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

[Aleksa Sarai]

On 2026-04-27, Christian Brauner <brauner@xxxxxxxxxx> wrote:
> On Sun, Apr 12, 2026 at 03:54:33PM +0200, Jori Koolstra wrote:
> > +	if (open && !error && !is_delegated(&delegated_inode)) {
> > +		const struct path new_path = { .mnt = path.mnt, .dentry = dentry };
> > +		filp = dentry_open(&new_path, O_DIRECTORY, current_cred());
> > +	}
> 
> So definitely a patchset worthing doing but this will be hairy. And
> Mateusz is right. As written this doesn't work. The canonical pattern
> how e.g., dentry_open() does it is to preallocate the file.
> 
> I do wonder though whether we shouldn't just make O_CREAT | O_DIRECTORY
> work. I remember that I had a vague comment about this in [1] a few
> years ago (cf. [1]). It might even be less hairy to get that one right
> as all the thinking for O_CREAT is already there.

That would be my preference, as it would also allow us to use RESOLVE_*
flags nicely.

> What was the rationale for mkdirat2() instead of threading this through
> openat()/openat2() with O_CREAT?

Mateusz said that he didn't like the idea of having more branches in
the open() paths, I think that ship has long since sailed tbh.

> And side-question: @Jeff, can nfs atomic open deal with O_CREAT |
> O_DIRECTORY?
> 
> [1]: 43b450632676 ("open: return EINVAL for O_DIRECTORY | O_CREAT")

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

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