Re: [PATCH v2 0/2] New fanotify API for ignoring events

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Jun 24, 2022 at 5:35 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> Hi Jan,
>
> As we discussed [1], here is the implementation of the new
> FAN_MARK_IGNORE API, to try and sort the historic mess of
> FAN_MARK_IGNORED_MASK.
>

Jan,

When we started talking about adding FAN_MARK_IGNORE
it was to address one specific flaw of FAN_MARK_IGNORED_MASK,
but after staring at the API for some time, I realized there are other
wrinkles with FAN_MARK_IGNORED_MASK that could be addressed
by a fresh new API.

I added more input validations following the EEXIST that you requested.
The new errors can be seen in the ERRORS section of the man page [3].
The new restrictions will reduce the size of the test matrix, but I did not
update the LTP tests [2] to check the new restrictions yet.

I do not plan to post v3 patches before improving the LTP tests,
but I wanted to send this heads up as an API proposal review.
The kernel commit that adds FAN_MARK_IGNORE [1] summarize the
new API restrictions as follows:

    The new behavior is non-downgradable.  After calling fanotify_mark() with
    FAN_MARK_IGNORE once, calling fanotify_mark() with FAN_MARK_IGNORED_MASK
    on the same object will return EEXIST error.

    Setting the event flags with FAN_MARK_IGNORE on a non-dir inode mark
    has no meaning and will return ENOTDIR error.

    The meaning of FAN_MARK_IGNORED_SURV_MODIFY is preserved with the new
    FAN_MARK_IGNORE flag, but with a few semantic differences:

    1. FAN_MARK_IGNORED_SURV_MODIFY is required for filesystem and mount
       marks and on an inode mark on a directory. Omitting this flag
       will return EINVAL or EISDIR error.

    2. An ignore mask on a non-directory inode that survives modify could
       never be downgraded to an ignore mask that does not survive modify.
       With new FAN_MARK_IGNORE semantics we make that rule explicit -
       trying to update a surviving ignore mask without the flag
       FAN_MARK_IGNORED_SURV_MODIFY will return EEXIST error.

    The conveniene macro FAN_MARK_IGNORE_SURV is added for
    (FAN_MARK_IGNORE | FAN_MARK_IGNORED_SURV_MODIFY), because the
    common case should use short constant names.

Comments and questions on the API changes are most welcome.
Suggestion for more restrictions as well.

Thanks,
Amir.

[1] https://github.com/amir73il/linux/commits/fan_mark_ignore
[2] https://github.com/amir73il/ltp/commits/fan_mark_ignore
[3] https://github.com/amir73il/man-pages/commits/fan_mark_ignore