Miklos Szeredi wrote:
Miklos Szeredi wrote:
I don't think Christoph will like the patch better, regardless of how
I change the description.
Of course, I'm open to suggestion on how to improve the interface, but
I think fundamentally this is the only way to correctly deal with the
below problem.
Anyway, here's the patch another time, please consider adding it to
-mm. For 2.6.25 obviously.
Thanks,
Miklos
----
From: Miklos Szeredi <mszeredi@xxxxxxx>
Add a new file operation: f_op->fgetattr(), that is invoked by
fstat(). Fall back to i_op->getattr() if it is not defined.
We need this because fstat() semantics can in some cases be better
implemented if the filesystem has the open file available.
Let's take the following example: we have a network filesystem, with
the server implemented as an unprivileged userspace process running on
a UNIX system (this is basically what sshfs does).
We want the filesystem to follow the familiar UNIX file semantics as
closely as possible. If for example we have this sequence of events,
we still would like fstat to work correctly:
1) file X is opened on client
2) file X is renamed to Y on server
3) fstat() is performed on open file descriptor on client
This is only possible if the filesystem server acutally uses fstat()
on a file descriptor obtained when the file was opened. Which means,
the filesystem client needs a way to get this information from the
VFS.
This true iff the protocol that this mythical
Not mythical at all. As noted in the description, there's sshfs, a
live and quite popular example of this sort of filesystem.
network file system uses the name of the file on the server to
actually identify the file on the server.
The constraint is that the server has to be an ordinary unprivileged
process. How should it identify the file, other than by name, or by
an open file descriptor?
I explained this. The fileid and the generation count along
with the file system id will uniquely identify the file.
Clearly, this is broken on many levels. It can't handle
situations as described nor can it handle different instances
of the same filename being used.
Can you please give concrete examples what it can't handle, and how
should the implementation be improved to be able to handle it, given
the above constraints?
This is why NFS, a network file system, does not use the filename
as part of the file handle.
And the nfs server isn't a userspace process, or if it is, it must use
horrible hacks to convert the file handle to a name, that don't work
half the time.
Nice try. Wrong. Try a different rationalization.
Wouldn't you be better off by attempting to implement an "open
by ino" operation and an operation to get the generation count
for the file and then modifying the network protocol of interest
to use these as identifiers for the file to be manipulated?
You mean an "open by inode" on the userspace API? My guess, it
wouldn't get very far.
This isn't a new idea and has been implemented on a variety of
different systems.
Anyway, that would still not work on old servers, and servers running
other OS's.
I didn't think that we were talking about old servers and other
OS's. My concern at the moment is Linux and the changes being
made to it.
Note, the point is _not_ to make a brand new NFS replacement
filesystem, that can use names instead of file handles. The point is
to use existing infrastructure, to make the setup as easy as ssh'ing
to a different machine. And sshfs does just that.
And the solution is limiting. It is not scalable nor particularly
interesting to anyone interested in security. Unless there is a
way of limiting access to a particular set of files, then it is
not generally useful outside of hackers or perhaps small groups
of users not concerned about too many aspects of security.
I am not interested in an extended discussion of this topic.
Thanx...
ps
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