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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     bfields@fieldses.org (J. Bruce Fields)
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Date:   Thu, 19 Aug 2021 08:56:52 -0500
In-Reply-To: <20210818154217.GB24115@fieldses.org> (J. Bruce Fields's message
        of "Wed, 18 Aug 2021 11:42:17 -0400")
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Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE
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bfields@fieldses.org (J. Bruce Fields) writes:

> On Fri, Aug 13, 2021 at 05:49:19PM -0700, Andy Lutomirski wrote:
>> I’ll bite.  How about we attack this in the opposite direction: remove
>> the deny write mechanism entirely.
>
> For what it's worth, Windows has open flags that allow denying read or
> write opens.  They also made their way into the NFSv4 protocol, but
> knfsd enforces them only against other NFSv4 clients.  Last I checked,
> Samba attempted to emulate them using flock (and there's a comment to
> that effect on the flock syscall in fs/locks.c).  I don't know what Wine
> does.
>
> Pavel Shilovsky posted flags adding O_DENY* flags years ago:
>
> 	https://lwn.net/Articles/581005/
>
> I keep thinking I should look back at those some day but will probably
> never get to it.
>
> I've no idea how Windows applications use them, though I'm told it's
> common.

I don't know in any detail.  I just have this memory of not being able
to open or do anything with a file on windows while any application has
it open.

We limit mandatory locks to filesystems that have the proper mount flag
and files that are sgid but are not executable.  Reusing that limit we
could probably allow such a behavior in Linux without causing chaos.

Without being very strict about which files can participate I can just
imagine someone hiding their presence by not allowing other applications
the ability to write to utmp or a log file.

In the windows world where everything evolved with those kinds of
restrictions it is probably fine (although super annoying).

Eric