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

[Arnd Bergmann]

On Tue, Mar 31, 2026, at 19:19, Jori Koolstra wrote:
> Currently there is no way to race-freely create and open a directory.
> For regular files we have open(O_CREAT) for creating a new file inode,
> and returning a pinning fd to it. The lack of such functionality for
> directories means that when populating a directory tree there's always
> a race involved: the inodes first need to be created, and then opened
> to adjust their permissions/ownership/labels/timestamps/acls/xattrs/...,
> but in the time window between the creation and the opening they might
> be replaced by something else.
>
> Addressing this race without proper APIs is possible (by immediately
> fstat()ing what was opened, to verify that it has the right inode type),
> but difficult to get right. Hence, mkdirat_fd() that creates a directory
> and returns an O_DIRECTORY fd is useful.
>
> This feature idea (and description) is taken from the UAPI group:
> https://github.com/uapi-group/kernel-features?tab=readme-ov-file#race-free-creation-and-opening-of-non-file-inodes
>
> Signed-off-by: Jori Koolstra <jkoolstra@xxxxxxxxx>

I checked that the calling conventions are fine, i.e. this will work
as expected across all architectures. I assume you are also aware
that the non-RFC patch will need to add the syscall number to all
.tbl files.

The hardest problem here does seem to be the naming of the
new syscall, and I'm sorry to not be able to offer any solution
either, just two observations:

- mkdirat/mkdirat_fd sounds similar to the existing
  quotactl/quotactl_fd pair, but quotactl_fd() takes a file
  descriptor argument rather than returning it, which makes
  this addition quite confusing.

- the nicest interface IMO would have been a variation of
  openat(dfd, filename, O_CREAT | O_DIRECTORY, mode)
  but that is a minefield of incompatible implementations[1],
  so we can't do that without changing the behavior for
  existing callers that currently run into an error.

       Arnd

[1] https://lwn.net/Articles/926782/