linux-kernel - Re: statx(2) API and documentation

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Date:   Thu, 18 Oct 2018 01:15:13 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: statx(2) API and documentation

On Wed, Oct 17, 2018 at 10:12 PM Miklos Szeredi <miklos@...redi.hu> wrote:

> >> - STATX_ALL definition is unclear, can this change, or is it fixed?
> >> If it's the former, than that's a backward compatibility nightmare.
> >> If it's the latter, then what's the point?
> >
> > The value can change over time.  It is intended to reflect the current
> > state of affairs at the time the userspace program and kernel are compiled.
> > The value sent from userspace lets the kernel know what fields are in
> > the userspace struct, so it doesn't try to set fields that aren't there.
>
> What's the point of a userspace program specifying STATX_ALL?  Without
> a way to programmatically query the interface definition it's useless:
> there's no way to guess which mask bit corresponds to which field, and
> what that field represents.
>
> And there will be programs out there which specify STATX_ALL without
> considering that in the future it may become slower as it is now due
> to a recompile.
>
> So what's the point exactly?
>
> > The value in the kernel allows masking off new fields from userspace that
> > it doesn't understand.
>
> Okay, but that has nothing to do with the UAPI.  Even as an internal
> flag we should be careful, as it might grow uses which can have
> similar issues as the userspace one above.
>

FYI, I identified a similar anti-pattern in fanotify UAPI when I wanted to
add new flags and did not want to change the UAPI _ALL_ constants.
This is how we plan to solve it:
https://github.com/amir73il/linux/commit/8c2b1acadb88ee4505ccc8bfdc665863111fb4cc

Thanks,
Amir.