Re: [PATCH net v2 1/3] class: add class_has_file_ns() helper function

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Apr 04, 2020 at 02:18:27PM +0000, Taehee Yoo wrote:
> The new helper function is to check whether the class file is existing
> or not. This function will be used by networking stack to
> check "/sys/class/net/*" file.
> 
> Reported-by: syzbot+830c6dbfc71edc4f0b8f@syzkaller.appspotmail.com
> Fixes: b76cdba9cdb2 ("[PATCH] bonding: add sysfs functionality to bonding (large)")
> Signed-off-by: Taehee Yoo <ap420073@gmail.com>
> ---
> 
> v1 -> v2:
>  - Implement class_has_file_ns() instead of class_find_and_get_file_ns().
>  - Change headline.
>  - Add kernel documentation comment.
> 
>  drivers/base/class.c         | 22 ++++++++++++++++++++++
>  include/linux/device/class.h |  3 ++-
>  2 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/class.c b/drivers/base/class.c
> index bcd410e6d70a..a2f2787f6aa7 100644
> --- a/drivers/base/class.c
> +++ b/drivers/base/class.c
> @@ -105,6 +105,28 @@ void class_remove_file_ns(struct class *cls, const struct class_attribute *attr,
>  		sysfs_remove_file_ns(&cls->p->subsys.kobj, &attr->attr, ns);
>  }
>  
> +/**
> + * class_has_file_ns - check whether file is existing or not
> + * @cls: the compatibility class
> + * @name: name to look for
> + * @ns: the namespace tag to use
> + */
> +bool class_has_file_ns(struct class *cls, const char *name,
> +		       const void *ns)

Why would you use this?  And what happens if the file shows up, or goes
away, instantly after this call is made?

This feels very broken.

greg k-h