Re: problems with changing UID/GID

[Mike Touloumtzis <miket@bluemug.com>]

On Mon, Aug 26, 2002 at 02:45:51PM +0100, Alan Cox wrote:
> On Mon, 2002-08-26 at 14:30, Zheng Jian-Ming wrote:
> 
> > But, the credentials are per-task in Linux, so it's possible to have
> > two tasks in a process running under different UIDs.
> 
> Really useful isnt it
> 
> There are other reasons for wanting refcounted credential structures (eg
> NFS writeback) so it may well be that once those go in for other reasons
> it makes sense to provide an option to do shared credentials for
> threaded apps. It is however nontrivial and you might want to see how
> your other systems respond to things like a file open on a slow device
> while a second thread is strobing the uid between two values. Does it
> change uid mid syscall, does it get the permissions checks right if so ?
> 
> Its non trivial stuff, if not plain crazy to implement a literal
> interpretation of (eg does a write fail half way if you change userid in
> another thread ?)

Does POSIX really require that granularity of change?  Would an
implementation which, say, stored all credentials per-thread/task
and only picked up changes from other threads at kernel entry/exit
not be in spec?  That seems like a lower overhead solution than
locking all access to UIDs.

miket