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

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

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.

	-hpa