Re: recursive commands

[Craig Allan Jeffree]

Oystein Viggen wrote:

> * [Craig Allan Jeffree] 
>
> > > old = open(".");
> > > chdir("foo);
> > > delete stuff
> > > fchdir(old);
> > > delete more stuff
> > Isn't this potentially racey as well?  I can see that fchdir(old); will
> > definately take you back to the parent of "foo" but how can you be sure
> > that the directories aren't moved around between step 1 and 2?  You
> > could end up in a different "foo" to what you intended, I guess this is
> > a different issue - but it is still an issue isn't it?
>
> I think the way rm already does that part is: 
>
> d=open("foo", 'r', O_DIRECTORY|O_NOFOLLOW); 
> fchdir(d);
>
> If you don't do that, even in regular unix, a user can rename the dir
> and exchange it with a symlink to / or whatever beneath your feet
> between steps 1 and 2.
>
> I've had other things to do in the easter, but I'll see if I can come up
> with some translator stuff on top of this soon  :)
>
> Oystein
This would just keep going back a step.  "foo" could be changed at any 
point between its discovery and the open syscall.  Obviously the user 
could look at the directory listing then run "rm -r foo", but in the 
time it takes for them to type the command someone else could change the 
directory tree - that is always going to be racey.  But internally to rm 
how can the discovery of a directory (from the directory recurse not the 
command line) be kept atomic with the entering of that directory?

Regards,
Craig.