linux-kernel - Re: [PATCH] vfs: new O

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Date:	Mon, 28 Sep 2009 14:28:45 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Valdis.Kletnieks@...edu, linux@...blig.org, agruen@...e.de,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: new O_NODE open flag

Miklos Szeredi wrote:
> BTW I just checked, and it is possible to re-open or promote an fd
> opened with O_NODE like this:
> 
> 	char tmp[64];
> 
> 	fd = open(filename, O_NODE | O_NOACCESS);
> 	/* ... */
> 	sprintf(tmp, "/proc/self/fd/%i", fd);
> 	fd_rw = open(tmp, O_RDWR);
> 
> Now fd_rw is guaranteed to refer to the same inode as fd.

If someone passes you a file descriptor opened with O_RDONLY, you
shouldn't be able to upgrade it to O_RDWR unless you have access to
the file and could do a normal open() on the file.

I hope the above cannot convert O_NOACCESS to O_RDWR without checking
that you have access to the file.

Hmm.  I have just tried, and you _can _use open("/proc/self/fd/%d",
O_RDWR) to re-open with more permissions when you can't access the
path which /proc/self/fd/%d pretends to link to.  It looks a bit
dubious, as you might have been passed an O_RDONLY descriptor with the
intention that you can't write to it...  Oh well!

-- Jamie
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