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

[Yann Droneaud <yann@droneaud.fr>]

Hi,

Le 31/03/2026 à 19:19, Jori Koolstra a écrit :
> 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@xs4all.nl>
> ---
>   arch/x86/entry/syscalls/syscall_64.tbl |  1 +
>   fs/internal.h                          |  1 +
>   fs/namei.c                             | 26 ++++++++++++++++++++++++--
>   include/linux/fcntl.h                  |  2 ++
>   include/linux/syscalls.h               |  2 ++
>   include/uapi/asm-generic/fcntl.h       |  3 +++
>   include/uapi/asm-generic/unistd.h      |  5 ++++-
>   scripts/syscall.tbl                    |  1 +
>   8 files changed, 38 insertions(+), 3 deletions(-)

> diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
> index a332e79b3207..d2f0fdb82847 100644
> --- a/include/linux/fcntl.h
> +++ b/include/linux/fcntl.h
> @@ -25,6 +25,8 @@
>   #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
>   #endif
>   
> +#define VALID_MKDIRAT_FD_FLAGS	(MKDIRAT_FD_NEED_FD)
> +

I don't see support for O_CLOEXEC-ish flag, is the file descriptor in 
close-on-exec mode by default ? If yes, it should be mentioned.


> diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> index 613475285643..621458bf1fbf 100644
> --- a/include/uapi/asm-generic/fcntl.h
> +++ b/include/uapi/asm-generic/fcntl.h
> @@ -95,6 +95,9 @@
>   #define O_NDELAY	O_NONBLOCK
>   #endif
>   
> +/* Flags for mkdirat_fd */
> +#define MKDIRAT_FD_NEED_FD	0x01
> +


Regards.