtuozzo.com>,Borislav Petkov <bp@alien8.de>,Josh Poimboeuf <jpoimboe@redhat.com>,Brian Gerst <brgerst@gmail.com>,Jan Beulich <JBeulich@suse.com>,Christian Borntraeger <borntraeger@de.ibm.com>,Fenghua Yu <fenghua.yu@intel.com>,He Chen <he.chen@linux.intel.com>,Russell King <linux@armlinux.org.uk>,Vladimir Murzin <vladimir.murzin@arm.com>,Will Deacon <will.deacon@arm.com>,Catalin Marinas <catalin.marinas@arm.com>,Mark Rutland <mark.rutland@arm.com>,James Morse <james.morse@arm.com>,"David A . Long" <dave.long@linaro.org>,Pratyush Anand <panand@redhat.com>,Laura Abbott <labbott@redhat.com>,Andre Przywara <andre.przywara@arm.com>,Chris Metcalf <cmetcalf@mellanox.com>,linux-s390 <linux-s390@vger.kernel.org>,LKML <linux-kernel@vger.kernel.org>,Linux API <linux-api@vger.kernel.org>,the arch/x86 maintainers
<x86@kernel.org>,"linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>,Kernel Hardening <kernel-hardening@lists.openwall.com>
From: hpa@zytor.com
Message-ID: <E0C934E7-6789-4598-AF55-468C84B8568B@zytor.com>

On March 22, 2017 2:11:12 PM PDT, Thomas Garnier <thgarnie@google.com> wrote:
>On Wed, Mar 22, 2017 at 1:49 PM, H. Peter Anvin <hpa@zytor.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.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.