linux-kernel - Re: Removing Mandatory Locks

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Message-ID: <18b073b95d692f4c7782c68de1f803681c15a467.camel@kernel.org>
Date:   Sat, 21 Aug 2021 08:45:54 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     "H. Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
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        Matthew Wilcox <willy@...radead.org>,
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        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
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        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: Removing Mandatory Locks

On Fri, 2021-08-20 at 17:29 -0400, Jeff Layton wrote:
> No, Windows has deny-mode locking at open time, but the kernel's
> mandatory locks are enforced during read/write (which is why they are
> such a pain). Samba will not miss these at all.
> 
> If we want something to provide windows-like semantics, we'd probably
> want to start with something like Pavel Shilovsky's O_DENY_* patches.
> 
> -- Jeff
> 

Doh! It completely slipped my mind about byte-range locks on windows...

Those are mandatory and they do block read and write activity to the
ranges locked. They have weird semantics vs. POSIX locks (they stack
instead of splitting/merging, etc.).

Samba emulates these with (advisory) POSIX locks in most cases. Using
mandatory locks is probably possible, but I think it would add more
potential for deadlock and security issues.
-- 
Jeff Layton <jlayton@...nel.org>