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SubjectRe: [RFC] correct flags to f_mode conversion in __dentry_open
On Wed, 21 May 2008, Michael Kerrisk wrote:

> Christoph Hellwig wrote:
> > On Sat, Mar 15, 2008 at 05:59:52PM -0400, Alan Cox wrote:
> >>> does 11 really mean and should it really always be mapped to (FMODE_READ
> >>> | FMODE_WRITE) or should it continue to get mapped to 'no permission?'
> >> We've always mapped 3 to "no permission" to read or write. It's a linuxism
> >
> > I've tripped over this recently aswell. It would for sure be useful
> > to add a sumbolic O_FOO constant for this magic value '3' and document
> > it in the manpage.
>
> Late follow-up to this thread
> (http://thread.gmane.org/gmane.linux.kernel/653123):
> I propose to add the following text to the open(2) man page.
>
> Unlike the other values that can be specified in flags,
> the access mode values O_RDONLY, O_WRONLY, and O_RDWR, do
> not specify individual bits. Rather, they define the low
> order two bits of flags, and are defined respectively as
> 0, 1, and 2. In other words, the combination O_RDONLY |
> O_WRONLY is a logical error, and certainly does not have
> the same meaning as O_RDWR. Linux reserves the special,
> non-standard access mode 3 (binary 11) in flags to mean:
> check for read and write permission on the file and
> return a descriptor that can't be used for reading or
> writing. This non-standard access mode is used by some
> Linux drivers to return a descriptor that is only to be
> used for device-specific ioctl(2) operations.
>
> Seem okay?

Looks good to me. We definitely need this documented.



- James
--
James Morris
<jmorris@namei.org>


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