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

[Paulo Alcantara]

Stefan Metzmacher <metze@xxxxxxxxx> writes:

> 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@xxxxxxxxx>
>>> ---
>>>   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.

Yes.  However cifs.ko will handle atomic open of regular files only.

IIRC, NFS also doesn't handle atomic opens of directories either.  Jeff
could confirm that.