Re: [PATCH]: NFS: fix inadvertent reversion of mode bits during chown

[Peter Staubach <staubach@redhat.com>]

Trond Myklebust wrote:

>On Thu, 2005-12-15 at 13:19 -0500, Peter Staubach wrote:
>  
>
>>It seems to me that there is a third option.  That would be to provide the
>>proper synchronization to prevent such races.  Any callers wishing to use
>>either the mode or owners of the file should have validated the attributes
>>first, and if there is such a setuid/setgid operation going on, then block
>>them until that operation has completed.  Perhaps something like the
>>revalidate hook can be used by the NFS client to synchronize things.
>>    
>>
>
>That also requires the VFS change to move the KILL_SUID/KILL_SGID stuff
>from notify_change() and into inode_setattr()/the filesystem code.
>
>  
>

Well, that isn't exactly what I had in mind, but I would need to look at
the implementation more in order to describe a design better.

>>We already know that it is not safe to combine changing the mode and the
>>uid or gid in a single NFS SETATTR request.  The server may or may not allow
>>the owners to be changed and so, may or may not perform the mode change.  In
>>fact, it is not terribly safe to combine changing the uid or gid with any
>>other attributes in a single call.  This is an unfortunate fact of life
>>when attempting to be interoperable across a large variety of platforms.
>>    
>>
>
>Yes, but that is why I'd argue that a server claiming to support
>suid/sgid mode bits had better have a _very_ good reason if it fails to
>clear them when a client changes the uid/gid.
>

The most common reason that I have seen is that the server may not be a 
POSIX
based server and may not emulate all of these semantics correctly.  
There are
times when the client has to interoperate with a "broken" server and for 
those
instances, we do not very aesthetic things in the client to make up for it.

    Thanx...

       ps


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