linux-kernel - Re: [PATCH RFC bpf-next 2/7] x86/ftrace: implement DYNAMIC_FTRACE_WITH

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Message-ID: <20251114113924.723f6fde@gandalf.local.home>
Date: Fri, 14 Nov 2025 11:39:24 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
 andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com,
 song@...nel.org, yonghong.song@...ux.dev, kpsingh@...nel.org,
 sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, mhiramat@...nel.org,
 mark.rutland@....com, mathieu.desnoyers@...icios.com, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next 2/7] x86/ftrace: implement
 DYNAMIC_FTRACE_WITH_JMP

On Fri, 14 Nov 2025 17:24:45 +0800
Menglong Dong <menglong8.dong@...il.com> wrote:

> --- a/arch/x86/kernel/ftrace_64.S
> +++ b/arch/x86/kernel/ftrace_64.S
> @@ -285,8 +285,18 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
>  	ANNOTATE_NOENDBR
>  	RET
>  
> +1:
> +	testb	$1, %al
> +	jz	2f
> +	andq $0xfffffffffffffffe, %rax
> +	movq %rax, MCOUNT_REG_SIZE+8(%rsp)
> +	restore_mcount_regs
> +	/* Restore flags */
> +	popfq
> +	RET
> +
>  	/* Swap the flags with orig_rax */
> -1:	movq MCOUNT_REG_SIZE(%rsp), %rdi
> +2:	movq MCOUNT_REG_SIZE(%rsp), %rdi
>  	movq %rdi, MCOUNT_REG_SIZE-8(%rsp)
>  	movq %rax, MCOUNT_REG_SIZE(%rsp)
>  

So in this case we have:

 original_caller:
 call foo -> foo:
             call fentry -> fentry:
                            [do ftrace callbacks ]
                            move tramp_addr to stack
                            RET -> tramp_addr
                                            tramp_addr:
                                            [..]
                                            call foo_body -> foo_body:
                                                             [..]
                                                             RET -> back to tramp_addr
                                            [..]
                                            RET -> back to original_caller

I guess that looks balanced.

-- Steve