[PATCH] seq_file: fix clang warning for NULL pointer arithmetic

[PATCH] seq_file: fix clang warning for NULL pointer arithmetic

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

Clang points out that adding something to NULL is notallowed
in standard C:

fs/kernfs/file.c:127:15: warning: performing pointer arithmetic on a
null pointer has undefined behavior [-Wnull-pointer-arithmetic]
                return NULL + !*ppos;
                       ~~~~ ^
fs/seq_file.c:529:14: warning: performing pointer arithmetic on a
null pointer has undefined behavior [-Wnull-pointer-arithmetic]
        return NULL + (*pos == 0);

Rephrase the function to do the same thing without triggering that
warning. Linux already relies on a specific binary representation
of NULL, so it makes no real difference here. The instance in
kernfs was copied from single_start, so fix both at once.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: c2b19daf6760 ("sysfs, kernfs: prepare read path for kernfs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/kernfs/file.c | 2 +-
 fs/seq_file.c    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index f277d023ebcd..b55e6ef4d677 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -124,7 +124,7 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
 		 * The same behavior and code as single_open().  Returns
 		 * !NULL if pos is at the beginning; otherwise, NULL.
 		 */
-		return NULL + !*ppos;
+		return (void *)(uintptr_t)!*ppos;
 	}
 }
 
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 31219c1db17d..d456468eb934 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -526,7 +526,7 @@ EXPORT_SYMBOL(seq_dentry);
 
 static void *single_start(struct seq_file *p, loff_t *pos)
 {
-	return NULL + (*pos == 0);
+	return (void *)(uintptr_t)(*pos == 0);
 }
 
 static void *single_next(struct seq_file *p, void *v, loff_t *pos)
-- 
2.27.0

Re: [PATCH] seq_file: fix clang warning for NULL pointer arithmetic

[Nathan Chancellor]

On Mon, Oct 26, 2020 at 10:52:56PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Clang points out that adding something to NULL is notallowed
> in standard C:
> 
> fs/kernfs/file.c:127:15: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>                 return NULL + !*ppos;
>                        ~~~~ ^
> fs/seq_file.c:529:14: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>         return NULL + (*pos == 0);
> 
> Rephrase the function to do the same thing without triggering that
> warning. Linux already relies on a specific binary representation
> of NULL, so it makes no real difference here. The instance in
> kernfs was copied from single_start, so fix both at once.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Fixes: c2b19daf6760 ("sysfs, kernfs: prepare read path for kernfs")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

> ---
>  fs/kernfs/file.c | 2 +-
>  fs/seq_file.c    | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> index f277d023ebcd..b55e6ef4d677 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -124,7 +124,7 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
>  		 * The same behavior and code as single_open().  Returns
>  		 * !NULL if pos is at the beginning; otherwise, NULL.
>  		 */
> -		return NULL + !*ppos;
> +		return (void *)(uintptr_t)!*ppos;
>  	}
>  }
>  
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 31219c1db17d..d456468eb934 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -526,7 +526,7 @@ EXPORT_SYMBOL(seq_dentry);
>  
>  static void *single_start(struct seq_file *p, loff_t *pos)
>  {
> -	return NULL + (*pos == 0);
> +	return (void *)(uintptr_t)(*pos == 0);
>  }
>  
>  static void *single_next(struct seq_file *p, void *v, loff_t *pos)
> -- 
> 2.27.0
> 

Re: [PATCH] seq_file: fix clang warning for NULL pointer arithmetic

[Christoph Hellwig]

> index f277d023ebcd..b55e6ef4d677 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -124,7 +124,7 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
>  		 * The same behavior and code as single_open().  Returns
>  		 * !NULL if pos is at the beginning; otherwise, NULL.
>  		 */
> -		return NULL + !*ppos;
> +		return (void *)(uintptr_t)!*ppos;

Yikes.  This is just horrible, why bnot the completely obvious:

	if (ops->seq_start) {
		...
		return next;
	}

	if (*ppos)
		return NULL;
	return ppos; /* random cookie */

>  static void *single_start(struct seq_file *p, loff_t *pos)
>  {
> -	return NULL + (*pos == 0);
> +	return (void *)(uintptr_t)(*pos == 0);

Same here.

Re: [PATCH] seq_file: fix clang warning for NULL pointer arithmetic

[Arnd Bergmann]

On Tue, Oct 27, 2020 at 11:45 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> > index f277d023ebcd..b55e6ef4d677 100644
> > --- a/fs/kernfs/file.c
> > +++ b/fs/kernfs/file.c
> > @@ -124,7 +124,7 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
> >                * The same behavior and code as single_open().  Returns
> >                * !NULL if pos is at the beginning; otherwise, NULL.
> >                */
> > -             return NULL + !*ppos;
> > +             return (void *)(uintptr_t)!*ppos;
>
> Yikes.  This is just horrible, why bnot the completely obvious:
>
>         if (ops->seq_start) {
>                 ...
>                 return next;
>         }
>
>         if (*ppos)
>                 return NULL;
>         return ppos; /* random cookie */

I was trying to not change the behavior, but I guess we can do better
than either the original version mine. Not sure I'd call your version
'obvious' either though, at least it was immediately clear to me that
returning an unrelated pointer here is the right thing to do (it works,
since it is guaranteed to be neither NULL nor an error pointer
and it is never dereferenced, but it's still odd).

I'd rather define something like

#define SEQ_OPEN_SINGLE (void *)1ul

and return that here. I'll send a patch doing that, let me know what
you think.

     Arnd