Re: Urgent help needed on an NFS question, please help!!!

[Xin Zhao]

I found where the server checks the generation number. It's in fh_verify(). :)

Many thanks for your help, Neil!

-x

On 8/10/06, Xin Zhao <uszhaoxin@xxxxxxxxx> wrote:
> I think nfs_compare_fh() might do the file handle verification task.
> However, it is still possible that AFTER C1 gets a valid file handle,
> BUT BEFORE C1 sends out the getattr() request, C2 deletes file X and
> creates a different file X1 which has the same inode number. Looks
> like the server side must verify the generation number carried in the
> file handle. Unfortunately, I didn't find this code at the server
> side. Any further insight on this?
>
> Thanks,
> Xin
>
> On 8/10/06, Neil Brown <neilb@xxxxxxx> wrote:
> > On Thursday August 10, uszhaoxin@xxxxxxxxx wrote:
> > > I just ran into a problem about NFS. It might be a fundmental problem
> > > of my current work. So please help!
> > >
> > > I am wondering how NFS guarantees a client didn't get wrong file
> > > attributes. Consider the following scenario:
> > >
> > > Suppose we have an NFS server S and two clients C1 and C2.
> > >
> > > Now C1 needs to access the file attributes of file X, it first does
> > > lookup() to get the file handle of file X.
> > >
> > > After C1 gets X's file handle and before C1 issues the getattr()
> > > request, C2 cuts in. Now C2 deletes file X and creates a new file X1,
> > > which has different name but the same inode number and device ID as
> > > the nonexistent file X.
> > >
> > > When C1 issues getattr() with the old file handle, it may get file
> > > attribute on wrong file X1. Is this true?
> > >
> > > If not, how NFS avoid this problem? Please direct me to the code that
> > > verifies this.
> >
> > Generation numbers.
> >
> > When the filesystem creates a new file it assigns a random number
> > as the 'generation' number and stores that in the inode.
> > This gets included in the filehandle, and checked when the filehandle
> > lookup is done.
> >
> > Look for references to 'i_generation' in fs/ext3/*
> >
> > Other files systems may approach this slightly differently, but the
> > filesystem is responsible for providing a unique-over-time filehandle,
> > and 'generation number' is the 'standard' way of doing this.
> >
> > NeilBrown
> >
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