Re: [Fwd: [Bug 136] New: FSID returned from statvfs always 0]

[Andries Brouwer <aebr@win.tue.nl>]

On Tue, Dec 03, 2002 at 03:53:27PM -0800, Andrew Morton wrote:

> I've been hit with a bugzilla record alleging that statfs
> is failing to initialise the statfs.f_fsid field.
> 
> Can anyone suggest whether we should be putting something in 
> there and if so, what?
> 
> The submitter suggests using part of the UUID.
> 
> What is is used for?

There are system calls statfs (Linux, FreeBSD, SunOS, HPUX, 4.4BSD, not POSIX)
and statvfs (Solaris, POSIX) that return a struct with a f_fsid field.

For statvfs the type is unsigned long. (In struct defined in <sys/statvfs.h>.)
For statfs the type is fsid_t with

	typedef struct { int val[2]; } fsid_t;

(In struct defined in <sys/vfs.h>, but FreeBSD uses <sys/mount.h>.)

The general idea is that f_fsid contains some random stuff such that
the pair (f_fsid,ino) uniquely determines a file.

Some OSes use (a variation on) the device number, or the device number
combined with the filesystem type.

Several OSes restrict giving out the f_fsid field to the superuser only,
(and zero it for nonprivileged users) because this field is used in the
filehandle of the filesystem when NFS-exported, and giving it out is a
security concern.

Under some OSes the fsid can be used as second parameter to the sysfs
system call.

Andries


[P.S. This is not used anywhere in Linux, I think.]