closedir - change in the libc

[Jilles Tjoelker]

On Fri, Aug 09, 2013 at 11:59:48AM +0200, Mario Oshogbo wrote:
> I'm a student attending in GSoC'13 in project "Write new features for
> Capsicum" [1] and I would like to propose a little change to the libc.

> In the libc we have the function opendir(3) and fdopendir(3) which are
> responsible for opening directories. We also have function closedir(3)
> which is responsible for free the structure allocated by two previous
> funcions.

> The problem with the closedir(3) is that he close fd used as argument of
> fdopendir(3). I think programmer should be able to make a choice if he
> want to close this descriptor automatically or not. Of course we are
> able to use for dup(2) function and save fd before calling fdopendir(3)
> to prevent closing but I think this is the one syscall which we could save.

> To support the propose I would like to give some example. I working now
> on making the fts(3) more sandbox friendly which means I would like to
> remove all fchdir(2) functions and operate only on fd. One of the steps
> I perform is changing the opendir(3) to the fdopendir(3). I also must
> remember the fd of currently opened directory after perform fclosedir.
> If we have N of directories we must perform N additional dup(2) and N
> additional close(2) functions to save this fd.

> So I would like to propose function called fdclosedir which will free
> DIR structure but will not close fd attached to DIR structure. This
> function also could return this fd which make it useful with the
> opendir(3) function, when we don't know the fd but we would like free
> DIR structure and perform some operations only on fd. I attach diff file
> with my proposal of change.

While this looks sensible, the only advantage is cleaner code and less
memory usage. Instead of additional dup()/close(), you could also keep
the DIR around as long as you need the fd. The ugliest part is that you
may also have regular open()s on directories (for example if the maximum
number of file descriptors had been exceeded and some directories had to
be closed and now need to be reopened). I think this should remain
doable, though.

-- 
Jilles Tjoelker