linux-kernel - Re: [PATCH] move IMMUTABLE|APPEND checks to notify

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Message-ID: <20060809141100.GP29920@ftp.linux.org.uk>
Date:	Wed, 9 Aug 2006 15:11:00 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Dmitry Mishin <dim@...nvz.org>
Cc:	Kirill Korotaev <dev@...ru>, Andrew Morton <akpm@...l.org>,
	viro@...iv.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] move IMMUTABLE|APPEND checks to notify_change()

On Wed, Aug 09, 2006 at 11:15:12AM +0400, Dmitry Mishin wrote:
> Do you meant utimes(file, NULL)?
> But is it correct behaviour? Why then do you get -EPERM on utimes(file, smth) 
> if the file is append-only? And why do you get -EACCESS on utimes(file, 
> NULL), if this file is immutable?
> 
> Could you explain, why is it done so?

RTFPOSIX...

Short version:
	* immutable files are immutable, including metadata
	* append-only files may be touched (when you write to the end), which
means that you can touch them.  Which is what utimes(file, NULL) does.
	* you can not truncate append-only file, overwrite already written
data or set timestamps to arbitrary values.

That's where the difference between utimes(file, NULL) and utimes(file, p)
is - the former basically is a write-without-write ("touch foo") and the
latter directly assigns to timestamps.  Permissions needed for these are
obviously different.

Please, read POSIX/SuS when modifying behaviour of syscalls.  Really.
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