Re: [PATCH v3] seq_file: fix clang warning for NULL pointer arithmetic

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Oct 27, 2020 at 11:18:24PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Clang points out that adding something to NULL is notallowed
                                                    not allowed
> 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 code to be extra explicit about the valid, giving

                                                   valid what?

> them named SEQ_OPEN_EOF and SEQ_OPEN_SINGLE definitions.
> 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>

Aside from the minor nits in the commit message, this should keep the
same behavior while eliminating the warning and making it a little
easier to understand.

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

> ---
> v2: add the named macros after Christoph Hellwig pointed out
> that my original logic was too ugly.
> Suggestions for better names welcome
> 
> v3: don't overload the NULL return, avoid ?: operator
> ---
>  fs/kernfs/file.c         | 9 ++++++---
>  fs/seq_file.c            | 5 ++++-
>  include/linux/seq_file.h | 2 ++
>  3 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> index f277d023ebcd..5a5adb03c6df 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -121,10 +121,13 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
>  		return next;
>  	} else {
>  		/*
> -		 * The same behavior and code as single_open().  Returns
> -		 * !NULL if pos is at the beginning; otherwise, NULL.
> +		 * The same behavior and code as single_open().  Continues
> +		 * if pos is at the beginning; otherwise, NULL.
>  		 */
> -		return NULL + !*ppos;
> +		if (*ppos)
> +			return NULL;
> +
> +		return SEQ_OPEN_SINGLE;
>  	}
>  }
>  
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 31219c1db17d..6b467d769501 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -526,7 +526,10 @@ EXPORT_SYMBOL(seq_dentry);
>  
>  static void *single_start(struct seq_file *p, loff_t *pos)
>  {
> -	return NULL + (*pos == 0);
> +	if (*pos)
> +	       return NULL;
> +
> +	return SEQ_OPEN_SINGLE;
>  }
>  
>  static void *single_next(struct seq_file *p, void *v, loff_t *pos)
> diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
> index 813614d4b71f..eb344448d4da 100644
> --- a/include/linux/seq_file.h
> +++ b/include/linux/seq_file.h
> @@ -37,6 +37,8 @@ struct seq_operations {
>  
>  #define SEQ_SKIP 1
>  
> +#define SEQ_OPEN_SINGLE	(void *)1
> +
>  /**
>   * seq_has_overflowed - check if the buffer has overflowed
>   * @m: the seq_file handle
> -- 
> 2.27.0
>