Re: [PATCH v4 2/5] fs: Add fchmodat2()

[Christian Brauner <brauner@kernel.org>]

On Thu, Jul 27, 2023 at 01:13:37PM -0400, dalias@libc.org wrote:
> On Thu, Jul 27, 2023 at 07:02:53PM +0200, Christian Brauner wrote:
> > On Thu, Jul 27, 2023 at 06:28:53PM +0200, Andreas Schwab wrote:
> > > On Jul 27 2023, David Laight wrote:
> > > 
> > > > From: Aleksa Sarai
> > > >> Sent: 25 July 2023 17:36
> > > > ...
> > > >> We almost certainly want to support AT_EMPTY_PATH at the same time.
> > > >> Otherwise userspace will still need to go through /proc when trying to
> > > >> chmod a file handle they have.
> > > >
> > > > That can't be allowed.
> > > 
> > > IIUC, fchmodat2(fd, "", m, AT_EMPTY_PATH) is equivalent to fchmod(fd,
> > > m).  With that, new architectures only need to implement the fchmodat2
> > > syscall to cover all chmod variants.
> > 
> > There's a difference though as fchmod() doesn't work with O_PATH file
> > descriptors while AT_EMPTY_PATH does. Similar to how fchown() doesn't
> > work with O_PATH file descriptors.
> > 
> > However, we do allow AT_EMPTY_PATH with fchownat() so there's no reason
> > to not allow it for fchmodat2().
> > 
> > But it's a bit of a shame that O_PATH looks less and less like O_PATH.
> > It came from can-do-barely-anything to can-do-quite-a-lot-of-things over
> > the years.
> > 
> > In any case, AT_EMPTY_PATH for fchmodat2() can be an additional patch on
> > top.
> 
> From a standpoint of implementing O_SEARCH/O_EXEC using it, I don't
> see any reason fchown/fchmod should not work on O_PATH file
> descriptors. And indeed when you have procfs available to emulate them
> via procfs, it already does. So I don't see this as unwanted

I'm really not talking about the fact that proc is a giant loophole for
basically everyhing related to O_PATH and reopening fds.

I'm saying that both fchmod() and fchown() don't work on O_PATH fds.
They explicitly reject them.

AT_EMPTY_PATH and therefore O_PATH for fchmodat2() is fine given that we
do it for fchownat() already.