[RFC PATCH v2 1/2] vfs: syscalls: add mkdirat2() that returns an O_DIRECTORY fd
From: Jori Koolstra
Date: Sun Apr 12 2026 - 09:56:48 EST
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());
+ }
end_creating_path(&path, dentry);
if (is_delegated(&delegated_inode)) {
error = break_deleg_wait(&delegated_inode);
@@ -5286,7 +5308,21 @@ int filename_mkdirat(int dfd, struct filename *name, umode_t mode)
lookup_flags |= LOOKUP_REVAL;
goto retry;
}
- return error;
+ if (error)
+ return ERR_PTR(error);
+ return filp;
+}
+
+#define VALID_MKDIRAT2_FLAGS (AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT)
+
+SYSCALL_DEFINE4(mkdirat2, int, dfd, const char __user *, pathname, umode_t, mode,
+ unsigned int, flags)
+{
+ CLASS(filename, name)(pathname);
+ if (flags & ~VALID_MKDIRAT2_FLAGS)
+ return -EINVAL;
+
+ return FD_ADD(O_CLOEXEC, do_file_mkdirat(dfd, name, mode, flags, true));
}
SYSCALL_DEFINE3(mkdirat, int, dfd, const char __user *, pathname, umode_t, mode)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 02bd6ddb6278..b3b4ae26dbdd 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -999,6 +999,8 @@ asmlinkage long sys_lsm_get_self_attr(unsigned int attr, struct lsm_ctx __user *
asmlinkage long sys_lsm_set_self_attr(unsigned int attr, struct lsm_ctx __user *ctx,
u32 size, u32 flags);
asmlinkage long sys_lsm_list_modules(u64 __user *ids, u32 __user *size, u32 flags);
+asmlinkage long sys_mkdirat2(int dfd, const char __user *pathname, umode_t mode,
+ unsigned int flags)
/*
* Architecture-specific system calls
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index a627acc8fb5f..6efc21779b62 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -863,8 +863,11 @@ __SYSCALL(__NR_listns, sys_listns)
#define __NR_rseq_slice_yield 471
__SYSCALL(__NR_rseq_slice_yield, sys_rseq_slice_yield)
+#define __NR_mkdirat2 472
+__SYSCALL(__NR_mkdirat2, sys_mkdirat2)
+
#undef __NR_syscalls
-#define __NR_syscalls 472
+#define __NR_syscalls 473
/*
* 32 bit systems traditionally used different
diff --git a/scripts/syscall.tbl b/scripts/syscall.tbl
index 7a42b32b6577..9d86f29762ae 100644
--- a/scripts/syscall.tbl
+++ b/scripts/syscall.tbl
@@ -412,3 +412,4 @@
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
--
2.53.0