Re: [RFC PATCH v2 1/2] vfs: syscalls: add mkdirat2() that returns an O_DIRECTORY fd

On 2026-05-11, Christian Brauner [off-list ref] wrote:

On Mon, May 04, 2026 at 07:41:15PM +0200, Jori Koolstra wrote:

quoted

Because of Mateusz' objection, but I agree with Aleksa (and you in 2023)
that this is intuitive and you mentioned POSIX allows for it.

But a more general issue, that also applies to this mkdirat2 patch,
is Linus' objection in that same thread.[1] However, the use-case of

mkdirat2() is objectively the worse api. It forces userspace to use a
separate system call without any reason whatsoever. If you can to
O_CREAT you should also be able to to O_DIRECTORY in the same system
call. If we support O_DIRECTORY | O_CREAT we get all the lookup
restriction niceties RESOLVE_* for free. Plus, it is supportable both in
openat() and openat2() because I made that combo return an errno.

UAPI design often is a nasty mix of performance (context switches),
separation of concerns and privileges, tastefulness, and compromises you
never thought or wanted to make.

I think here it is pretty clear that O_DIRECTORY | O_CREAT is the right
thing to do. Instead of restructuring a bunch of codepaths so it can be
plumbed through to the filesystems we just reuse the existing codepaths
that give us the right context for free.

FWIW, I completely agree.

And during LSFMM the VFS maintains all agreed to proceed with
O_DIRECTORY | O_CREAT.

(With the caveat that we do not copy the dangling symlink behaviour from
O_CREAT, as there be dragons.)

-- 
Aleksa Sarai
https://www.cyphar.com/