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

[H. Peter Anvin]

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From: hpa@xxxxxxxxx
Message-ID: <E0C934E7-6789-4598-AF55-468C84B8568B@xxxxxxxxx>

On March 22, 2017 2:11:12 PM PDT, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>On Wed, Mar 22, 2017 at 1:49 PM, H. Peter Anvin <hpa@xxxxxxxxx> 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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