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

[Christian Brauner]

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.