linux-kernel - Re: [PATCH v2 3/3] powerpc/mm: Implement CONFIG_DEBUG

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Date:   Fri, 21 Apr 2017 13:32:10 +0000
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 3/3] powerpc/mm: Implement CONFIG_DEBUG_RODATA on PPC32

Excerpts from Christophe Leroy's message of April 21, 2017 18:32:
> This patch implements CONFIG_DEBUG_RODATA on PPC32.
> 
> As for CONFIG_DEBUG_PAGEALLOC, it deactivates BAT and LTLB mappings
> in order to allow page protection setup at the level of each page.
> 
> As BAT/LTLB mappings are deactivated, their might be performance
> impact. For this reason, we keep it OFF by default.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>

[snip]

> diff --git a/arch/powerpc/kernel/ftrace.c 
> b/arch/powerpc/kernel/ftrace.c
> index 32509de6ce4c..06d2ac53f471 100644
> --- a/arch/powerpc/kernel/ftrace.c
> +++ b/arch/powerpc/kernel/ftrace.c
> @@ -46,6 +46,7 @@ static int
>  ftrace_modify_code(unsigned long ip, unsigned int old, unsigned int new)
>  {
>  	unsigned int replaced;
> +	int err;
> 
>  	/*
>  	 * Note:
> @@ -67,10 +68,11 @@ ftrace_modify_code(unsigned long ip, unsigned int old, unsigned int new)
>  	}
> 
>  	/* replace the text with the new text */
> -	if (patch_instruction((unsigned int *)ip, new))
> -		return -EPERM;
> +	set_kernel_text_rw(ip);
> +	err = patch_instruction((unsigned int *)ip, new);
> +	set_kernel_text_ro(ip);

Is there a reason to not put those inside patch_instruction()?

- Naveen