Re: move all statfs f_type magic numbers to linux/magic.h

[Karel Zak <kzak@redhat.com>]

On Tue, Mar 04, 2014 at 09:58:07PM +1100, Dave Chinner wrote:
> On Tue, Mar 04, 2014 at 11:00:36AM +0100, Karel Zak wrote:
> > The file <linux/magic.h> is incomplete and some magic numbers that kernel
> > returns by statfs f_type to userspace are not defined there. This situation
> > makes statfs f_type usage in userpace painful as you have to "ifndef-define"
> > many magic numbers in userspace code.
> > 
> > All the patches are trivial and just move *_SUPER_MAGIC stuff from 
> > filesystem specific files to the generic <linux/magic.h>.
> 
> IMO, this is the wrong way to fix the statfs magic numbers into the
> user API. You're changing the code to define magic numbers that are
> on disk to be defined - and fixed permanently - by the statfs
> syscall API.

 Well, my idea was to follow the current kernel manner :-) and add the 
 missing magic numbers to the API.

 I agree that linux/magic.h is mess, and if there is call for something 
 better than I can send v2 with API cleanup.

> We need to be able to change the on-disk magic numbers whenever and
> however we want without affecting userspace

 It's still possible, just define a new FOO_SUPER_MAGIC2 in fs code 
 and keep linux/magic.h unchanged. 
 
 And not in all cases *_SUPER_MAGIC are used for on-disk stuff, very
 often it's magic number for s_magic and f_type only.
 
> If you want all the magic numbers that filesystems expose to
> userspace in statfs to be defined in one place, then you need to
> create the definitions for only that purpose that match what is

 Yes, I thought about it, but I was not sure if such change will be
 accepted.

> currently documented in the statfs() man page and use those in the
> filesystem statfs implementation rather than the on-disk superblock
> magic number definitions.
> 
> e.g. include/uapi/linux/statfs_magic.h:
> 
> #define __STATFS_ADFS_SUPER_MAGIC	0xadf5
> #define __STATFS_AFFS_SUPER_MAGIC	0xADFF
> ...

 Now filesystems usually share statfs f_type with superblock s_magic. 
 It would be probably good enough to use something like

 include/uapi/linux/statfs_magic.h:

    #define __STATFS_FOO_SUPER_MAGIC 0xadf5

 fs/foo/foo_sb.h:

    #define FOO_SUPER_MAGIC     __STATFS_FOO_SUPER_MAGIC

 to avoid multiple definitions of the same numbers.

> and then convert all the kernel filesystem statfs code to use these
> types rather than their on-disk magic numbers in f_type.
> 
> Then, for userspace, wrap them appropriately in glibc sys/statfs.h:
> 
> #include <linux/statfs_magic.h>
> 
> #define ADFS_SUPER_MAGIC	__STATFS_ADFS_SUPER_MAGIC
> #define AFFS_SUPER_MAGIC	__STATFS_AFFS_SUPER_MAGIC
 
 and what about the old linux/magic.h? I guess it's necessary
 to keep it for backward compatibility.

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com