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

Stefan Metzmacher
2026-04-28T13:39:37Z

Am 27.04.26 um 17:48 schrieb Christian Brauner:
> On Sun, Apr 12, 2026 at 03:54:33PM +0200, 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, mkdirat2() 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 | 2 ++
>> fs/namei.c | 44 +++++++++++++++++++++++---
>> include/linux/syscalls.h | 2 ++
>> include/uapi/asm-generic/unistd.h | 5 ++-
>> scripts/syscall.tbl | 1 +
>> 6 files changed, 50 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
>> index 524155d655da..e200ca2067a4 100644
>> --- a/arch/x86/entry/syscalls/syscall_64.tbl
>> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
>> @@ -396,6 +396,7 @@
>> 469 common file_setattr sys_file_setattr
>> 470 common listns sys_listns
>> 471 common rseq_slice_yield sys_rseq_slice_yield
>> +472 common mkdirat2 sys_mkdirat2
>>
>> #
>> # Due to a historical design error, certain syscalls are numbered differently
>> diff --git a/fs/internal.h b/fs/internal.h
>> index cbc384a1aa09..c6a79afadacf 100644
>> --- a/fs/internal.h
>> +++ b/fs/internal.h
>> @@ -59,6 +59,8 @@ int may_linkat(struct mnt_idmap *idmap, const struct path *link);
>> int filename_renameat2(int olddfd, struct filename *oldname, int newdfd,
>> struct filename *newname, unsigned int flags);
>> int filename_mkdirat(int dfd, struct filename *name, umode_t mode);
>> +struct file *do_file_mkdirat(int dfd, struct filename *name, umode_t mode,
>> + unsigned int flags, bool open);
>> int filename_mknodat(int dfd, struct filename *name, umode_t mode, unsigned int dev);
>> int filename_symlinkat(struct filename *from, int newdfd, struct filename *to);
>> int filename_linkat(int olddfd, struct filename *old, int newdfd,
>> diff --git a/fs/namei.c b/fs/namei.c
>> index a880454a6415..6451e96dc225 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -5255,18 +5255,36 @@ struct dentry *vfs_mkdir(struct mnt_idmap *idmap, struct inode *dir,
>> }
>> EXPORT_SYMBOL(vfs_mkdir);
>>
>> -int filename_mkdirat(int dfd, struct filename *name, umode_t mode)
>> +static int mkdirat_lookup_flags(unsigned int flags)
>> +{
>> + int lookup_flags = LOOKUP_DIRECTORY;
>> +
>> + if (!(flags & AT_SYMLINK_NOFOLLOW))
>> + lookup_flags |= LOOKUP_FOLLOW;
>> + if (!(flags & AT_NO_AUTOMOUNT))
>> + lookup_flags |= LOOKUP_AUTOMOUNT;
>> +
>> + return lookup_flags;
>> +}
>> +
>> +int filename_mkdirat(int dfd, struct filename *name, umode_t mode) {
>> + return PTR_ERR_OR_ZERO(do_file_mkdirat(dfd, name, mode, 0, false));
>> +}
>> +
>> +struct file *do_file_mkdirat(int dfd, struct filename *name, umode_t mode,
>> + unsigned int flags, bool open)
>> {
>> struct dentry *dentry;
>> struct path path;
>> int error;
>> - unsigned int lookup_flags = LOOKUP_DIRECTORY;
>> + struct file *filp = NULL;
>> + unsigned int lookup_flags = mkdirat_lookup_flags(flags);
>> struct delegated_inode delegated_inode = { };
>>
>> retry:
>> dentry = filename_create(dfd, name, &path, lookup_flags);
>> if (IS_ERR(dentry))
>> - return PTR_ERR(dentry);
>> + return ERR_CAST(dentry);
>>
>> error = security_path_mkdir(&path, dentry,
>> mode_strip_umask(path.dentry->d_inode, mode));
>> @@ -5276,6 +5294,10 @@ int filename_mkdirat(int dfd, struct filename *name, umode_t mode)
>> if (IS_ERR(dentry))
>> error = PTR_ERR(dentry);
>> }
>> + 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.
>
> What was the rationale for mkdirat2() instead of threading this through
> openat()/openat2() with O_CREAT?
>
> And side-question: @Jeff, can nfs atomic open deal with O_CREAT |
> O_DIRECTORY?

If it helps the SMB2/3 protocol only has a single SMB2 Create operation
that uses FILE_CREATE+FILE_NON_DIRECTORY_FILE or FILE_CREATE+FILE_DIRECTORY_FILE.

Given all the openat() ignores unknown flags or combinations, maybe this
should be openat2 only and even a new flag (at the for the userspace interface).
or do_sys_open() will reject it for open and openat.

While we're there an O_TMPDIR would also be wonderful to have.
Currently samba works around it by using a hidden directory name, invisible
for SMB clients, but nfs and local users see it.

metze