Re: [RFC] correct flags to f_mode conversion in __dentry_open

[James Morris <jmorris@namei.org>]

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>