linux-kernel - Re: [PATCH] UBIFS: fill f

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Date:	Tue, 02 Sep 2008 23:32:07 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Andreas Dilger <adilger@....com>,
	Artem Bityutskiy <dedekind@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Adrian Hunter <ext-adrian.hunter@...ia.com>
Subject: Re: [PATCH] UBIFS: fill f_fsid

On Tue, 2008-09-02 at 17:02 -0400, Christoph Hellwig wrote:
> Umm, different things.  f_fsid in stat(v)fs is just a cookie exported to
> userspac that has never really been documented.

Our man page for statfs(2) says...

 The f_fsid field

   Solaris,  Irix  and  POSIX have a system call statvfs(2) that returns a
   struct statvfs (defined in <sys/statvfs.h>) containing an unsigned long
   f_fsid.   Linux,  SunOS, HP-UX, 4.4BSD have a system call statfs() that
   returns a struct statfs (defined in <sys/vfs.h>)  containing  a  fsid_t
   f_fsid,  where  fsid_t  is defined as struct { int val[2]; }.  The same
   holds for FreeBSD, except that it uses the include file  <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 unprivileged 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.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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