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Date:   Fri, 28 Apr 2023 09:37:19 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     "Hou Wenlong" <houwenlong.hwl@antgroup.com>
Cc:     linux-kernel@vger.kernel.org,
        "Thomas Garnier" <thgarnie@chromium.org>,
        "Lai Jiangshan" <jiangshan.ljs@antgroup.com>,
        "Kees Cook" <keescook@chromium.org>,
        "Masami Hiramatsu" <mhiramat@kernel.org>,
        "Mark Rutland" <mark.rutland@arm.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
        "Dave Hansen" <dave.hansen@linux.intel.com>, <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        <linux-trace-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC 21/43] x86/ftrace: Adapt assembly for PIE support
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On Fri, 28 Apr 2023 17:51:01 +0800
"Hou Wenlong" <houwenlong.hwl@antgroup.com> wrote:

> Change the assembly code to use only relative references of symbols for
> the kernel to be PIE compatible.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> Cc: Thomas Garnier <thgarnie@chromium.org>
> Cc: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> Cc: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/kernel/ftrace_64.S | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
> index eddb4fabc16f..411fa4148e18 100644
> --- a/arch/x86/kernel/ftrace_64.S
> +++ b/arch/x86/kernel/ftrace_64.S
> @@ -315,7 +315,14 @@ STACK_FRAME_NON_STANDARD_FP(ftrace_regs_caller)
>  SYM_FUNC_START(__fentry__)
>  	CALL_DEPTH_ACCOUNT
>  
> +#ifdef CONFIG_X86_PIE
> +	pushq %r8
> +	leaq ftrace_stub(%rip), %r8
> +	cmpq %r8, ftrace_trace_function(%rip)
> +	popq %r8
> +#else
>  	cmpq $ftrace_stub, ftrace_trace_function
> +#endif
>  	jnz trace
>  	RET
>  
> @@ -329,7 +336,7 @@ trace:
>  	 * ip and parent ip are used and the list function is called when
>  	 * function tracing is enabled.
>  	 */
> -	movq ftrace_trace_function, %r8
> +	movq ftrace_trace_function(%rip), %r8
>  	CALL_NOSPEC r8
>  	restore_mcount_regs
>  

I really don't want to add more updates to !DYNAMIC_FTRACE. This code only
exists to make sure I don't break it for other architectures.

How about

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 442eccc00960..ee4d0713139d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -37,7 +37,7 @@ config X86_64
 
 config FORCE_DYNAMIC_FTRACE
 	def_bool y
-	depends on X86_32
+	depends on X86_32 || X86_PIE
 	depends on FUNCTION_TRACER
 	select DYNAMIC_FTRACE
 	help


?

-- Steve