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

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Message-ID: <61e165241e4b98cd655ac79cd110f3f08e389838.camel@perches.com>
Date:   Sun, 10 Nov 2019 10:13:50 -0800
From:   Joe Perches <joe@...ches.com>
To:     Zheng Yongjun <zhengyongjun3@...wei.com>, keescook@...omium.org,
        arnd@...db.de, gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org, Hulk Robot <hulkci@...wei.com>
Subject: Re: [PATCH] lkdtm: Remove set but not used variable 'byte'

On Sun, 2019-11-10 at 17:22 +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.

I believe "hulk robot" needs to be updated instead.

It seems a generically bad idea to elide as byte
is in fact used because it's volatile and the
compiler was forced to perform the access of *ptr.


> 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;
> -
>  	pr_err("FAIL: accessed page before stack!\n");
>  }
>  
> @@ -263,12 +260,9 @@ void lkdtm_STACK_GUARD_PAGE_TRAILING(void)
>  {
>  	const unsigned char *stack = task_stack_page(current);
>  	const unsigned char *ptr = stack + THREAD_SIZE;
> -	volatile unsigned char byte;
>  
>  	pr_info("attempting bad read from page above current stack\n");
>  
> -	byte = *ptr;
> -
>  	pr_err("FAIL: accessed page after stack!\n");
>  }
>