Re: [PATCH v2 14/39] x86/ibt,ftrace: Make function-graph play nice

[Peter Zijlstra]

On Thu, Feb 24, 2022 at 03:51:52PM +0100, Peter Zijlstra wrote:
> @@ -316,10 +317,12 @@ SYM_FUNC_START(return_to_handler)
>  
>  	call ftrace_return_to_handler
>  
> -	movq %rax, %rdi
> +	movq %rax, 16(%rsp)
>  	movq 8(%rsp), %rdx
>  	movq (%rsp), %rax
> -	addq $24, %rsp
> -	JMP_NOSPEC rdi
> +
> +	addq $16, %rsp
> +	UNWIND_HINT_FUNC
> +	RET
>  SYM_FUNC_END(return_to_handler)
>  #endif

While talking about this with Mark, an alternative solution is something
like this, that would keep the RSB balanced and only mess up the current
return.

No idea it if makes an appreciatable difference on current hardware,
therefore I went with the simpler option above.

@@ -307,7 +315,7 @@ EXPORT_SYMBOL(__fentry__)
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 SYM_FUNC_START(return_to_handler)
-	subq  $24, %rsp
+	subq  $16, %rsp
 
 	/* Save the return values */
 	movq %rax, (%rsp)
@@ -319,7 +327,13 @@ SYM_FUNC_START(return_to_handler)
 	movq %rax, %rdi
 	movq 8(%rsp), %rdx
 	movq (%rsp), %rax
-	addq $24, %rsp
-	JMP_NOSPEC rdi
+
+	addq $16, %rsp
+	ANNOTATE_INTRA_FUNCTION_CALL
+	call .Ldo_rop
+.Ldo_rop:
+	mov %rdi, (%rsp)
+	UNWIND_HINT_FUNC
+	RET
 SYM_FUNC_END(return_to_handler)
 #endif