linux-kernel - Re: [PATCH v6 3/5] samples: ftrace: Save required argument registers in sample trampolines

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Date:   Thu, 6 Apr 2023 11:22:09 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Florent Revest <revest@...omium.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kpsingh@...nel.org, jolsa@...nel.org,
        xukuohai@...weicloud.com, lihuafei1@...wei.com
Subject: Re: [PATCH v6 3/5] samples: ftrace: Save required argument registers
 in sample trampolines

On Wed, Apr 05, 2023 at 08:02:48PM +0200, Florent Revest wrote:
> The ftrace-direct-too sample traces the handle_mm_fault function whose
> signature changed since the introduction of the sample. Since:
> commit bce617edecad ("mm: do page fault accounting in handle_mm_fault")
> handle_mm_fault now has 4 arguments. Therefore, the sample trampoline
> should save 4 argument registers.
> 
> s390 saves all argument registers already so it does not need a change
> but x86_64 needs an extra push and pop.
> 
> This also evolves the signature of the tracing function to make it
> mirror the signature of the traced function.
> 
> Signed-off-by: Florent Revest <revest@...omium.org>

Reviewed-by: Mark Rutland <mark.rutland@....com>

Thanks for this!

Mark.

> ---
>  samples/ftrace/ftrace-direct-too.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/samples/ftrace/ftrace-direct-too.c b/samples/ftrace/ftrace-direct-too.c
> index f28e7b99840f..71ed4ee8cb4a 100644
> --- a/samples/ftrace/ftrace-direct-too.c
> +++ b/samples/ftrace/ftrace-direct-too.c
> @@ -5,14 +5,14 @@
>  #include <linux/ftrace.h>
>  #include <asm/asm-offsets.h>
>  
> -extern void my_direct_func(struct vm_area_struct *vma,
> -			   unsigned long address, unsigned int flags);
> +extern void my_direct_func(struct vm_area_struct *vma, unsigned long address,
> +			   unsigned int flags, struct pt_regs *regs);
>  
> -void my_direct_func(struct vm_area_struct *vma,
> -			unsigned long address, unsigned int flags)
> +void my_direct_func(struct vm_area_struct *vma, unsigned long address,
> +		    unsigned int flags, struct pt_regs *regs)
>  {
> -	trace_printk("handle mm fault vma=%p address=%lx flags=%x\n",
> -		     vma, address, flags);
> +	trace_printk("handle mm fault vma=%p address=%lx flags=%x regs=%p\n",
> +		     vma, address, flags, regs);
>  }
>  
>  extern void my_tramp(void *);
> @@ -34,7 +34,9 @@ asm (
>  "	pushq %rdi\n"
>  "	pushq %rsi\n"
>  "	pushq %rdx\n"
> +"	pushq %rcx\n"
>  "	call my_direct_func\n"
> +"	popq %rcx\n"
>  "	popq %rdx\n"
>  "	popq %rsi\n"
>  "	popq %rdi\n"
> -- 
> 2.40.0.577.gac1e443424-goog
>