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SubjectRe: [RFC] O_NOACC: open without any access


On Tue, 23 Jun 2009, David Howells wrote:

> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > Define O_NOACC as 3. On open(..., O_FILESYSTEM | O_NOACC) require no
> > privileges on the file.
>
> It must also work with O_NOFOLLOW, which I think your suggestion will.

This does sound like a fairly natural extension of what we already do.

We essentially already have O_NOACCESS (3), and use it exactly because we
need to do operations on a file descriptor without "real" accesses
(notably things like accessing /dev/cdrom without waiting/checking for the
disk being present etc).

O_FILESYSTEM I don't like as a name (to me, it doesn't say _what_ it is
doing - of course an open works on a filesystem!), but the concept of
saying "don't follow device nodes - just open the node itself" makes
perfect sense. Together with O_NOFOLLOW it also fairly naturally means
"give me the actual symlink _node_, don't return error or follow it".

And we can trivially test at a higher level that O_FILESYSTEM (with a
better name, please), is always paired with O_NOACCESS (not O_NOACC: we do
not try to save three letters, there is no shortage). Because the raw node
obviously must never really be "accessed" (ie you can't do read/write etc
on it).

That said, I do _not_ like the notion of

> Add a new inode->i_filesystem_fop pointer

regardless of whether it's in inode->i_op or wherever. I think we should
just handle this in the regular "inode->f_op->open" routine, the same way
we handle FMODE_EXCLUSIVE (O_EXCL), FMODE_NDELAY (O_NONBLOCK) and lack of
access rights (O_NOACCESS) in the driver open routines that currently
handle those specially (O_NDELAY is spe

Al?

Linus


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