On Fri, Oct 05, 2018 at 08:10:32PM +0200, Adam Borowski wrote:
> On Fri, Oct 05, 2018 at 09:06:00AM -0700, Darrick J. Wong wrote:
> > On Fri, Oct 05, 2018 at 02:27:41PM +0200, Adam Borowski wrote:
> > > Needed by users of fstatfs().
> >
> > NAK.
> >
> > The XFS superblock magic number is part of the on-disk format definition
> > of XFS. It belongs with all the other on-disk format definitions in this
> > file.
> >
> > I don't think it's a good idea for userspace to associate s_magic with
> > userspace API feature sets, though the cat's long escaped the bag on
> > that one.
> >
> > With that being reality, "the XFS superblock magic number" has a
> > different semantic meaning than "the agreed upon statfs.f_type value for
> > XFS", which means the latter should have a different symbol name to
> > reflect that difference.
>
> Ie, you'd be ok with the same value to be defined in two places -- do I
> understand this right?
>
> > > --- a/fs/xfs/libxfs/xfs_format.h
> > > -#define XFS_SB_MAGIC 0x58465342 /* 'XFSB' */
> > > +#define XFS_SB_MAGIC XFS_SUPER_MAGIC /* 0x58465342 = 'XFSB' */
> > > --- a/include/uapi/linux/magic.h
> > > #define JFFS2_SUPER_MAGIC 0x72b6
> > > +#define XFS_SUPER_MAGIC 0x58465342 /* "XFSB" */
> > > #define PSTOREFS_MAGIC 0x6165676C
Sorry, hit send too fast.
include/uapi/linux/magic.h would get:
#define XFS_STATFS_MAGIC 0x58465342 /* 'XFSB' */
fs/xfs/libxfs/xfs_format.h would not be changed at all:
#define XFS_SB_MAGIC 0x58465342 /* 'XFSB' */
fs/xfs/xfs_super.c would then be changed to:
xfs_fs_statfs(...) {
statp->f_type = XFS_STATFS_MAGIC;
}
xfs_fs_fill_super(...) {
sb->s_magic = XFS_STATFS_MAGIC;
}
We're making two separate promises here:
"The on-disk superblock magic is XFS_SB_MAGIC."
"When XFS is the filesystem, statfs will return XFS_STATFS_MAGIC."
Two completely independent #defines. It just happens to be coincidental
that they have the same numeric value.
--D
>
>
> Meow!
> --
> ⢀⣴⠾⠻⢶⣦⠀
> ⣾⠁⢰⠒⠀⣿⡁ 10 people enter a bar: 1 who understands binary,
> ⢿⡄⠘⠷⠚⠋⠀ 1 who doesn't, D who prefer to write it as hex,
> ⠈⠳⣄⠀⠀⠀⠀ and 1 who narrowly avoided an off-by-one error.
