linux-kernel - Re: [PATCH 05/11] vfs: add RENAME

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Date:	Wed, 15 Jan 2014 10:26:23 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Zach Brown <zab@...hat.com>, Jan Kara <jack@...e.cz>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>
Subject: Re: [PATCH 05/11] vfs: add RENAME_NOREPLACE flag

On Wed, Jan 15, 2014 at 10:19 AM, J. Bruce Fields <bfields@...ldses.org> wrote:
> On Wed, Jan 08, 2014 at 11:10:09PM +0100, Miklos Szeredi wrote:
>> From: Miklos Szeredi <mszeredi@...e.cz>
>>
>> If this flag is specified and the target of the rename exists then the
>> rename syscall fails with EEXIST.
>
> Why is this useful?

The trivial answer: to eliminate the race condition from 'mv -i'.

Another answer: there's a common pattern to atomically create a file
with contents: open a temporary file, write to it, optionally fsync
it, close it, then link(2) it to the final name, then unlink the
temporary file.

The reason to use link(2) is because it won't silently clobber the destination.

This is annoying:
 - It requires an extra system call that shouldn't be necessary.
 - It doesn't work on (IMO sensible) filesystems that don't support
hard links (e.g. vfat).
 - It's not atomic -- there's an intermediate state where both files exist.
 - It's ugly.

The new rename flag will make this totally sensible.

To be fair, on new enough kernels, you can also use O_TMPFILE and
linkat to achieve the same thing even more cleanly.

--Andy
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