[PATCH] lkdtm: Remove set but not used variable 'byte'

[PATCH] lkdtm: Remove set but not used variable 'byte'

[Zheng Yongjun]

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@huawei.com>
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.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");
 }
 
-- 
2.23.0

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

[Joe Perches]

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@huawei.com>
> Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.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");
>  }
>  

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

[Greg KH]

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@huawei.com>
> Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.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