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

[Thomas Garnier <thgarnie@google.com>]

On Tue, Mar 14, 2017 at 2:40 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 03/13/17 17:04, H. Peter Anvin wrote:
>> On 03/11/17 01:42, Ingo Molnar wrote:
>>>>
>>>> +   /*
>>>> +    * Check user-mode state on fast path return, the same check is done
>>>> +    * under the slow path through syscall_return_slowpath.
>>>> +    */
>>>> +#ifdef CONFIG_BUG_ON_DATA_CORRUPTION
>>>> +   call    verify_pre_usermode_state
>>>> +#else
>>>> +   /*
>>>> +    * Similar to set_fs(USER_DS) in verify_pre_usermode_state without a
>>>> +    * warning.
>>>> +    */
>>>> +   movq    PER_CPU_VAR(current_task), %rax
>>>> +   movq    $TASK_SIZE_MAX, %rcx
>>>> +   cmp     %rcx, TASK_addr_limit(%rax)
>>>> +   jz      1f
>>>> +   movq    %rcx, TASK_addr_limit(%rax)
>>>> +1:
>>>> +#endif
>>>> +
>>
>> How about simply doing...
>>
>>       movq    PER_CPU_VAR(current_task), %rax
>>       movq    $TASK_SIZE_MAX, %rcx
>> #ifdef CONFIG_BUG_ON_DATA_CORRUPTION
>>       cmpq    %rcx, TASK_addr_limit(%rax)
>>       jne     syscall_return_slowpath
>> #else
>>       movq    %rcx, TASK_addr_limit(%rax)
>> #endif
>>
>> ... and let the slow path take care of BUG.  This should be much faster,
>> even with the BUG, and is simpler to boot.
>>
>
> In fact, we could even to the cmpq/jne unconditionally.  I'm guessing
> the occasional branch mispredict will be offset by occasionally touching
> a clean cacheline in the case of an unconditional store.
>
> Since this is something that should never happen, performance doesn't
> matter.

Ingo: Which approach do you favor? I want to keep the fast path as
fast as possible obviously.

>
>         -hpa
>
>



-- 
Thomas