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

[Nathan Chancellor <nathan@kernel.org>]

Hi Maíra,

On Mon, Jan 31, 2022 at 03:22:42PM -0300, Maíra Canal wrote:
> Implement conditional logic in order to replace NULL pointer arithmetic.
> 
> The use of NULL pointer arithmetic was pointed out by clang with the
> following warning:
> 
> fs/kernfs/file.c:128:15: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>                 return NULL + !*ppos;
>                        ~~~~ ^
> fs/seq_file.c:559:14: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>         return NULL + (*pos == 0);
> 
> Signed-off-by: Maíra Canal <maira.canal@usp.br>

Thanks a lot for the patch!

It looks like Arnd sent a similar patch but it was never picked up:

https://lore.kernel.org/r/20201028151202.3074398-1-arnd@kernel.org/

This is a cleaner solution as it uses existing defines and functions to
deduplicate the code.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
> V1 -> V2:
> - Use SEQ_START_TOKEN instead of open-coding it
> - kernfs_seq_start call single_start instead of open-coding it
> V2 -> V3:
> - Remove the EXPORT of the single_start symbol
> ---
>  fs/kernfs/file.c         | 7 +------
>  fs/seq_file.c            | 4 ++--
>  include/linux/seq_file.h | 1 +
>  3 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> index 9414a7a60a9f..7aefaca876a0 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -120,13 +120,8 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
>  		if (next == ERR_PTR(-ENODEV))
>  			kernfs_seq_stop_active(sf, next);
>  		return next;
> -	} else {
> -		/*
> -		 * The same behavior and code as single_open().  Returns
> -		 * !NULL if pos is at the beginning; otherwise, NULL.
> -		 */
> -		return NULL + !*ppos;
>  	}
> +	return single_start(sf, ppos);
>  }
>  
>  static void *kernfs_seq_next(struct seq_file *sf, void *v, loff_t *ppos)
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index f8e1f4ee87ff..7ab8a58c29b6 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -554,9 +554,9 @@ int seq_dentry(struct seq_file *m, struct dentry *dentry, const char *esc)
>  }
>  EXPORT_SYMBOL(seq_dentry);
>  
> -static void *single_start(struct seq_file *p, loff_t *pos)
> +void *single_start(struct seq_file *p, loff_t *pos)
>  {
> -	return NULL + (*pos == 0);
> +	return *pos ? NULL : SEQ_START_TOKEN;
>  }
>  
>  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 88cc16444b43..60820ab511d2 100644
> --- a/include/linux/seq_file.h
> +++ b/include/linux/seq_file.h
> @@ -162,6 +162,7 @@ int seq_dentry(struct seq_file *, struct dentry *, const char *);
>  int seq_path_root(struct seq_file *m, const struct path *path,
>  		  const struct path *root, const char *esc);
>  
> +void *single_start(struct seq_file *, loff_t *);
>  int single_open(struct file *, int (*)(struct seq_file *, void *), void *);
>  int single_open_size(struct file *, int (*)(struct seq_file *, void *), void *, size_t);
>  int single_release(struct inode *, struct file *);
> -- 
> 2.34.1
>