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.
