Re: [PATCH v7 22/28] fanotify: Report FID entry even for zero-length file_handle

[Amir Goldstein <amir73il@xxxxxxxxx>]

On Fri, Oct 15, 2021 at 12:39 AM Gabriel Krisman Bertazi
<krisman@xxxxxxxxxxxxx> wrote:
>
> Non-inode errors will reported with an empty file_handle.  In
> preparation for that, allow some events to print the FID record even if
> there isn't any file_handle encoded
>
> Even though FILEID_ROOT is used internally, make zero-length file
> handles be reported as FILEID_INVALID.
>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
> ---
>  fs/notify/fanotify/fanotify_user.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 5324890500fc..39cf8ba4a6ce 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -127,6 +127,16 @@ static int fanotify_fid_info_len(int fh_len, int name_len)
>                        FANOTIFY_EVENT_ALIGN);
>  }
>
> +static bool fanotify_event_allows_empty_fh(struct fanotify_event *event)
> +{
> +       switch (event->type) {
> +       case FANOTIFY_EVENT_TYPE_FS_ERROR:
> +               return true;
> +       default:
> +               return false;
> +       }
> +}
> +
>  static size_t fanotify_event_len(unsigned int info_mode,
>                                  struct fanotify_event *event)
>  {
> @@ -157,7 +167,7 @@ static size_t fanotify_event_len(unsigned int info_mode,
>         if (info_mode & FAN_REPORT_PIDFD)
>                 event_len += FANOTIFY_PIDFD_INFO_HDR_LEN;
>
> -       if (fh_len)
> +       if (fh_len || fanotify_event_allows_empty_fh(event))
>                 event_len += fanotify_fid_info_len(fh_len, dot_len);
>
>         return event_len;
> @@ -338,9 +348,6 @@ static int copy_fid_info_to_user(__kernel_fsid_t *fsid, struct fanotify_fh *fh,
>         pr_debug("%s: fh_len=%zu name_len=%zu, info_len=%zu, count=%zu\n",
>                  __func__, fh_len, name_len, info_len, count);
>
> -       if (!fh_len)
> -               return 0;
> -
>         if (WARN_ON_ONCE(len < sizeof(info) || len > count))
>                 return -EFAULT;
>
> @@ -375,6 +382,11 @@ static int copy_fid_info_to_user(__kernel_fsid_t *fsid, struct fanotify_fh *fh,
>
>         handle.handle_type = fh->type;
>         handle.handle_bytes = fh_len;
> +
> +       /* Mangle handle_type for bad file_handle */
> +       if (!fh_len)
> +               handle.handle_type = FILEID_INVALID;
> +
>         if (copy_to_user(buf, &handle, sizeof(handle)))
>                 return -EFAULT;
>
> @@ -467,7 +479,8 @@ static int copy_info_records_to_user(struct fanotify_event *event,
>                 total_bytes += ret;
>         }
>
> -       if (fanotify_event_object_fh_len(event)) {
> +       if (fanotify_event_object_fh_len(event) ||
> +           fanotify_event_allows_empty_fh(event)) {
>                 const char *dot = NULL;
>                 int dot_len = 0;
>

I don't like this fanotify_event_allows_empty_fh() implementation so much.

How about this instead:

static inline struct fanotify_fh *fanotify_event_object_fh(
                                                struct fanotify_event *event)
{
        struct fanotify_fh *fh = NULL;

        /* An error event encodes (a FILEID_INVAL) fh for an empty fh */
        if (event->type == FANOTIFY_EVENT_TYPE_FS_ERROR)
                return &FANOTIFY_EE(event)->object_fh;
        else if (event->type == FANOTIFY_EVENT_TYPE_FID)
                fh = &FANOTIFY_FE(event)->object_fh;
        else if (event->type == FANOTIFY_EVENT_TYPE_FID_NAME)
                fh = fanotify_info_file_fh(&FANOTIFY_NE(event)->info);

        if (!fh && !fh->len)
                return NULL;

        return fh;
}

        struct fanotify_fh *object_fh = fanotify_event_object_fh(event);
...

-       if (fanotify_event_object_fh_len(event)) {
+       if (object_fh) {
                const char *dot = NULL;
...
                ret = copy_fid_info_to_user(fanotify_event_fsid(event),
-                                           fanotify_event_object_fh(event),
+                                          object_fh,
                                            info_type, dot, dot_len,
                                            buf, count);
...

And similar change to fanotify_event_len()

This way, the logic of whether to report fh or not is encoded in
fanotify_event_object_fh() and fanotify_event_object_fh_len()
goes back to being a property of the the fh report.

Thanks,
Amir.