Re: [PATCH RFC 09/15] fanotify: Introduce generic error record

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Apr 26, 2021 at 9:42 PM Gabriel Krisman Bertazi
<krisman@collabora.com> wrote:
>
> This record describes a fs error in a fs agnostic way.  It will be send
> back to userspace in response to a FSNOTIFY_EVENT_ERROR for groups with
> the FAN_ERROR mark.

It's not a mark, it's an event, so:
"...for groups with the FAN_ERROR event in their mark mask"

>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
> ---
>  fs/notify/fanotify/fanotify.h      | 16 ++++++++++++++++
>  fs/notify/fanotify/fanotify_user.c | 28 ++++++++++++++++++++++++++++
>  include/uapi/linux/fanotify.h      | 10 ++++++++++
>  3 files changed, 54 insertions(+)
>
> diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
> index 47299e3d6efd..4cb9dd31f084 100644
> --- a/fs/notify/fanotify/fanotify.h
> +++ b/fs/notify/fanotify/fanotify.h
> @@ -179,6 +179,22 @@ FANOTIFY_NE(struct fanotify_event *event)
>         return container_of(event, struct fanotify_name_event, fae);
>  }
>
> +struct fanotify_error_event {
> +       struct fanotify_event fae;
> +       int error;
> +       __kernel_fsid_t fsid;
> +
> +       int fs_data_size;
> +       /* Must be the last item in the structure */
> +       char fs_data[0];
> +};
> +
> +static inline struct fanotify_error_event *
> +FANOTIFY_EE(struct fanotify_event *event)
> +{
> +       return container_of(event, struct fanotify_error_event, fae);
> +}
> +
>  static inline __kernel_fsid_t *fanotify_event_fsid(struct fanotify_event *event)
>  {
>         if (event->type == FANOTIFY_EVENT_TYPE_FID)
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 5031198bf7db..21162d347bd1 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -64,6 +64,11 @@ static int fanotify_fid_info_len(int fh_len, int name_len)
>         return roundup(FANOTIFY_INFO_HDR_LEN + info_len, FANOTIFY_EVENT_ALIGN);
>  }
>
> +static size_t fanotify_error_info_len(struct fanotify_error_event *fee)
> +{
> +       return sizeof(struct fanotify_event_info_error);
> +}
> +
>  static size_t fanotify_event_len(struct fanotify_event *event,
>                                  unsigned int fid_mode)
>  {
> @@ -232,6 +237,29 @@ static int process_access_response(struct fsnotify_group *group,
>         return -ENOENT;
>  }
>
> +static size_t copy_error_info_to_user(struct fanotify_error_event *fee,
> +                                     char __user *buf, int count)
> +{
> +       struct fanotify_event_info_error info;
> +
> +       info.hdr.info_type = FAN_EVENT_INFO_TYPE_ERROR;
> +       info.hdr.pad = 0;
> +       info.hdr.len = fanotify_error_info_len(fee);
> +
> +       if (WARN_ON(count < info.hdr.len))
> +               return -EFAULT;
> +
> +       info.version = FANOTIFY_EVENT_INFO_ERROR_VERS_1;
> +       info.error = fee->error;
> +       info.fsid = fee->fsid;
> +
> +       if (copy_to_user(buf, &info, sizeof(info)))
> +               return -EFAULT;
> +
> +       return info.hdr.len;
> +
> +}
> +
>  static int copy_info_to_user(__kernel_fsid_t *fsid, struct fanotify_fh *fh,
>                              int info_type, const char *name, size_t name_len,
>                              char __user *buf, size_t count)
> diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
> index b283531549f1..cc9a1fa80e30 100644
> --- a/include/uapi/linux/fanotify.h
> +++ b/include/uapi/linux/fanotify.h
> @@ -124,6 +124,7 @@ struct fanotify_event_metadata {
>  #define FAN_EVENT_INFO_TYPE_FID                1
>  #define FAN_EVENT_INFO_TYPE_DFID_NAME  2
>  #define FAN_EVENT_INFO_TYPE_DFID       3
> +#define FAN_EVENT_INFO_TYPE_ERROR      4
>
>  /* Variable length info record following event metadata */
>  struct fanotify_event_info_header {
> @@ -149,6 +150,15 @@ struct fanotify_event_info_fid {
>         unsigned char handle[0];
>  };
>
> +#define FANOTIFY_EVENT_INFO_ERROR_VERS_1   1

Honestly, this struct is too simple to have a 'version'.
The format of this simple struct is already defined by
FAN_EVENT_INFO_TYPE_ERROR and if we want to change
the reported info in the future, we can use
FAN_EVENT_INFO_TYPE_ERROR_V2.
In fact, I suggest to name the type
FAN_EVENT_INFO_TYPE_FS_ERROR
to differentiate from a future
FAN_EVENT_INFO_TYPE_WB_ERROR

> +
> +struct fanotify_event_info_error {
> +       struct fanotify_event_info_header hdr;
> +       int version;
> +       int error;
> +       __kernel_fsid_t fsid;
> +};

I suggest to put an error seq counter in this struct.
The per-sb seq counter can be provided by the filesystem
or by fsnotify.

Thanks,
Amir.