Re: [RFC PATCH 1/2] vfs: syscalls: add mkdirat_fd()

[Mateusz Guzik]

On Thu, Apr 2, 2026 at 4:52 AM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
>
> On 2026-04-01, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
> > Trying to handle this in open() is a no-go. openat2 is rather
> > problematic.
>
> I'm interested in what makes you say that. It would be very nice to be able
> to do mkdir + RESOLVE_IN_ROOT and get an fd back all in one syscall. :D
>

Not handling this in either of open or openat2 does not preclude mkdir
+ RESOLVE_IN_ROOT + getting a fd in one go from existing.

Creating a directory was always a different syscall than creating a
file. I don't see any benefit to squeezing it into open. I do see a
downside because of an extra branchfest to differentiate the cases.

> > The routine would have to start with validating the passed O_ flags, for
> > now only allowing O_CLOEXEC and EINVAL-ing otherwise.
>
> Please do not use O_* flags! O_CLOEXEC takes up 3 flag bits on different
> architectures which makes adding new flags a nightmare.
>

With my proposal there are no new flags added so I don't think that's relevant.

> I think this should take AT_* flags and (like most newer syscalls)
> O_CLOEXEC should be automatically set. Userspace can unset it with
> fnctl(F_SETFD) in the relatively rare case where they don't want
> O_CLOEXEC. Alternatively, we could just bite the bullet and make
> AT_NO_CLOEXEC a thing...
>

I would say that's a pretty weird discrepancy vs what normally happens
with other syscalls, but perhaps it would be fine.