Re: [PATCH V2 2/7] x86/entry: Switch the stack after error_entry() returns

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 03, 2022 at 11:54:29AM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> 
> error_entry() calls sync_regs() to settle/copy the pt_regs and switches
> the stack directly after sync_regs().  But error_entry() itself is also
> a function call, the switching has to handle the return address of it
> together, which causes the work complicated and tangly.
> 
> Switching to the stack after error_entry() makes the code simpler and
> intuitive.
> 
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> ---
>  arch/x86/entry/entry_64.S | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 24846284eda5..a51cad2b7fff 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -323,6 +323,8 @@ SYM_CODE_END(ret_from_fork)
>  .macro idtentry_body cfunc has_error_code:req
>  
>  	call	error_entry
> +	movq	%rax, %rsp			/* switch stack settled by sync_regs() */
> +	ENCODE_FRAME_POINTER
>  	UNWIND_HINT_REGS
>  
>  	movq	%rsp, %rdi			/* pt_regs pointer into 1st argument*/
> @@ -980,14 +982,10 @@ SYM_CODE_START_LOCAL(error_entry)
>  	/* We have user CR3.  Change to kernel CR3. */
>  	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
>  
> +	leaq	8(%rsp), %rdi			/* arg0 = pt_regs pointer */
>  .Lerror_entry_from_usermode_after_swapgs:
>  	/* Put us onto the real thread stack. */
> -	popq	%r12				/* save return addr in %12 */
> -	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
>  	call	sync_regs
> -	movq	%rax, %rsp			/* switch stack */
> -	ENCODE_FRAME_POINTER
> -	pushq	%r12
>  	RET
>  
>  	/*
> @@ -1019,6 +1017,7 @@ SYM_CODE_START_LOCAL(error_entry)
>  	 */
>  .Lerror_entry_done_lfence:
>  	FENCE_SWAPGS_KERNEL_ENTRY
> +	leaq	8(%rsp), %rax			/* return pt_regs pointer */
>  	RET
>  
>  .Lbstep_iret:
> @@ -1039,12 +1038,9 @@ SYM_CODE_START_LOCAL(error_entry)
>  	 * Pretend that the exception came from user mode: set up pt_regs
>  	 * as if we faulted immediately after IRET.
>  	 */
> -	popq	%r12				/* save return addr in %12 */
> -	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
> +	leaq	8(%rsp), %rdi			/* arg0 = pt_regs pointer */
>  	call	fixup_bad_iret
> -	mov	%rax, %rsp
> -	ENCODE_FRAME_POINTER
> -	pushq	%r12
> +	mov	%rax, %rdi
>  	jmp	.Lerror_entry_from_usermode_after_swapgs
>  SYM_CODE_END(error_entry)

I can't seem to follow; perhaps I need more wake-up-juice?

Suppose we have .Lerror_bad_iret, then the code flow is something like:


	old						new

.Lerror_bad_iret
				SWAWPGS business

	popq	%r12					leaq	8(%rsp), %rsi
	movq	%rsp, %rdi
	call	fixup_bad_iret				call	fixup_bad_iret
	mov	%rax, %rsp				mov	%rax, %rdi
	ENCODE_FRAME_POINTER
	pushq	%r12

		jmp	.Lerror_entry_from_usermode_after_swapgs

.Lerror_entry_from_usermode_after_swapgs
	pop	%r12
	movq	%rsp, %rdi
	call	sync_regs				call	sync_regs
	mov	%rax, %rsp
	ENCODE_FRAME_POINTER
	push	%r12
	RET						RET


The thing that appears to me is that syn_regs is called one stack switch
short. This isn't mentioned in the Changelog nor in the comments. Why is
this OK?