linux-kernel - Re: [git pull] vfs.git part 2

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Date:	Fri, 12 Jul 2013 12:02:45 +0000
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	linux-kernel@...r.kernel.org
Cc:	linux-fsdevel@...r.kernel.org
Subject: Re: [git pull] vfs.git part 2

Al Viro <viro@...IV.linux.org.uk> writes:

> On Thu, Jul 11, 2013 at 02:42:54PM -0700, Linus Torvalds wrote:
>> But with an *old* kernel, O_TMPFILE will just silently be ignored as
>> an unrecognized flag, and things won't work. If you do
>> 
>>   fd = open("dirname", O_CREAT | O_TMPFILE | O_RDWR, 0666);
>> 
>> it may be that it ends up acting as a "create file at specified
>> directory path" instead of what the user *meant* for it to do, which
>> was "create unnamed temporary file in the specified directory".
>> 
>
> It's slightly less painful than that - if dirname exists, the old kernels
> will fail; O_CREAT for existing directory means an error.

But isn't the problem the case where dirname does not exist? I.e., the
application has to make sure that "/some/where" exists and is a directory
before open("/some/where", O_CREAT | O_TMPFILE | O_RDWR, 0666) can be
relied upon to fail on kernels not recognizing O_TMPFILE, instead of
just creating "where" in "/some".

Just thinking out loud, and please tell me to shut up if it doesn't make
sense: The documentation for O_DIRECTORY seems to imply that one could
require O_DIRECTORY to be given when using O_TMPFILE. The "If pathname
is not a directory, cause the open to fail" certainly seems to make
sense when O_TMPFILE is used, and older kernels should complain when
seeing the O_CREAT|O_DIRECTORY combination. It is a hack, though.

Rasmus

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