linux-kernel - Re: [PATCH] lkdtm: Remove set but not used variable 'byte'

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Date:   Sun, 10 Nov 2019 19:18:33 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Zheng Yongjun <zhengyongjun3@...wei.com>
Cc:     keescook@...omium.org, arnd@...db.de, linux-kernel@...r.kernel.org,
        Hulk Robot <hulkci@...wei.com>
Subject: Re: [PATCH] lkdtm: Remove set but not used variable 'byte'

On Sun, Nov 10, 2019 at 05:22:49PM +0800, Zheng Yongjun wrote:
> Fixes gcc '-Wunused-but-set-variable' warning:
> 
> drivers/misc/lkdtm/bugs.c: In function lkdtm_STACK_GUARD_PAGE_LEADING:
> drivers/misc/lkdtm/bugs.c:236:25: warning: variable byte set but not used [-Wunused-but-set-variable]
> drivers/misc/lkdtm/bugs.c: In function lkdtm_STACK_GUARD_PAGE_TRAILING:
> drivers/misc/lkdtm/bugs.c:250:25: warning: variable byte set but not used [-Wunused-but-set-variable]
> 
> byte is never used, so remove it.
> 
> Reported-by: Hulk Robot <hulkci@...wei.com>
> Signed-off-by: Zheng Yongjun <zhengyongjun3@...wei.com>
> ---
>  drivers/misc/lkdtm/bugs.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
> index 7284a22b1a09..fcd943725b66 100644
> --- a/drivers/misc/lkdtm/bugs.c
> +++ b/drivers/misc/lkdtm/bugs.c
> @@ -249,12 +249,9 @@ void lkdtm_STACK_GUARD_PAGE_LEADING(void)
>  {
>  	const unsigned char *stack = task_stack_page(current);
>  	const unsigned char *ptr = stack - 1;
> -	volatile unsigned char byte;
>  
>  	pr_info("attempting bad read from page below current stack\n");
>  
> -	byte = *ptr;
> -

You do realize just what this code is trying to do _is_ a bug, so it's
ok to leave it, right?  You just broke the code here with your change as
you are trying to "fix" it :)

thanks,

greg k-h