Re: [PATCH 2/3 v4] ftrace/x86: Allow for arguments to be passed in to ftrace_regs by default

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 06, 2020 at 04:42:36PM -0500, Steven Rostedt wrote:
> +static __always_inline struct pt_regs *
> +arch_ftrace_get_regs(struct ftrace_regs *fregs)
> +{
> +	/* Only when FL_SAVE_REGS is set, cs will be non zero */
> +	if (!fregs->regs.cs)
> +		return NULL;
> +	return &fregs->regs;
> +}
> +#endif
> +
>  #endif /*  CONFIG_DYNAMIC_FTRACE */
>  #endif /* __ASSEMBLY__ */
>  #endif /* CONFIG_FUNCTION_TRACER */
> diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
> index ac3d5f22fe64..60e3b64f5ea6 100644
> --- a/arch/x86/kernel/ftrace_64.S
> +++ b/arch/x86/kernel/ftrace_64.S
> @@ -140,12 +140,19 @@ SYM_FUNC_START(ftrace_caller)
>  	/* save_mcount_regs fills in first two parameters */
>  	save_mcount_regs
>  
> +	/* Stack - skipping return address of ftrace_caller */
> +	leaq MCOUNT_REG_SIZE+8(%rsp), %rcx
> +	movq %rcx, RSP(%rsp)
> +
>  SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL)
>  	/* Load the ftrace_ops into the 3rd parameter */
>  	movq function_trace_op(%rip), %rdx
>  
> -	/* regs go into 4th parameter (but make it NULL) */
> -	movq $0, %rcx
> +	/* regs go into 4th parameter */
> +	leaq (%rsp), %rcx
> +
> +	/* Only ops with REGS flag set should have CS register set */
> +	movq $0, CS(%rsp)
>  
>  SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
>  	call ftrace_stub

You now seem to be relying on save_mcount_regs() resulting in a cleared
CS, however, AFAICT CS is uninitialized stack garbage.