Re: [PATCH RFC 21/43] x86/ftrace: Adapt assembly for PIE support

["Hou Wenlong" <houwenlong.hwl@antgroup.com>]

On Fri, Apr 28, 2023 at 09:37:19PM +0800, Steven Rostedt wrote:
> 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
> 
> 
> ?
>
OK, I'll drop it. Actually, I select DYNAMIC_FTRACE when
CONFIG_RETPOLINE is enabled for PIE due to the indirect call for
__fentry__() in patch 34.

Thanks.
> -- Steve