linux-kernel - Re: [PATCH v4 13/16] arm/ftrace: Use __patch_text

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Date:   Mon, 28 Oct 2019 16:25:26 +0000
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com, jeyu@...nel.org,
        rabin@....in, Mark Rutland <mark.rutland@....com>,
        james.morse@....com
Subject: Re: [PATCH v4 13/16] arm/ftrace: Use __patch_text_real()

Hi Peter,

On Fri, Oct 18, 2019 at 09:35:38AM +0200, Peter Zijlstra wrote:
> Instead of flipping text protection, use the patch_text infrastructure
> that uses a fixmap alias where required.
> 
> This removes the last user of set_all_modules_text_*().
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: ard.biesheuvel@...aro.org
> Cc: rabin@....in
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: james.morse@....com
> ---
>  arch/arm/kernel/ftrace.c |   16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> --- a/arch/arm/kernel/ftrace.c
> +++ b/arch/arm/kernel/ftrace.c
> @@ -22,6 +22,7 @@
>  #include <asm/ftrace.h>
>  #include <asm/insn.h>
>  #include <asm/set_memory.h>
> +#include <asm/patch.h>
>  
>  #ifdef CONFIG_THUMB2_KERNEL
>  #define	NOP		0xf85deb04	/* pop.w {lr} */
> @@ -31,13 +32,15 @@
>  
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  
> +static int patch_text_remap = 0;
> +
>  static int __ftrace_modify_code(void *data)
>  {
>  	int *command = data;
>  
> -	set_kernel_text_rw();
> +	patch_text_remap++;
>  	ftrace_modify_all_code(*command);
> -	set_kernel_text_ro();
> +	patch_text_remap--;
>  
>  	return 0;
>  }
> @@ -59,13 +62,13 @@ static unsigned long adjust_address(stru
>  
>  int ftrace_arch_code_modify_prepare(void)
>  {
> -	set_all_modules_text_rw();
> +	patch_text_remap++;
>  	return 0;
>  }
>  
>  int ftrace_arch_code_modify_post_process(void)
>  {
> -	set_all_modules_text_ro();
> +	patch_text_remap--;
>  	/* Make sure any TLB misses during machine stop are cleared. */
>  	flush_tlb_all();
>  	return 0;
> @@ -97,10 +100,7 @@ static int ftrace_modify_code(unsigned l
>  			return -EINVAL;
>  	}
>  
> -	if (probe_kernel_write((void *)pc, &new, MCOUNT_INSN_SIZE))
> -		return -EPERM;
> -
> -	flush_icache_range(pc, pc + MCOUNT_INSN_SIZE);
> +	__patch_text_real((void *)pc, new, patch_text_remap);

Why can't you just pass 'true' for patch_text_remap? AFAICT, the only
time you want to pass false is during early boot when the text is
assumedly still writable without the fixmap.

Will